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BGP4 Case Studies/Tutorial

Sam Halabi-cisco Systems

The purpose of this paper is to introduce the reader to the latest in BGP4 terminology and
design issues. It is targeted to the novice as well as the experienced user. For any clarifica-
tion or comments please send e-mail to

Copyright 1995 ©Cisco Systems Inc.

1/26/96-Rev: A1.2 Page 1 Sam Halabi-cisco Systems

1.0 Introduction..............................................................................................................4
1.1 How does BGP work ...........................................................................................................4
1.2 What are peers (neighbors) ..................................................................................................4
1.3 Information exchange between peers...................................................................................4
2.0 EBGP and IBGP ......................................................................................................5
3.0 Enabling BGP routing..............................................................................................6
3.1 BGP Neighbors/Peers ..........................................................................................................7
4.0 BGP and Loopback interfaces ...............................................................................10
5.0 EBGP Multihop .....................................................................................................11
5.1 EBGP Multihop (Load Balancing) ....................................................................................12
6.0 Route Maps ............................................................................................................13
7.0 Network command.................................................................................................17
7.1 Redistribution.....................................................................................................................18
7.2 Static routes and redistribution ..........................................................................................20
8.0 Internal BGP ..........................................................................................................22
9.0 The BGP decision algorithm..................................................................................23
10.0 As_path Attribute...................................................................................................24
11.0 Origin Attribute......................................................................................................25
12.0 BGP Nexthop Attribute..........................................................................................27
12.1 BGP Nexthop (Multiaccess Networks)..............................................................................29
12.2 BGP Nexthop (NBMA) .....................................................................................................30
12.3 Next-hop-self .....................................................................................................................31
13.0 BGP Backdoor .......................................................................................................32
14.0 Synchronization .....................................................................................................34
14.1 Disabling synchronization .................................................................................................35
15.0 Weight Attribute.....................................................................................................37
16.0 Local Preference Attribute.....................................................................................39
17.0 Metric Attribute .....................................................................................................41
18.0 Community Attribute .............................................................................................44
19.0 BGP Filtering .........................................................................................................45
19.1 Route Filtering ...................................................................................................................45
19.2 Path Filtering......................................................................................................................47
19.2.1 AS-Regular Expression .......................................................................................49
19.3 BGP Community Filtering.................................................................................................50
20.0 BGP Neighbors and Route maps ...........................................................................53
20.1 Use of set as-path prepend .................................................................................................55
20.2 BGP Peer Groups...............................................................................................................56
21.0 CIDR and Aggregate Addresses ............................................................................58

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21.1 Aggregate Commands........................................................................................................59
21.2 CIDR example 1 ................................................................................................................61
21.3 CIDR example 2 (as-set)....................................................................................................63
22.0 BGP Confederation................................................................................................65
23.0 Route Reflectors.....................................................................................................68
23.1 Multiple RRs within a cluster ............................................................................................71
23.2 RR and conventional BGP speakers ..................................................................................73
23.3 Avoiding looping of routing information...........................................................................74
24.0 Route Flap Dampening ..........................................................................................75
25.0 How BGP selects a Path ........................................................................................79
26.0 Practical design example: ......................................................................................80

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BGP keeps a version number of the BGP table and it should be the same for all of its BGP peers. 1. 1. From then on incremental updates are sent as the routing table changes. this information is mainly an indication of the full paths (BGP AS numbers) that a route should take in order to reach the destination network. BGP routers will exchange network reachability information.0 Introduction The Border Gateway Protocol (BGP).2 What are peers (neighbors) Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers. An autonomous system is a set of routers under a single technical administration. 1/26/96-Rev: A1.3 Information exchange between peers BGP peers will initially exchange their full BGP routing tables. defined in RFC 1771.2 Page 4 Sam Halabi-cisco Systems . 1. The version number will change whenever BGP updates the table due to some routing information changes. This information will help in constructing a graph of ASs that are loop free and where routing policies can be applied in order to enforce some restrictions on the routing behavior. Keepalive packets are sent to ensure that the connection is alive between the BGP peers and notification packets are sent in response to errors or special conditions.1. Routers in an AS can use multiple interior gateway protocols to exchange routing information inside the AS and an exterior gateway protocol to route packets outside the AS.1 How does BGP work BGP uses TCP as its transport protocol (port 179). they are also called neighbors. allows you to create loop free interdomain routing between autonomous systems. Two BGP speaking routers form a TCP connection between one another (peer routers) and exchange messages to open and confirm the connection parameters.

As far as this paper is concerned. AS200 is a transit autonomous system for AS100 and AS300. This is done by a combination of Internal BGP peering between routers inside an AS and by redistributing BGP information to Internal Gateway protocols running in the AS. It is necessary to ensure reachability for networks within an AS before sending the information to other external ASs.2. EBGP AS100 IBGP AS300 AS200 1/26/96-Rev: A1. it could be used as a transit service for other ASs. when BGP is running between routers belonging to two different ASs we will call it EBGP (Exterior BGP) and for BGP running between routers in the same AS we will call it IBGP (Interior BGP).2 Page 5 Sam Halabi-cisco Systems . As you see below.0 EBGP and IBGP If an Autonomous System has multiple BGP speakers.

The next step in the configuration process is to define BGP neighbors. In the first example RTA and RTB are in different autonomous systems and in the second example both routers belong to the same AS. We start by defining the router process and define the AS number that the routers belong to: The command used to enable BGP on a router is: router bgp autonomous-system RTA# router bgp 100 RTB# router bgp 200 The above statements indicate that RTA is running BGP and it belongs to AS100 and RTB is running BGP and it belongs to AS200 and so on. The neighbor definition indicates which routers we are trying to talk to with BGP.2 Page 6 Sam Halabi-cisco Systems . Let us assume you want to have two routers RTA and RTB talk BGP.3.0 Enabling BGP routing Here are the steps needed to enable and configure BGP. 1/26/96-Rev: A1. The next section will introduce you to what is involved in forming a valid peer connection.

the BGP router ID and the keepalive hold time. the BGP version they are running (version 3 or 4). etc. The TCP connection is essential in order for the two peer routers to start exchanging routing updates. 1/26/96-Rev: A1.A special case for loopback interfaces is discussed later. It is essential that the two IP addresses used in the neighbor command of the peer routers be able to reach one another. 1. 2. One sure way to verify reachability is an extended ping between the two IP addresses. Two BGP speaking routers trying to become neighbors will first bring up the TCP connection between one another and then send open messages in order to exchange values such as the AS number. Any state other than established is an indication that the two routers did not become neighbors and hence the BGP updates will not be exchanged.A special case (EBGP multihop) will be discussed later when the external BGP peers are not directly connected. The ip-address is the next hop directly connected address for EBGP1 and any IP address2 on the other router for IBGP. After these values are confirmed and accepted the neighbor connection will be established.3. The neighbor command used to establish a TCP connection is: neighbor ip-address remote-as number The remote-as number is the AS number of the router we are trying to connect to via BGP. the extended ping forces the pinging router to use as source the IP address specified in the neighbor command rather than the IP address of the interface the packet is going out from.1 BGP Neighbors/Peers Two BGP routers become neighbors or peers once they establish a TCP connection between one another.2 Page 7 Sam Halabi-cisco Systems .

BGP sessions begin using BGP Version 4 and negotiating downward to earlier versions if necessary.213.1 remote-as 200 RTB# router bgp 200 neighbor 129.2 AS200 RTA# router bgp 100 neighbor 129. perform the following task in router configuration mode: neighbor {ip address|peer-group-name} version value An example of the neighbor command configuration follows: RTA RTD EBGP 129. To prevent negotiations and force the BGP version used to communicate with a neighbor. is important to reset the neighbor connection in case any bgp configuration changes are made in order for the new parameters to take effect.2 remote-as 200 RTC# router bgp 200 neighbor 175.1.1 RTB IBGP RTC 175.2 AS100 AS300 129.2 remote-as 100 neighbor 175.220.1 remote-as 200 1/26/96-Rev: A1.1. clear ip bgp address (where address is the neighbor address) clear ip bgp * (clear all neighbor connections) By default.2 Page 8 Sam Halabi-cisco Systems .220.220.

212.1. dropped 10 In the next section we will discuss special situations such as EBGP multihop and loopback addresses. remote router ID 175. Anything other than state established indicates that the peers are not up. 1/26/96-Rev: A1. 0 notifications. hold time is 180. The following is an example of the information that the command “sh ip bgp neighbors” will show you. 0 in queue Sent 2826 messages. keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 2828 messages. pay special attention to the BGP state. The difference between EBGP and IBGP is manifested by having the remote-as number pointing to either an external or an internal AS. as long as there is some IGP running that allows the two neighbors to reach one another. table version = 3. IBGP routers do not have to be directly connected. external link BGP version 4. up for 0:10:59 Last read 0:00:29. the remote router ID (highest IP address on that box or the highest loopback interface in case it exists) and the table version (this is the state of the table. 0 notifications. Any time new information comes in. RTB and RTC are run- ning IBGP. Also. 0 in queue Connections established 11.1 BGP state = Established. the EBGP peers are directly connected and the IBGP peers are not.213.2 Page 9 Sam Halabi-cisco Systems .In the above example RTA and RTB are running EBGP. You should also note the BGP is version 4.220. remote AS 200. the table will increase the version and a version that keeps incrementing indicates that some route is flapping causing routes to keep getting updated).1. #SH IP BGP N BGP neighbor is 129.

11.4. in this case RTA has to force BGP to use the loopback IP address as the source in the TCP neighbor connection.212.11. Loopback Interface 1 150.1 update-source int loopback 1) and this statement forces BGP to use the IP address of its loopback interface when talking to neighbor 190. If the IP address of a loopback interface is used in the neighbor com- mand.212.225. The command used to indicate a loopback interface is: neighbor ip-address update-source interface The following example should illustrate the use of this command.1 AS100 RTA# router bgp 100 neighbor 190.225.212. The neighbor router needs to tell BGP that it is using a loopback interface rather than a physical interface to initiate the BGP neighbor TCP connection.1). 1/26/96-Rev: A1.1 remote-as 100 In the above example.1.1. some extra configuration needs to be done on the neighbor router. In the case of EBGP.1.225.2 Page 10 Sam Halabi-cisco Systems . most of the time the peer routers are directly connected and loopback does not apply.1 remote-as 100 neighbor 190.1. RTA and RTB are running internal BGP inside autonomous system 100.11.225. RTB is using in its neighbor command the loopback interface of RTA (150. Normally the loopback interface is used to make sure that the IP address of the neighbor stays up and is independent of an interface that might be flaky. RTA will do so by adding the update-source int loopback configuration (neighbor update-source int loopback 1 RTB# router bgp 100 neighbor 150.11.0 BGP and Loopback interfaces Using a loopback interface to define neighbors is commonly used with IBGP rather than EBGP.1 RTB RTA 190.

11.3 remote-as 100 RTA is indicating an external neighbor that is not directly connected.1 ebgp-multihop RTB# router bgp 300 neighbor 129.2 remote-as 300 neighbor 180. 1/26/96-Rev: A1. The following example shows how to achieve load balancing with BGP in a particular case where we have EBGP over parallel lines.213.213. In this case EBGP multihop is used to allow the neighbor connection to be established between two non directly con- nected external peers.1 AS100 AS300 RTA# router bgp 100 neighbor 180.225. RTA RTB 129.213.0 EBGP Multihop In some special cases. there could be a requirement for EBGP speakers to be not directly connected.2 Page 11 Sam Halabi-cisco Systems . The multihop is used only for external BGP and not for internal BGP. RTA needs to indicate that it will be using ebgp-multihop.11. Some IGP or static routing should also be configured in order to allow the non directly connected neighbors to reach one another. On the other hand. RTB is indicating a neighbor that is directly connected (129.Note that RTA has used the physical interface IP address ( and that is why it does not need the ebgp-multihop command. 5.225.1) of RTB as a neighbor and that is why RTB does not need to do any special configuration. The following example gives a better illustration of EBGP multihop.225.

5.1 EBGP Multihop (Load Balancing) loopback loopback

AS 100 AS 200

int loopback 0
ip address

router bgp 100
neighbor remote-as 200
neighbor ebgp-multihop
neighbor update-source loopback 0

ip route
ip route

int loopback 0
ip address

router bgp 200
neighbor remote-as 100
neighbor update-source loopback 0
neighbor ebgp-multihop

ip route
ip route

The above example illustrates the use of loopback interfaces,
update-source and ebgp-multihop. This is a workaround in order to achieve
load balancing between two EBGP speakers over parallel serial lines. In
normal situations, BGP will pick one of the lines to send packets on and
load balancing would not take place. By introducing loopback interfaces,
the next hop for EBGP will be the loopback interface. Static routes (it
could be some IGP also) are used to introduce two equal cost paths to
reach the destination. RTA will have two choices to reach next hop one via and the other one via and the same
for RTB.

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6.0 Route Maps

At this point I would like to introduce route maps because they will be
used heavily with BGP. In the BGP context, route map is a method used to
control and modify routing information. This is done by defining condi-
tions for redistributing routes from one routing protocol to another or
controlling routing information when injected in and out of BGP. The for-
mat of the route map follows:

route-map map-tag [[permit | deny] | [sequence-number]]

The map-tag is just a name you give to the route-map. Multiple instances
of the same route map (same name-tag) can be defined. The sequence number
is just an indication of the position a new route map is to have in the
list of route maps already configured with the same name.

For example, if I define two instances of the route map, let us call it
MYMAP, the first instance will have a sequence-number of 10, and the
second will have a sequence number of 20.

route-map MYMAP permit 10
(first set of conditions goes here.)

route-map MYMAP permit 20
(second set of conditions goes here.)

When applying route map MYMAP to incoming or outgoing routes, the first
set of conditions will be applied via instance 10. If the first set of
conditions is not met then we proceed to a higher instance of the route

The conditions that we talked about are defined by the match and set
configuration commands. Each route map will consist of a list of match
and set configuration. The match will specify a match criteria and set
specifies a set action if the criteria enforced by the match command are

For example, I could define a route map that checks outgoing updates and
if there is a match for IP address then the metric for that
update will be set to 5. The above can be illustrated by the following

match ip address
set metric 5

Now, if the match criteria are met and we have a permit then the routes
will be redistributed or controlled as specified by the set action and we
break out of the list.

If the match criteria are met and we have a deny then the route will not
be redistributed or controlled and we break out of the list.

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If the match criteria are not met and we have a permit or deny then the
next instance of the route map (instance 20 for example) will be checked,
and so on until we either break out or finish all the instances of the
route map. If we finish the list without a match then the route we are
looking at will not be accepted nor forwarded.

One restriction on route maps is that when used for filtering BGP updates
(as we will see later) rather than when redistributing between protocols,
you can NOT filter on the inbound when using a “match” on the ip address.
Filtering on the outbound is OK.

The related commands for match are:

match as-path
match community
match clns
match interface
match ip address
match ip next-hop
match ip route-source
match metric
match route-type
match tag

The related commands for set are:

set as-path
set automatic-tag
set community
set clns
set interface
set default interface
set ip next-hop
set ip default next-hop
set ip precedence
set tos
set level
set local-preference
set metric
set metric-type
set next-hop
set origin
set tag
set weight

Let’s look at some route-map examples:

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10.3.0 0.0 with a metric of 2 and all other routes with a metric of 5 then we might use the following configuration: RTA# router rip network 3.3 remote-as 300 network AS 100 RTA is getting updates via BGP and redistributing them to rip.2 Page 15 Sam Halabi-cisco Systems . 150.0.3 RTC passive-interface Serial0 redistribute bgp 100 route-map SETMETRIC router bgp 100 neighbor 2.0 network 2.0 network 150.0.0 route-map SETMETRIC permit 10 match ip-address 1 set metric 2 route-map SETMETRIC permit 20 set metric 5 access-list 1 permit 170.3 3.255 1/26/96-Rev: A1.0. If RTA wants to redistribute to RTB routes about 170.0 AS 300 Example 1: Assume RTA and RTB are running rip. RTA and RTC are running BGP.2.0 RTA RTB 3.2.0.

let’s look at how to start exchanging network information.2.In the above example if a route matches the IP address 170.2 route-map STOPUPDATES out route-map STOPUPDATES permit 10 match ip address 1 access-list 1 deny 170.0.2. There are multiple ways to send network information using BGP.0. Since route maps cannot be applied on the inbound when matching based on an ip address.255.255. set everything else to metric 5.0 0.255 access-list 1 permit 0.0.0. It is always very important to ask the question.10.0 255.0.0 it will have a metric of 2 and then we break out of the route map list.0. we have to use an outbound route map on RTC: RTC# router bgp 300 network 170. 1/26/96-Rev: A1. what will happen to routes that do not match any of the match statements because they will be dropped by default.255 Now that you feel more comfortable with how to start BGP and how to define a neighbor.2 Page 16 Sam Halabi-cisco Systems .2.0 neighbor 2. Example 2: Suppose in the above example we did not want AS100 to accept updates about 170. If there is no match then we go down the route map list which says.10.2.2 remote-as 100 neighbor I will go through these methods one by one.10.

rather we are trying to indicate to BGP what networks it should originate from this box.255.0 Network command The format of the network command follows: network network-number [mask network-mask] The network command controls what networks are originated by this box. The network command will work if the network you are trying to advertise is known to the router. A maximum of 200 entries of the network command are accepted. 1/26/96-Rev: A1.0 because the static route will put a matching entry in the routing table. An example of the network command follows: RTA# router bgp 1 network 192.0.0 ip route 192.213.2 Page 17 Sam Halabi-cisco Systems . static or learned dynamically.0/16.0 null 0 The above example indicates that router A.0. whether connected.213.213. Note that we need the static route to get the router to generate The mask portion is used because BGP4 can handle subnetting and supernetting.0. The /16 indicates that we are using a supernet of the class C address and we are advertizing the first two octets (the first 16 bits). This is a different concept from what you are used to configuring with IGRP and RIP.7. With this command we are not trying to run BGP on a certain interface.0 mask 255.0 255. will generate a network entry for 192.0.255.

1.2 RTB RTA RTC AS100 175. Look at RTC’s configuration: RTD AS300 1. etc.213.0.) into BGP.1. RTA is announcing 129.1 Redistribution The network command is one way to advertise your networks via BGP.1. Careful filtering should be applied to make sure you are sending to the internet only routes that you want to advertise and not everything you have.0) If you use redistribution instead you will have: 1/26/96-Rev: A1.1 remote-as 300 network 175.0.0. OSPF. (this will limit the networks originated by your AS to 175. Let us look at the example below. This sounds scary because now you are dumping all of your internal routes into BGP. RIP.1.0.220. some of these routes might have been learned via BGP and you do not need to send them out again. Another way is to redistribute your IGP (IGRP.220.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1.0 129.220.0 mask and RTC is announcing AS200 If you use a network command you will have: RTC# router eigrp 10 network 175.2 Page 18 Sam Halabi-cisco Systems .1.255. EIGRP. The correct configuration would be: RTC# router eigrp 10 network 175.0. 1/26/96-Rev: A1.0.2 Page 19 Sam Halabi-cisco Systems .1. This is misleading because you are not the source of The access-list is used to control what networks are to be originated from AS200.0 0. So you would have to use filters to prevent that network from being sourced out by your AS.0 but AS100 is.1 distribute-list 1 out redistribute eigrp 10 access-list 1 permit 175.220.RTC# router eigrp 10 network again into BGP) This will cause 129.1.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor remote-as 300 neighbor 1.1 remote-as 300 redistribute eigrp 10 (eigrp will inject 129.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1.213.0 to be originated by your AS.

0 Please remember that these routes are generated in addition to other BGP routes that BGP has learned via neighbors (internal or external). will indicate your AS as the origin for these networks. We have discussed how we can use different methods to originate routes out of our autonomous system. The only difference is that BGP will consider these routes as having an origin of incomplete (unknown).1 remote-as 300 redistribute static ip route 175.2 Static routes and redistribution You could always use static routes to originate a network or a subnet. This is a nice way to advertise a supernet. Page 20 Sam Halabi-cisco Systems . So if I get the packet and there is a more specific match than 175.255. or redistribution or static.7.0.255. BGP passes on information that it learns from one peer to other peers.0 (which exists of course) the router will send it to the specific match otherwise it will disregard it. Injecting BGP into IGP is always done by redistribution. The difference is that routes generated by the network command. 1/26/96-Rev: A1.0 null0 The null 0 interface means to disregard the packet.1.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1. In the above example the same could have been accomplished by doing: RTC# router eigrp 10 network 175.

1/26/96-Rev: A1. RTC 150.2 remote-as 300 network 160.0.0 in RTC unless you want RTC to also generate these networks on top of passing them on as they come in from AS100 and AS200.20.20.00 Note that you do not need network 150.10.0 RTB# router bgp 200 neighbor AS 200 remote-as 100 neighbor 160.0 or network 160.0 RTB 150.2 remote-as 300 network An important point to remember is that BGP will not accept updates that have originated from its own AS.10.10.0 RTC# router bgp 300 neighbor remote-as 200 network 170.0. Again the difference is that the network command will add an extra advertisement for these same networks indicating that AS300 is also an origin for these routes.2 160.0 AS 300 RTA# router bgp 100 neighbor 150.10. This is to insure a loop free interdomain topology.Example: 150.0 AS 100 RTA 160.20.2 Page 21 Sam Halabi-cisco Systems .10.10.0.

0.2 RTA AS 100 170. 8.2 remote-as 300 network 1/26/96-Rev: A1.20.10.2 Page 22 Sam Halabi-cisco Systems .0.10. then RTC will pass this route to AS200 with the origin kept as AS100.50.0. why can’t we do the same thing by learning via EBGP redistributing into IGP and then redistributing again into another AS? We can. RTC 175. RTA will notice that the update has originated from its own AS and will ignore it.1 150. for example IBGP provides us with ways to control what is the best exit point out of the AS by using local preference (will be discussed later). IBGP 175.0.0 AS300 RTA# router bgp 100 neighbor to AS100 with origin still AS100.50.10. You might ask.10.For example. assume AS200 above had a direct BGP connection into AS100.1 RTE 175.0 and will send it to AS300.10. RTA will generate a route 150.1 remote-as 100 neighbor 170. RTB will pass 150.30.1 170.40.0 AS500 AS400 170. but IBGP offers more flexibility and more efficient ways to exchange information within an AS. RTD IBGP RTB 190.0 Internal BGP IBGP is used if an AS wants to act as a transit system to other ASs.

1/26/96-Rev: A1.0 An important point to remember.1 remote-as 400 network 190.1 remote-as 100 neighbor 175. In the above diagram. metric and so on. RTA and RTB are running IBGP and RTA and RTD are running IBGP also. local preference. The decision process is based on different attributes.2 remote-as 100 network 175. The receiving BGP speaker will redistribute that information to other BGP speakers outside of its AS.10.RTB# router bgp 100 neighbor 150.10.10. This is why an IBGP peering should be made between RTB and RTD in order not to break the flow of the updates. the protocol will have to decide which paths to choose in order to reach a specific destination. BGP will choose only a single path to reach a specific destination. 9. That is why it is important to sustain a full mesh between the IBGP speakers within an AS.10.0 The BGP decision algorithm After BGP receives updates about different destinations from different autonomous systems. We will start with the path attribute. BGP will always propagate the best path to its neighbors. origin code. is that when a BGP speaker receives an update from other BGP speakers in its own AS (IBGP). such as next hop. the route origin. path length.0 RTC# router bgp 400 neighbor 175. In the following section I will try to explain these attributes and show how they are used.40. The BGP updates coming from RTB to RTA will be sent to RTE (outside of the AS) but not to RTD (inside of the AS). administrative weights. the receiving BGP speaker will not redistribute that information to other BGP speakers in its own AS.2 Page 23 Sam Halabi-cisco Systems .

10.0 is advertised by RTB in AS200.0 reaches RTA it will have two AS numbers attached to it: first 200 then 300.0. In the above example.e.0.10.0 is (300. The same applies for 170.10.0 190.100) i.10.200). 1/26/96-Rev: A1.0. RTB will have to take path (300.0. An AS-SET is an ordered mathematical set {} of all the ASs that have been traversed.0 As_path Attribute 170. An example of AS-SET is given later.10. network and path (100) in order to reach 170.0. traverse AS300 and then AS100 in order to reach 170.0. RTC will have to traverse path (200) in order to reach 190.0 and 180.10.2 Page 24 Sam Halabi-cisco Systems . The AS_path attribute is actually the list of AS numbers that a route has traversed in order to reach a destination.10.0 AS 100 AS200 RTB RTA AS 300 RTC 100 200 180. when that route traverses AS300 and RTC will append its own AS number to it.0. So as far as RTA is concerned the path to reach 190.10. So when 300 Whenever a route update passes through an AS.10.0. the AS number is prepended to that update.10.0.0.

10.20.0 AS300 1/26/96-Rev: A1.30.10. Example: RTA RTB 190.1 175.20. The origin attribute can assume three values: IGP: Network Layer Reachability Information (NLRI) is interior to the originating AS.2 170.10. This is indicated with an “i” in the BGP table.50. INCOMPLETE: NLRI is unknown or learned via some other means. This normally happens when we use the bgp network command or when IGP is redistributed into BGP.10.40.10. This is indicated with an “?” in the BGP table.1 150.11.2 RTE 170. then the origin of the path info will be IGP.0. This usually occurs when we redistribute a static route into BGP and the origin of the route will be incomplete.10. EGP: NLRI is learned via EGP (Exterior Gateway Protocol). This is indicated with an “e” in the BGP table.2 Page 25 Sam Halabi-cisco Systems .1 IBGP AS 100 170.0 Origin Attribute The origin is a mandatory attribute that defines the origin of the path information.

1 remote-as 100 network 170.RTA# router bgp 100 neighbor 190.0 255.2 Page 26 Sam Halabi-cisco Systems .1 remote-as 100 neighbor 170.0 null0 RTB# router bgp 100 neighbor 150. RTE will reach 150.255. RTE will also reach 190.10.1 remote-as 100 network 190.0 via: 100 i (the next AS is 100 and the origin is IGP).0 redistribute static ip route RTA will also reach the entry is in the same AS and the origin is IGP).0.0 RTA will reach 170. coming from a static route).10.0 via: i (which means. RTE# router bgp 300 neighbor 170.10.0 via: 300 i (which means the next AS path is 300 and the origin of the route is IGP). remote-as 300 network 150.0. 1/26/96-Rev: A1. via: 100 ? (the next AS is 100 and the origin is incomplete “?”.10.

2 and RTA will advertise the protocol states that the next hop advertised by EBGP should be carried into IBGP.10. if RTB is running igrp you could also run IGRP on RTA network 170.0 BGP Nexthop Attribute 150. You would want to make IGRP passive on the link to RTC so BGP is only exchanged. RTA will advertise 170.0.10. 1.2 RTC 170. So according to RTB.10. In the above example.0.10. For example. to RTA with a next hop of 170. Because of that rule.1 150.0 to its IBGP peer RTB with a next hop of 170.50.2 and NOT 150.0 to RTC with a next hop of 170.20. You should make sure that RTB can reach is not true if the next hop is on a multiaccess media. the next hop to reach 170.0 RTA RTB 150.0.1. For IBGP.2 via IGP. otherwise RTB will drop packets destined to 170.10.0 is 170.20.2 Page 27 Sam Halabi-cisco Systems . This is described in the following sections.10. 1/26/96-Rev: A1.10. For EBGP. RTC will advertise 170.10.20. the next hop is always the IP address of the neighbor specified in the neighbor command1.12.1 IBGP 170.0.0 because the next hop address would be inaccessible.1 AS 100 170.10.0 AS 300 The BGP nexthop attribute is the next hop IP address that is going to be used to reach a certain destination.10. in which case the nexthop will be the ip address of the router that is closest to the destination.10.10.10.


router bgp 100
neighbor remote-as 300
neighbor remote-as 100

router bgp 100
neighbor remote-as 100

router bgp 300
neighbor remote-as 100

*RTC will advertise to RTA with a NextHop =
*RTA will advertise to RTB with a NextHop=
(The external NextHop via EBGP is sent via IBGP)

Special care should be taken when dealing with multiaccess and NBMA
networks as described in the following sections.

1/26/96-Rev: A1.2 Page 28 Sam Halabi-cisco Systems

12.1 BGP Nexthop (Multiaccess Networks) AS 100

AS 300

The following example shows how the nexthop will behave on a multiaccess
network such as ethernet.

Assume that RTC and RTD in AS300 are running OSPF. RTC is running BGP
with RTA. RTC can reach network via When RTC
sends a BGP update to RTA regarding it will use as next hop and not its own IP address ( This is because the
network between RTA, RTC and RTD is a multiaccess network and it makes
more sense for RTA to use RTD as a next hop to reach rather
than making an extra hop via RTC.

*RTC will advertise to RTA with a NextHop =

If the common media to RTA, RTC and RTD was not multiaccess, but NBMA
(Non Broadcast Media Access) then further complications will occur.

1/26/96-Rev: A1.2 Page 29 Sam Halabi-cisco Systems

12.2 BGP Nexthop (NBMA) AS 100

FR AS 400

AS 300

If the common media as you see in the shaded area above is a frame relay
or any NBMA cloud then the exact behavior will occur as if we were
connected via ethernet. RTC will advertise to RTA with a next
hop of

The problem is that RTA does not have a direct PVC to RTD, and cannot
reach the next hop. In this case routing will fail.

In order to remedy this situation a command called NextHopself is

1/26/96-Rev: A1.2 Page 30 Sam Halabi-cisco Systems

3 Next-hop-self Because of certain situations with the nexthop as we saw in the previous example.0 with a NextHop = the syntax is: neighbor {ip-address|peer-group-name1} next-hop-self The next-hop-self command will allow us to force BGP to use a specified IP address as the next hop rather than letting the protocol choose the nexthop.20. a command called next-hop-self is created.1 remote-as 100 neighbor Page 31 Sam Halabi-cisco Systems .2 1.We will discuss peer-group-names later on 1/26/96-Rev: A1.10.20.1 next-hop-self RTC will advertise 180. In the previous example the following will solve our problem: RTC# router bgp 300 neighbor 170.

0.1 3.2 3. but that could be changed by the distance command: distance bgp external-distance internal-distance local-distance external-distance:20 internal-distance:200 local-distance:200 RTA will pick EBGP via RTC because of the lower distance. EBGP updates have a distance of 20 which is lower than the IGP distances.3.0 AS 300 Consider the above diagram.0. etc. BGP has the following distances.0 BGP Backdoor 150.0 via two routing protocols: EBGP with a distance of 20 and IGP with a distance higher than RTC 170. 100 for IGRP. Default distance is 120 for RIP. then we have two options: 1/26/96-Rev: A1. By default.2.3.0 160.2 Page 32 Sam Halabi-cisco Systems . via RTB (IGP).13. 90 for EIGRP and 110 for OSPF.10. RTA and RTC are running EBGP and RTB and RTC are running EBGP.) By definition.3.2.2. IGRP.3 AS 200 AS 100 2.0 RTA RTB IGP 2. If we want RTA to learn about RTA and RTB are running some kind of IGP (RIP. RTA will receive updates about 160.

0.2. For BGP this network will be treated as a locally assigned network except it will not be advertised in bgp updates. RTA will learn 160.0 router bgp 100 neighbor 2. The configured network is the network that we would like to reach via IGP.10.0.0 backdoor Network from RTB via EIGRP with distance 90. Example: RTA# router eigrp 10 network 160.1 remote-as 300 network the preferred route.Use BGP backdoor BGP backdoor will make the IGP route. Use the following command: network address backdoor.2 Page 33 Sam Halabi-cisco Systems . Normally EBGP will be preferred.Change EBGP’s external distance or IGP’s distance which is NOT recommended. but because of the backdoor command EIGRP will be preferred.10. 1/26/96-Rev: A1.2.0 will be treated as a local entry but will not be advertised as a normal network entry would. 2. and will also learn it from RTC via EBGP with distance 20.

10.0 RTE Was 170.2 Page 34 Sam Halabi-cisco Systems .0 then traffic coming from RTD to RTB with destination 170.2.0. Assume that RTA has not redistributed network 170.0 even exists.2. RTB will have to send the traffic to RTE.2 170.0. If RTB starts advertising to AS400 that he can reach 170.0. In order to reach the next hop. 1.10.2. Synchronization states: If your autonomous system is passing traffic from another AS to a third AS.2. so at this point RTE has no idea that 170.14. RTA and RTB are running IBGP.0.0 propagated within IGP? RTA RTB IBGP 2. BGP should not advertise a route before all routers in your AS have learned about the route via IGP1. will flow in and get dropped at RTE.10. BGP will wait until IGP has propagated the route within the AS and then will advertise it to external peers. 1/26/96-Rev: A1.0. This could be done by defining a static route.1 RTD RTC AS400 170.10.0 into IGP.0.0 via next hop 2.As far as the router is concerned.1 (remember that the next hop is carried via IBGP).2.0 AS 100 170. This is called synchronization.0 AS 300 Before we discuss synchronization let us look at the following scenario. RTC in AS300 is sending updates about Synchronization 150. so RTB will get the update and will be able to reach 170.10. we will check to see if we have a route in the ip routing table.0.

0.0. (Make sure you do a clear ip bgp address to reset the session) 1/26/96-Rev: A1.10. if you have all your routers in the AS running BGP and you are not running any IGP. or if all routers in your AS will be running BGP.10. and your router will be waiting forever for an IGP update about a certain route before sending it to external peers.0. Disabling this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly.1 Disabling synchronization In some cases you do not need synchronization. router bgp 100 no synchronization. We can fool RTB into thinking that IGP has propagated the information by adding a static route in RTB pointing to 170.2 Page 35 Sam Halabi-cisco Systems . Disabling synchronization is not automatic. If you will not be passing traffic from a different autonomous system through your AS. 14.10.0. You have to disable synchronization manually in this case for routing to work correctly. you can disable synchronization. RTB will wait to hear about 170.0 via IGP before it starts sending the update to RTD. the router has no way of knowing that.0 otherwise we will have a problem reaching that network.In the above example. Care should be taken to make sure that other routers can reach 170.

2.2 RTC RTD AS400 175.0) RTD# router bgp 400 neighbor 170.3.0 AS300 RTB# router bgp 100 network remote-as 400 neighbor 3.10.0 RTA# router bgp 100 network 150.3 remote-as 100 no synchronization (RTB will put Page 36 Sam Halabi-cisco Systems . neighbor 1.10.2.Example: 150.1 remote-as 100 network 175.0 AS100 170.3.0 propagated within IGP? RTA RTB IBGP 170.0.3 1.0 neighbor 3.0 in its ip routing table and will advertise it to RTD even if it does not have an IGP path to 170.4 remote-as 100 1/26/96-Rev: A1.1.0 Was 170.10.0 2.1

10.0. The weight is assigned locally to the router.0. RTB has also learned about network 175.10.0 190.0.0) w=200 RTC w= 100 AS300 The weight attribute is a Cisco defined attribute.2 Page 37 Sam Halabi-cisco Systems .Using AS path access-lists ip as-path access-list access-list-number {permit|deny} as-regular- expression neighbor ip-address filter-list access-list-number weight weight 1/26/96-Rev: A1.10.0. Routes with a higher weight are preferred when multiple routes exist to the same destination.0 AS100 175. A weight can be a number from 0 to 65535.0.0. RTA has learned about network 175.2 1.2.0. This is achieved by using multiple methods: 1. The weight is used for a best path selection process. (175. It is a value that only makes sense to the specific router and which is not propagated or carried through any of the route updates.0 and has to decide which way to go.10.0 Weight Attribute Let us study the above example. Paths that the router originates have a weight of 32768 by default and other paths have a weight of zero.1.0.0 from AS4 and will propagate it to RTC.10.Using the neighbor command neighbor {ip-address|peer-group} weight weight 2.0 from AS4 and will propagate the update to RTC. RTC has now two ways for reaching 175.0.0 RTA AS200 RTB AS4 2.15.1 (175. then we will force RTC to use RTA as a next hop to reach 175. If on RTC we can set the weight of the updates coming from RTA to be higher than the weight of updates coming from RTB.

2 Page 38 Sam Halabi-cisco Systems . would have weight 200) route-map setweightin permit 20 set weight 100 (anything else would have weight 100) 1/26/96-Rev: A1.10.1 remote-as 100 neighbor 1.0 from RTA will have 200 weight) neighbor 2.2 remote-as 200 neighbor 2. RTC# router bgp 300 neighbor 1.1.2 remote-as 200 neighbor 2.2 filter-list 6 weight 100 ip as-path access-list 5 permit ^100$(this will only permit path 100) ip as-path access-list 6 permit ^200$ The same outcome as above can be achieved by using routmaps. i.1 weight 200 (route to remote-as 100 neighbor 1.3-Using route-maps example: RTC# router bgp 300 neighbor The same outcome can be achieved via ip as-path and filter lists.2.1 filter-list 5 weight 200 neighbor route-map setweightin in ip as-path access-list 5 permit ^100$ route-map setweightin permit 10 match as-path 5 set weight 200 (anything that applies to access-list 5.1.2 weight 100 (route to 175. packets from AS100.1. RTA will be preferred as the next hop.1.1 route-map setweightin in neighbor 2.2 remote-as 200 neighbor 2.2.0 from RTB will have 100 weight) *Routes with higher weight are preferred when multiple routes exist to the same destination.2.0. RTC# router bgp 300 neighbor 1.1.1 remote-as 100 neighbor

0 from two different sides of the organization.16.3 IBGP 128.1 AS100 AS300 set local pref 200 set local pref 150 AS256 1.0 RTB RTA 3. In the above diagram.4 1. A path with a higher local preference is more preferred.213.2 AS34 RTC 3. local preference is an attribute that is exchanged among routers in the same AS.1. The default value for local preference is 100. Local preference is set via the “bgp default local-preference <value>” command or with route-maps as will be demonstrated in the following example: The bgp default local-preference <value> command will set the local pref- erence on the updates out of the router going to peers in the same AS. AS256 is receiving updates about 170. Unlike the weight attribute which is only relevant to the local router. Local preference will help us determine which way to exit AS256 in order to reach that network.3. The following configuration will set the local preference for updates coming from AS300 to 200 and those coming from AS100 to 150.0 Local Preference Attribute RTD Local Pref= 150 Local Pref = 200 Local preference is an indication to the AS about which path is preferred to exit the AS in order to reach a certain network.3.0. 1/26/96-Rev: A1.2 Page 39 Sam Halabi-cisco Systems .213. Let us assume that RTD is the preferred exit point.0.

10.213.1 remote-as 100 neighbor 128.11.11. Since local preference is exchanged within AS256.213.1. 1/26/96-Rev: A1. In the above example.213. both RTC and RTD will realize that network 170. any update coming from AS300 will be set with a local preference of 200. This is why we can use route maps to specify what specific updates need to be tagged with a specific local preference as shown below: RTD# router bgp 256 neighbor 3.2 Page 40 Sam Halabi-cisco Systems .3.0 has a higher local preference when coming from AS300 rather than when coming from AS100.3.4 remote-as 300 neighbor 128. This means that updates coming from AS34 will also be tagged with the local preference of 200.3.2 remote-as 256 bgp default local-preference 150 RTD# router bgp 256 neighbor 3. More flexibility is provided by using route maps. The same RTD will set the local preference of all updates to 200. Any other updates such as those coming from AS34 will be set with a value of 150. All traffic in AS256 addressed to that network will be sent to RTD as an exit point.1 remote-as 256 ip as-path 7 permit ^300$ route-map setlocalin permit 10 match as-path 7 set local-preference 400 route-map setlocalin permit 20 set local-preference 150 With this configuration.4 remote-as 300 neighbor 3.3.1 remote-as 256 bgp default local-preference 200 In the above configuration RTC will set the local preference of all updates to 150.4 setlocalin in neighbor 128. This might not be needed. router bgp 256 neighbor 1. all updates received by RTD will be tagged with local preference 200 when they reach RTD.

1 AS300 3.2 Page 41 Sam Halabi-cisco Systems . RTC and 1/26/96-Rev: A1. In the above diagram.1.1. When the same update is passed on to a third AS.17.0 via three different routers: RTC.0 set metric 50 4.2 RTD In order for the router to compare metrics from neighbors coming from different ASs the special configuration command “bgp always-compare-med” should be configured on the router. that metric will be set back to 0 as shown in the above diagram. that metric is used for decision making inside the AS.0.3.0. Unlike local preference. metric is exchanged between ASs.3 RTC 1.3 AS400 set metric 200 set metric 120 2.0 The metric attribute which is also called Multi_exit_discriminator (MED.0.4 RTB 2.3.10. This is a dynamic way to influence another AS on which way to choose in order to reach a certain route given that we have multiple entry points into that AS. Unless otherwise specified.1.4. When an update enters the AS with a certain metric. A metric is carried into an AS but does not leave the AS. A lower value of a metric is more preferred.10.0. AS100 is getting information about network 180.3.0 Metric Attribute metric=0 METRIC (MULTI_EXIT_DISC) (INTER_AS) 170.0 AS100 RTA 180. RTD and RTB. a router will compare metrics for paths from neighbors in the same AS.2 4. BGP4) or Inter-As (BGP3) is a hint to external neighbors about the pre- ferred path into an AS. The Metric default value is 0.2 3.1 1.10.

1.3 remote-as 400 RTC# router bgp 300 neighbor remote-as 100 neighbor 4.2.2 remote-as 300 route-map setmetricout permit 10 set metric 120 RTD# router bgp 300 neighbor 3. Given that by default a router compares metrics coming from neighbors in the same AS. considering all other attributes are the same. RTA will pick RTC as next hop.2 remote-as 100 neighbor 2.4.2 route-map setmetricout out neighbor 1. This is illustrated in the configs below: RTA# router bgp 100 neighbor 2.RTD are in AS300 and RTB is in AS400. In order to force RTA to compare the metrics we have to add bgp always-compare-med to RTA.1.4 route-map setmetricout out route-map setmetricout permit 10 set metric 50 With the above configs.2 Page 42 Sam Halabi-cisco Systems .3.2. 1/26/96-Rev: A1. When RTA gets an update from RTB with metric 50.1 remote-as 300 route-map setmetricout permit 10 set metric 200 RTB# router bgp 400 neighbor 4.3 remote-as 300 neighbor remote-as 300 neighbor 3.4. RTA can only compare the metric coming from RTC to the metric coming from RTD and will pick RTC as the best next hop because 120 is less than 200. he can not compare it to 120 because RTC and RTB are in different ASs (RTA has to choose based on some other attributes).2 remote-as 100 neighbor 3.3. the metric coming from RTD to 200 and the metric coming from RTB to Assume that we have set the metric coming from RTC to 120.2 route-map setmetricout out neighbor 1.1. 255.3 remote-as 300 neighbor 4.0.2 Page 43 Sam Halabi-cisco Systems .4. we have to configure RTA as follows: RTA# router bgp 100 neighbor Metric can also be set while redistributing routes into BGP.3 remote-as 400 bgp always-compare-med In this case RTA will pick RTB as the best next hop in order to reach network 180.0 with a metric of 50. the command is: default-metric number Assume in the above example that RTB is injecting a network via static into AS100 then the following configs: RTB# router bgp 400 redistribute static default-metric 50 ip route 180.21 remote-as 300 neighbor 3.10.0 null 0 will cause RTB to send out 180.2.In order to have RTB included in the metric comparison.3. 1/26/96-Rev: A1.

In order to send the attribute to our neighbor we have to use the following: neighbor {ip-address|peer-group-name} send-community Example: RTA# router bgp 100 neighbor 3. We can use route maps to set the community attributes. The community attribute is a way to group destinations in a certain community and apply routing decisions (accept.2 Page 44 Sam Halabi-cisco Systems .3 send-community neighbor 3. if we use the keyword additive then the 200 will be added to the community.3 route-map setcommunity out 1/26/96-Rev: A1.3. this attribute will not be sent to neighbors by default.3.18.3. Even if we set the community attribute.294. prefer. optional attribute in the range 0 to 4.0 Community Attribute The community attribute is a transitive.3 remote-as 300 neighbor 3. redistribute.3. any router belongs to it) An example of route maps where community is set is: route-map communitymap match ip address 1 set community no-advertise or route-map setcommunity match as-path 1 set community 200 additive If the additive keyword is not set.200. 200 will replace any old community that already exits.967.3.) according to those communities. etc. The route map set command has the following syntax: set community community-number [additive] A few predefined well known communities (community-number) are: -no-export (Do not advertise to EBGP peers) -no-advertise (Do not advertise this route to any peer) -internet (Advertise this route to the internet community.3.

we would have to apply an access-list to filter those updates and apply it when talking to RTA: 1/26/96-Rev: A1.0 and sending it to RTC.0 BGP Filtering Sending and receiving BGP updates can be controlled by using a number of different filtering methods.2 Page 45 Sam Halabi-cisco Systems . 3.1 Route Filtering 150. All methods will achieve the same results. Use the following com- mand in the router configuration mode: Neighbor {ip-address|peer-group-name} distribute-list access-list-number {in | out} In the following example.10.19.10. In order to achieve this.1 RTC 170. choosing one over the other depends on the spe- cific network configuration. If RTC wanted to stop those updates from propagating to AS100. BGP updates can be filtered based on route information.2. 19.3 AS 200 AS 100 2.2. you can filter BGP based on routing updates to or from a particular neighbor.10.3.2 3.3. RTB is originating network 160.2.0 RTA RTB 2. an access-list is defined and applied to the updates to or from a neighbor.10. on path information or on communities.0.0.0 160.0 AS 300 In order to restrict the routing information that the router learns or advertises.0.

0/8.0. this is equivalent to 160.255 255.x) Using access-lists is a bit tricky when we are dealing with supernets that might cause some conflicts.0 0.0/8 we have to use an extended access list of the following format: access-list <number> permit ip <ip address> <ip address don’t care bits> <mask> <mask don’t care bits> ex: access-list 101 permit ip and so on. Assume in the above example that RTB has different subnets of 160.X.0.0/8 (this notation means that we are using 8 bits of subnet mask starting from the far left of the IP address. 255.255 will permit 160.0.0 255.0.RTC# router bgp 300 network 170.x. In order to restrict the update to only 1/26/96-Rev: A1.0.X and our goal is to filter updates and advertise only 160.255 (filter out all routing updates about 160.0.0 0.0.2 Page 46 Sam Halabi-cisco Systems .0 neighbor 3.0 distribute-list 1 out access-list 1 deny access-list 1 permit The following access list: access-list 1 permit 160.0.2 remote-as 100 neighbor 2.3 remote-as 200 neighbor 0.0/8 only. Another type of filtering.2.2.0 This list will permit 160.0. is path filtering which is described in the next section.

To do this use the following statements.0 AS 300 You can specify an access list on both incoming and outgoing updates based on the BGP autonomous system paths information.0 AS400 160.0 from going to AS100 by defining an access list on RTC that prevents any updates that have originated from AS 200 from being sent to AS100.0.2 In the above figure we can block updates about RTC# router bgp 300 neighbor 3.2 Path Filtering 150.10. This term will be discussed shortly 1/26/96-Rev: A1.2 remote-as 100 neighbor 2.1 3.2 Page 47 Sam Halabi-cisco Systems .2.3. ip as-path access-list access-list-number {permit|deny} as-regular- expression1 neighbor {ip-address|peer-group-name} filter-list access-list-number {in|out} The following example will stop RTC from sending RTA updates about AS 200 AS 100 2.0 RTB RTA filter-list 1 out (the 1 is the access list number below) RTC 170.3.3 remote-as 200 neighbor 2.

The . 1/26/96-Rev: A1. Since RTB sends updates about 160. So . Those updates will match the access list ^200 because they start with 200 and will be prevented from being sent to RTA which is not the required behavior.* In the above example.* is actually any path information.0 with path information starting with 200 and ending with 200. The ^200$ is called a regular expression.10.* is another regular expression with the dot meaning any character and the * meaning the repetition of that character. updates originated by AS400 will have path information of the form (200. Regular expressions sound a bit complicated but actually they are not. 400) with 200 being first and 400 being last. with ^ meaning starts with and $ meaning ends with. then this update will match the access list and will be denied. This will show us all the path that has matched the configured regular expression.2 Page 48 Sam Halabi-cisco Systems .0. access-list 1 states: deny any updates with path information that start with 200 (^) and end with 200 ($). which is needed to permit all other updates to be sent. What would happen if instead of using ^200$ we have used ^200 If you have an AS400 (see figure above).ip as-path access-list 1 deny ^200$ ip as-path access-list 1 permit . A good way to check whether we have implemented the correct regular expression is to do: sh ip bgp regexp <regular expression>. The next section will explain what is involved in creating a regular expression.

* (coming from AS100) ^$ (originated from this AS) 1/26/96-Rev: A1. Examples of regular expressions follow: a* any occurrence of the letter a. ex: [abcd] B. the end of the input string.Ranges: A range is a sequence of characters contained within left and right square brackets.Atoms An atom is a single character .2 Page 49 Sam Halabi-cisco Systems . right brace (}). In the previous example we specified the string ^200$ and wanted path information coming inside updates to match it in order to perform a decision. The regular expression is composed of the following: A.).2. By building a regular expression we specify a string that input must match. C-Pieces A piece is an atom followed by one of the symbols: * (Matches 0 or more sequences of the atom) + (Matches 1 or more sequences of the atom) ? (Matches the atom or the null string) D.Branch A branch is a 0 or more concatenated pieces. In case of BGP we are specifying a string consisting of path information that an input should match. including none a+ at least one occurrence of a should be present ab?a this will match aa or aba ex: _100_(via AS100) ^100$ (origin AS100) ^100 .19.(Matches a comma (. or a space.1 AS-Regular Expression A regular expression is a pattern to match against an input string. (Matches any single character) ^ (Matches the beginning of the input string) $ (Matches the end of the input string) \character (Matches the character) . left brace ({). the beginning of the input string.

The no-export community attribute is used: RTB# router bgp 200 network BGP Community Filtering We have already seen route filtering and as-path filtering.10.2.2 3.0 AS 300 We would like RTB above to set the community attribute to the bgp routes it is advertising such that RTC would not propagate these routes to its external peers.10.10. Community has been discussed in section 19.3.1 remote-as 300 neighbor 3.0. 150.3.0 160.1 route-map setcommunity out route-map setcommunity match ip address 1 set community no-export access-list 1 permit 0.0.0 RTA RTB 2. Another method is community filtering.1 RTC 170.0 neighbor Page 50 Sam Halabi-cisco Systems . 255.3 AS 200 AS 100 1/26/96-Rev: A1.0.0.1 send-community neighbor 3.0 and here are few examples of how we can use it.

255. Example 2: RTB# router bgp 200 network 160.3.0. If RTC wants to set the weight based on those values we could do the following: 1/26/96-Rev: A1.1 remote-as 300 neighbor 3.1 send-community neighbor 3. match-on-community: route-map match-on-community match community 10 (10 is the community-list number) set weight 20 ip community-list 10 permit 200 300 (200 300 is the community number) We can use the above in order to filter or set certain parameters like weight and metric based on the community value in certain updates.0 255. it will not propagate them to its external peer RTA. Note also that we had to use the “neighbor send-community” command in order to send this attribute to RTC. The value 100 200 will be added to any existing community value before being sent to RTC.3.2 Page 51 Sam Halabi-cisco Systems .3. In example two above.3. A community list is a group of communities that we use in a match clause of a route map which allows us to do filtering or setting attributes based on different lists of community numbers.255 In the above example.10.0 neighbor 3. When RTC gets the updates with the attribute no-export. RTB has set the community attribute to 100 200 additive. RTB was sending updates to RTC with a community of 100 200.255.1 route-map setcommunity out route-map setcommunity match ip address 2 set community 100 200 additive access-list 2 permit ip community-list community-list-number {permit|deny} community-number For example we can define the following route map.0.Note that we have used the route-map setcommunity in order to set the community to no-export.

3. The last community list is here to make sure that other updates are not dropped.2 Page 52 Sam Halabi-cisco Systems . Remember that anything that does not match.3.3.RTC# router bgp 300 neighbor 3. The key- word exact states that community should consist of 200 only and nothing else. 1/26/96-Rev: A1.3 route-map check-community in route-map check-community permit 10 match community 1 set weight 20 route-map check-community permit 20 match community 2 exact set weight 10 route-map check-community permit 30 match community 3 ip community-list 1 permit 100 ip community-list 2 permit 200 ip community-list 3 permit internet In the above example. any route that has 100 in its community attribute will match list 1 and will have the weight set to 20. Any route that has only 200 as community will match list 2 and will have weight 20.3 remote-as 200 neighbor 3. The keyword internet means all routes because all routes are members of the internet community. will be dropped by default.3. 150.2 Page 53 Sam Halabi-cisco Systems .3 AS 200 2.10. Also. Route maps associated with the neighbor statement have no affect on incoming updates when matching based on the IP address: neighbor ip-address route-map route-map-name Assume in the above diagram we want RTC to learn from AS200 about networks that are local to AS200 and nothing else.1 3.20.2 AS 100 3.0 RTB 2.0 neighbor 3. we want to set the weight on the accepted routes to AS 300 The neighbor command can be used in conjunction with route maps to perform either filtering or parameter setting on incoming and outgoing updates. route-map stamp in 1/26/96-Rev: A1.0.3.0 BGP Neighbors and Route maps 190.10.3 remote-as 200 neighbor RTC 170.10.3. We can achieve this with a combination of neighbor and as-path access lists.0 AS600 AS400 RTA 160.0. Example 1: RTC# router bgp 300 network 170.

* The above statement will set a weight of 20 for updates that are local to AS200.2 Page 54 Sam Halabi-cisco Systems . Example 2: Assume that we want the following: 1. and will set a weight of 10 for updates that are behind AS400 and will drop updates coming from AS400.3 route-map stamp in route-map stamp permit 10 match as-path 1 set weight 20 route-map stamp permit 20 match as-path 2 set weight 10 ip as-path access-list 1 permit ^200$ ip as-path access-list 2 permit ^200 600 .3.3 remote-as 200 neighbor 3. RTC# router bgp 300 network 170.0. Any other updates will be dropped.3. 1/26/96-Rev: A1.3.Other updates to have a weight of 10. 2.0 neighbor 3.10. 3.3.Updates originating from AS200 to be accepted with weight 20.Updates originating from AS400 to be dropped.route-map stamp match as-path 1 set weight 20 ip as-path access-list 1 permit ^200$ Any updates that originate from AS200 have a path information that starts with 200 and ends with 200 and will be permitted.

0. Assuming that all other attributes are the same. Suppose in the above diagram that RTC is advertising its own network 170.2.2. 200.1 Use of set as-path prepend In some situations we are forced to manipulate the path information in order to manipulate the BGP decision process. 300) and the second one is via AS400 with path (400.2 Page 55 Sam Halabi-cisco Systems . A common practice is to repeat our own AS number using the following: RTC# router bgp 300 network 170.10. If we want to influence this decision from the AS300 end we can make the path through AS100 look like it is longer than the path going through AS400.300).10.. AS600 will pick the shortest path and will choose the route via AS100.0 neighbor 2. route-map SETPATH out route-map SETPATH set as-path prepend 300 300 Because of the above configuration. When the information is propagated to AS600. via AS100 with a path information of: (100. the first route is via AS100 with path (100. 300) which is longer than (400. 300.2 remote-as 100 neighbor 2.. the routers in AS600 will have network reachability information about 150.0 via two different routes. 300) received from AS100. 1/26/96-Rev: A1.0 to two different ASs: AS100 and AS200. AS600 will receive updates about 170. AS300 will be getting all its traffic via AS100. 300. The command that is used with a route map is: set as-path prepend <as-path#> <as-path#> .20.0. We can do this by prepending autonomous system numbers to the existing path info adver- tised to AS100.

0 RTH RTA RTB 4.0.1 5. 1/26/96-Rev: A1. Page 56 Sam Halabi-cisco Systems .6.0 RTG AS300 A BGP peer group. Members of the peer group inherit all of the configuration options of the peer group.20. distribute-lists and filter-lists.1.0. we define a peer group name and we assign these policies to the peer group.10.5. Instead of defining the same policies for each separate neighbor. you can only override options set on the inbound. BGP Peer Groups 150.2 1.1 RTF RTC 5.2 AS100 RTE 2.1 6.6.2 AS600 AS200 1.4.1 3.6. To define a peer group use the following: neighbor peer-group-name peer-group In the following example we will see how peer groups are applied to internal and external BGP neighbors.2.2.5. etc. Update policies are usually set by route maps.10.2 2.2 170.1.2. Members can also be configured to override these options if these options do not affect outbound updates.3.2 6. is a group of BGP neighbors with the same update policies.1.

RTF and RTG.3. Example 2: RTC# router bgp 300 neighbor externalmap peer-group neighbor externalmap route-map SETMETRIC neighbor externalmap filter-list 1 out neighbor externalmap filter-list 2 in neighbor 2. Also we did an override for the inbound updates of neighbor 1.3.2 peer-group internalmap neighbor peer-group externalmap neighbor 1.2. and this will override filter-list 2 inside the peer group.2 by assigning filter-list 3. We have applied the peer group to all internal neighbors RTE. remote-as 200 neighbor 1. such as a route map SETMETRIC to set the metric to 5 and two different filter lists 1 and 2.2. In the same diagram we will configure RTC with a peer-group externalmap and we will apply it to external neighbors.2 peer-group internalmap neighbor remote-as 600 neighbor 4.2 filter-list 3 in Note that in the above configs we have defined the remote-as statements outside of the peer group because we have to define different external ASs. let us look at how we can use peer groups with external neighbors.1. 1/26/96-Rev: A1.4.2 remote-as 100 neighbor 2.1.6. Now.1.Example 1: RTC# router bgp 300 neighbor internalmap peer-group neighbor internalmap remote-as 300 neighbor internalmap route-map SETMETRIC out neighbor internalmap filter-list 1 out neighbor internalmap filter-list 2 in neighbor 5.2 peer-group externalmap neighbor 1. We have defined a separate filter-list 3 for neighbor RTE.2 filter-list 3 in In the above configuration. Page 57 Sam Halabi-cisco Systems .1.2.5. Note that we could only override options that affect inbound updates. we have defined a peer group named internalmap and we have defined some policies for that group.2 peer-group externalmap neighbor 4.2 peer-group internalmap neighbor 3.

0.0.1 RTC 170.2 Page 58 Sam Halabi-cisco Systems . RTB# router bgp 200 neighbor AS 200 AS 100 2. This is similar to 192.0 aggregate-address 160.213.0 CIDR and Aggregate Addresses 150. RTB is generating network AS 300 One of the main enhancements of BGP4 over BGP3 is CIDR (Classless Interdomain Routing).0. There is no notion of classes anymore (class A.0.0.1 remote-as 200 neighbor 2.0/16 where the 16 is the number of bits in the subnet mask counting from the far left of the IP address.3.3.0 RTC# router bgp 300 neighbor 3.0.3. B or C). to RTA.0. Aggregates are used to minimize the size of routing tables.2 3. network 1/26/96-Rev: A1. We will configure RTC to propagate a supernet of that route 160.0 RTC will propagate the aggregate address remote-as 300 network 160. For example. CIDR or supernetting is a new way of looking at IP addresses. Aggregation is the process of combining the characteristics of several different routes in such a way that a single route can be advertised. remote-as 100 network which used to be an illegal class C network is now a legal supernet represented by 192.0 255. In the example below.0 RTA RTB 2.0 to RTA.

0.0 have been propagated to RTA.0.0.1 Aggregate Commands There is a wide range of aggregate commands.0.0.0 from being also propagated to RTA. In case we would like RTC to propagate network 160.0. 1/26/96-Rev: A1.0 on RTB) then the network entry is always injected into BGP updates even though we are using the “aggregate summary-only” command.0 will propagate an addi- tional network 160.0). all the more specific routes are suppressed. Please note that you can not aggregate an address if you do not have a more specific route of that address in the BGP routing table.0 only and NOT the more specific route then we would have to use the following: aggregate-address address mask summary-only This will a advertise the prefix only. The command aggregate 160.0.0. RTB can not generate an aggregate for 160. from redistributing an IGP or static into BGP or via the network command (network 160.0 and 160.0.0 255.0 if it does not have a more specific entry of 160. The command aggregate-address The upcoming CIDR example discusses this situation.0 but will not prevent 160. This is what we mean by advertising the prefix and the more specific route.0 and will suppress the more specific route 160.0.0. For example.21.0 in its BGP table.0. The first command is the one used in the previous example: aggregate-address address mask This will advertise the prefix route. and all of the more specific routes.0. The outcome of this is that both networks It is important to understand how each one works in order to have the desired aggregation behavior.0 summary-only will propagate network 160. The more specific route could have been injected into the BGP table via incoming updates from other ASs.0.0. Please note that if we are aggregating a network that is injected into our BGP via the network statement (ex: network Page 59 Sam Halabi-cisco Systems .0.10.

0.0 255. This will allow us to be selective about which more specific routes to suppress.0. In the previous diagram.0 as-set.20.0 aggregate-address 160.0 and suppress the more specific route Page 60 Sam Halabi-cisco Systems .0 suppress-map CHECK Another variation is the: aggregate-address address mask attribute-map map-name This allows us to set the attributes (metric. RTC# router bgp 300 neighbor ex: aggregate 129. This will be discussed in an example by itself in the following sections.0.0. The following route map when applied to the aggregate attribute-map command will set the origin of the aggregates to IGP.0.0 255.0.255 Then we apply the route-map to the aggregate statement.10.255.3 remote-as 200 neighbor 2.0 attribute-map SETORIGIN 1/26/96-Rev: A1. and allow 160.0 0.0.255 access-list 1 permit 0.0 to be propagated.20. aggregate-address address-mask suppress-map map-name This advertises the prefix and the more specific routes but it suppresses advertisement according to a route-map.aggregate-address address mask as-set This advertises the prefix and the more specific routes but it includes as-set information in the path information of the routing updates. route-map SETMETRIC set origin igp aggregate-address 160.0 255.0.0. we can use the following route map: route-map CHECK permit 10 match ip address 1 access-list 1 deny 160.0.2. if we would like to aggregate 160.0. In case we would like to suppress more specific routes when doing the aggregation we can define a route map and apply it to the aggregates.2 remote-as 100 network 170.) when aggregates are sent out.0. etc.

3.0 even if you use the “aggregate summary-only” command because RTB is the originator of 160.0 null0 1/26/96-Rev: A1.0.0 and suppress all the more specific routes.0 AS 300 Request: Allow RTB to advertise the prefix 3.2.1 remote-as 300 redistribute static (This will generate an update for 160. The outcome is that RTB will advertise the aggregate with an origin of incomplete (?). RTB# router bgp 200 neighbor CIDR example 1 AS200 is the originator of 160.0 is local to AS200 i.0.1 RTC 160.0 with the origin path as *incomplete*) ip route 160.0.0 AS 100 AS 200 2.10.10. You cannot have RTB generate a prefix for 160. The problem here is that network 3.e.2 Page 61 Sam Halabi-cisco Systems .0.0.0. Solution 1: The first solution is to use a static route and redistribute it into BGP.2.0 RTA RTB 2.2.0 without generating an entry for 160.

0. this will have the same effect except that the origin of the update will be set to IGP. Page 62 Sam Halabi-cisco Systems .0.0 null0 1/26/96-Rev: A1.0.0 mask 255. RTB# router bgp 200 network 160.1 remote-as 300 redistribute static ip route 160.Solution 2: In addition to the static route we add an entry for the network command.0 (this will mark the update with origin IGP) neighbor 3.

21.0 RTB RTA 2.20.3 CIDR example 2 (as-set) AS-SETS are used in aggregation to reduce the size of the path information by listing the AS number only once.0.10.0 160.10. The as-set aggregate command is used in situations were aggregation of information causes loss of information regarding the path attribute.2 Page 63 Sam Halabi-cisco Systems . Suppose RTC wants to aggregate network AS200 AS 100 2.3.1 remote-as 300 1/26/96-Rev: A1.2.0.1 RTD RTC 4.4.0. All the path information is included in that set irrespective of which path came first.0 from RTB.3.0 from RTA and updates about 160.1 4.3. regardless of how many times it may have appeared in multiple paths that were aggregated.2.1 neighbor 3.10.0 neighbor 2.2 3. By adding the aggregate as-set statement we force RTC to generate path information in the form of a set {}.0. RTD would not know what the origin of that route is.1 remote-as 300 RTA# router bgp 100 network 160.4.0/8 and send it to RTD. AS300 AS400 170. RTB# router bgp 200 network In the following example RTC is getting updates about 160.

0.0/8to RTD with path information (300) as if the route has originated from AS300. this may create loops if RT4 has an entry back into AS100 Case 2: RTC# router bgp 300 neighbor 3.0.0 belongs to a set {100 200}) 1/26/96-Rev: A1.0.2 Page 64 Sam Halabi-cisco Systems .2.Case 1: RTC does not have an as-set statement. remote-as 200 neighbor 2.0 as-set (causes RTC to send RTD updates about 160.0 255.2.4 remote-as 400 aggregate 255.2.0/8 with no indication that 160.4 remote-as 400 aggregate 160.2.0. RTC# router bgp 300 neighbor 255. RTC will send an update 160.0.0 is actually coming from two different autonomous systems.0/8 with an indication that 160.3 remote-as 200 neighbor 2.2 remote-as 100 neighbor remote-as 100 neighbor 4.0 summary-only (this causes RTC to send RTD updates about 160.0 summary-only aggregate 160.0.

Each AS by itself will have IBGP fully meshed and has connections to other ASs inside the confederation. Peering within the confederation between multiple ASs is done via the following command: bgp confederation peers autonomous-system [autonomous-system. Even though these ASs will have EBGP peers to ASs within the confedera- tion. metric and local preference information are preserved.2 Page 65 Sam Halabi-cisco Systems .0 BGP Confederation BGP confederation is implemented in order to reduce the IBGP mesh inside an AS. The trick is to divide an AS into multiple ASs and assign the whole group to a single confederation. “confederation” and “route reflectors” are designed for ISPs who would like to further control the explosion of IBGP peering inside their autonomous systems. next hop. The group of ASs will look to the outside world as one AS with the AS number being the confederation identifier.The next two subjects.] The following is an example of confederation: 1/26/96-Rev: A1. 22. the confederation (the group of ASs) will look as a single AS. they exchange routing as if they were using IBGP. To the outside world. To configure a BGP confederation use the following: bgp confederation identifier autonomous-system The confederation identifier will be the AS number of the confederation group.

but we are only interested in the BGP speakers that have EBGP connections to other ASs). 1/26/96-Rev: A1. I will show a sample configuration of routers RTC.4 RTC RTD AS60 128. The outside world will see only one AS500.14.6.1 128.5 AS600 AS 100 5. RTA has only knowledge of AS500.10.213. 8 IBGP peers and one EBGP peer to external ASs. For each AS50. Note that RTA has no knowledge of ASs 50.30.2 Page 66 Sam Halabi-cisco Systems .1 AS 500 Let us assume that you have an autonomous system 500 consisting of nine BGP speakers (other non BGP speakers exist also. If you want to make a full IBGP mesh inside AS500 then you would need nine peer connections for each router.Example: RTA 6.212. AS60 and AS70.11. By using confederation we can divide AS500 into multiple ASs: AS50. RTD and RTA. 128. We give the AS a confederation identifier of 500.5.20.1 AS70 AS50 129.2 135. AS60 and AS70 we define a full mesh of IBGP peers and we define the list of confederation peers using the bgp confederation peers command.210. 60 or 70.5.6 5.

RTC# router bgp 50 bgp confederation identifier 500 bgp confederation peers 60 70 neighbor remote-as 70 (BGP connection with confederation peer 70) neighbor remote-as 100 (EBGP connection to external AS100) RTD# router bgp 60 bgp confederation identifier 500 bgp confederation peers 50 70 neighbor 129.20.1 remote-as 60 (BGP connection with confederation peer 60) neighbor 135.6 remote-as 600 (EBGP connection to external AS600) RTA# router bgp 100 neighbor 5.2 Page 67 Sam Halabi-cisco Systems . remote-as 500 (EBGP connection to confederation 500) 1/26/96-Rev: A1.1 remote-as 50 (IBGP connection within AS50) neighbor 128.1 remote-as 70 (BGP connection with confederation peer 70) neighbor 6.5.1 remote-as 50 (IBGP connection within AS50) neighbor 129.5.1 remote-as 50(BGP connection with confederation peer 50) neighbor remote-as 60 (IBGP connection within AS60) neighbor 128.6.

RTC could be elected as a RR and have a partial IBGP peering with RTA and RTB. Configuring a route reflector is done using the following BGP router sub- command: neighbor <ip-address> route-reflector-client The router with the above command would be the RR and the neighbors pointed at would be the clients of that RR. Peering between RTA and RTB is not needed because RTC will be a route reflector for the updates coming from RTA and RTB.0 Route Reflectors Another solution for the explosion of IBGP peering within an autonomous system is Route Reflectors (RR). By utilizing the route reflector concept. we can allow a router to advertise (reflect) IBGP learned routes to other IBGP speakers. RTC would be configured with the “neighbor route-reflector-client” command point- ing at RTA and RTB’s IP addresses. 1/26/96-Rev: A1. RTB and RTC above would form a cluster with a single RR within AS100. RTA. In our example. As demonstrated in section 9. Other IBGP peers of the RR that are not clients are called non-clients.2 Page 68 Sam Halabi-cisco Systems . RTB and RTC within AS100. This will reduce the number of IBGP peers within an AS. The combination of the RR and its cli- ents is called a cluster. By relaxing this restriction a bit and by providing additional control.23. a full IBGP mesh should be maintained between RTA. Example: RTA (RR) RTC AS 100 RTB In normal cases. a BGP speaker will not advertise a route learned via another IBGP speaker to a third IBGP speaker.0 (Internal BGP).

RTA and RTB are clients and anything else is a non-client. Remember that clients of an RR are pointed at using the “neighbor <ip-address> route-reflector-client” command.4 3. The same RTD is the RR for its clients RTE and RTF.4.2. When a route is received by a RR. a RR would treat other RRs just like any other IBGP speaker.Example: AS200 8. the AS could be divided into multiple clusters. RTA.2 1.12.6 5. RTC and RTG are fully meshed but routers within a cluster are not. Clients should not peer with IBGP speakers outside their cluster.7 4. it will do the following depending on the peer type: 1/26/96-Rev: A1.7.8 AS 100 RTG (RR) 7.2 Page 69 Sam Halabi-cisco Systems . each RR will be configured with other RRs as non-client peers in a fully meshed topol- ogy.6.1. RTG is a RR in a third cluster.3 RTD (RR) RTC (RR) AS300 An autonomous system can have more than one route reflector.3.12.7. Note that RTD. Other RRs could belong to the same cluster (client group) or to other clusters. According to RTC.8.6.1 RTE RTA RTB RTF 12. Consider the above diagram.5 2. RTB and RTC form a single cluster with RTC being the RR. In a simple con- figuration.

RTD and RTB: RTC# router bgp 100 neighbor 2.3.7 remote-as 100 neighbor The Route-Reflector scheme has a few methods to avoid this: remote-as 300 RTD# router bgp 100 neighbor 6.3 remote-as 100 As the IBGP learned routes are reflected.1 remote-as 100 neighbor 1.7 remote-as 100 neighbor 4.Route from a client peer: reflect to all the non-client peers and also to the client peers. if the routing information comes back to the originator.3.1.Originator-id: this is an optional. 1/26/96-Rev: A1.12.2. due to poor configuration.3.2. Thus.12.6 route-reflector-client neighbor 5.1.5 remote-as 100 neighbor 5. 3.5 route-reflector-client neighbor 7.2 route-reflector-client neighbor 1.Route from an EBGP peer: send the update to all client and non-client peers.Cluster-list: this will be discussed in the next section. This attribute will carry the router-id (RID) of the originator of the route in the local AS.5.3.2.Route from a non-client peer: reflect to all the clients within the cluster. 2.5. The following is the relative BGP configuration of routers RTC.6.8 remote-as 200 RTB# router bgp 100 neighbor 3.1 route-reflector-client neighbor 7.6 remote-as 100 neighbor it is possible to have the routing information loop.2 Page 70 Sam Halabi-cisco Systems .2 remote-as 100 neighbor 2.4.2. 2. it will be ignored.6.7.4 remote-as 100 neighbor 8. non transitive BGP attribute that is four bytes long and is created by a RR.3 remote-as 100 neighbor 12.1.8.

13 Usually. RTH will take its place.10 1. Note the redundancy in that RTH has a fully meshed peering with all the RRs. All RRs in the same cluster need to be configured with a 4 byte cluster-id so that a RR can recognize updates from RRs in the same cluster. In order to increase redundancy and avoid single points of failure.2.3 RTD (RR) RTC (RR) 6.8. In the above diagram. it will append the local cluster-id to the cluster-list. If this update has an empty cluster-list the RR will create one. Multiple RRs within a cluster AS400 9.5.6 (RR) RTE.13.7.11 8. When a RR reflects a route from its clients to non-clients outside of the cluster. If the local cluster-id is found in the cluster-list. the cluster will be identified by the router-id of the RR.4.23.1 RTE RTA 5. In this case. RTD.10.10. A cluster-list is a sequence of cluster-ids that the route has passed.4.4 RTH 3.9 AS200 AS300 11.2 Page 71 Sam Halabi-cisco Systems .1. RTF and RTH belong to one cluster with both RTD and RTH being RRs for the same cluster.7 4. Using this attribute.9. a RR can identify if the routing information is looped back to the same cluster due to poor configuration.2. a cluster of clients will have a single RR. RTF and RTC: 1/26/96-Rev: A1.11. In case RTD goes down.3. The following are the configuration of RTH.2 10. a cluster might have more than one RR.5 RTB RTF AS500 13.3.8 AS 100 RTG (RR) 7. the advertisement will be ignored. RTD.

4 remote-as 100 neighbor remote-as 100 neighbor 7.7 remote-as 100 neighbor 3.8 remote-as 200 Note that we did not need the cluster command for RTC because only one RR exists in that cluster. remote-as 100 neighbor 8.5.6 remote-as 100 neighbor 6.RTH# router bgp 100 neighbor remote-as 100 neighbor 9. 1/26/96-Rev: A1. remote-as 100 neighbor 13.7.9 remote-as 300 bgp route-reflector 10 (This is the cluster-id) RTD# router bgp 100 neighbor 10.10.3 remote-as 100 neighbor route-reflector-client neighbor remote-as 400 bgp route-reflector 10 (This is the cluster-id) RTF# router bgp 100 neighbor remote-as 100 neighbor 6.13.1 route-reflector-client neighbor 2.2 route-reflector-client neighbor remote-as 100 neighbor remote-as 100 neighbor remote-as 100 neighbor 1.3.13 remote-as 500 RTC# router bgp 100 neighbor 1.11.6 route-reflector-client neighbor remote-as 100 neighbor 5.9.5 remote-as 100 neighbor Page 72 Sam Halabi-cisco Systems .4.6.7 remote-as 100 neighbor remote-as 100 neighbor 5.1.5 route-reflector-client neighbor 6.5 route-reflector-client neighbor 6.10 remote-as 100 neighbor

3 RTD RTC (RR) 1.If BGP client-to-client reflection were turned off on the RR and redundant BGP peering was made between the clients. Example: AS300 AS200 13.14.An important thing to note.1 RTE RTA RTB RTF AS400 14.6 5.14. One could start creating clusters by configuring a single router as RR and making other RRs and their clients normal IBGP peers. RR and conventional BGP speakers It is normal in an AS to have BGP speakers that do not understand the concept of route reflectors.5.3. then a potential withdrawal to the source of a route on the RR would be sent to all clients inside the cluster and could cause problems.4 3. The router sub-command bgp client-to-client reflection is enabled by default on the RR.13.6. These routers could be either members of a client group or a non-client group. is that peer-groups were not used in the above configuration. Page 73 Sam Halabi-cisco Systems .1.13.4. Then more clusters could be cre- ated gradually. We will call these routers conventional BGP speakers.13 8. then using peer groups would be alright. peer-goups should not be used. If peer groups were to be configured.8 AS 100 4.14 1/26/96-Rev: A1. The route reflector scheme will allow such conventional BGP speakers to coexist.5 2.4. 23. This would allow easy and gradual migration from the current IBGP model to the route reflector model. If the clients inside a cluster do not have direct IBGP peers among one another and they exchange updates through the RR.

3 Avoiding looping of routing information We have mentioned so far two attributes that are used to prevent poten- tial information looping: the originator-id and the cluster-list. The following is the configuration of RTD and RTC: RTD# router bgp 100 neighbor 6. The set clause for out-bound route-maps does not affect routes reflected to IBGP peers.13 remote-as 300 RTC# router bgp 100 neighbor 4.14. When used on RRs the nexthop-self will only affect the nexthop of EBGP learned routes because the nexthop of reflected routes should not be changed. remote-as 100 neighbor 6. Later on.2 Page 74 Sam Halabi-cisco Systems .1. RTE and RTF have the concept of route reflec- tion.2 remote-as 100 neighbor 1. RTC could be made a RR with clients RTA and RTB.13.In the above diagram. RTC. when we are ready to upgrade. Another means of controlling loops is to put more restrictions on the set clause of out-bound route-maps. More restrictions are also put on the nexthop-self which is a per neigh- bor configuration option.14 remote-as 400 When we are ready to upgrade RTC and make it a RR.5.5 route-reflector-client neighbor 3. we would remove the IBGP full mesh and have RTA and RTB become clients of RTC.6 route-reflector-client neighbor 5.3 remote-as 100 neighbor 2. RTD. it is only the RRs that would have to be upgraded. remote-as 100 neighbor 13. 1/26/96-Rev: A1.2.2 remote-as 100 neighbor 1.6.4 remote-as 100 neighbor 2.1 remote-as 100 neighbor 14. RTA and RTB are what we call conventional routers and cannot be configured as RRs.5. Clients do not have to understand the route reflection scheme.2.2. Normal IBGP mesh could be done between these rout- ers and RTD.5 remote-as 100 neighbor 5.

range is 1-255. <suppress-value> range is 1-20000. This might change if there is a need to have this feature enabled by default. <max-suppress-time> maximum duration a route can be suppressed. A command that sets all parameters at the same time is: bgp dampening <half-life-time> <reuse> <suppress> <maximum-suppress-time> <half-life-time> range is 1-45 min. no bgp dampening . As soon as the cumulative penalty reaches a predefined “sup- press-limit”. default is 4 times half-life-time. Once the penalty decreases below a predefined “reuse-limit”.will turn off dampening. at that point the information is purged from the router.24. 1/26/96-Rev: A1. default is 750.change the half-life time. bgp dampening <half-life-time> . The dampening information is kept until the pen- alty becomes less than half of “reuse-limit”. To accomplish this. current default is 15 min.0 Route Flap Dampening Route dampening (introduced in Cisco IOS version 11. the advertisement of the route will be suppressed.0) is a mechanism to minimize the instability caused by route flapping and oscillation over the network. Routes. learned via IBGP will not be dampened. A route which is flapping gets a pen- alty for each flap (1000). <reuse-value> range is 1-20000. Initially. This is to avoid the IBGP peers having higher penalty for routes external to the AS.2 Page 75 Sam Halabi-cisco Systems . The following are the commands used to control route damp- ening: bgp dampening . The penalty will be exponentially decayed based on a preconfigured “half-time”. default is 2000. The penalty will be decayed at a granularity of 5 seconds and the routes will be un-sup- pressed at a granularity of 10 seconds. the route advertisement will be un-suppressed. external to an AS. criteria are defined to identify poorly behaved routes. dampening will be off by default.will turn on dampening.

192 interface Serial0/0 ip address 255.208.2 RTB S0/0 192.255.0 neighbor 192.255.0 neighbor 255. RTB’s BGP table would look like this: 1/26/96-Rev: A1.174 RTB# hostname RTB interface Serial0 ip address 203.2 Page 76 Sam Halabi-cisco Systems .5 remote-as 300 RTD# hostname RTD interface Loopback0 ip address 192.208.6 255.250.Example: S0 203.5 RTD AS100 AS300 L0 192.255.10. Assuming the EBGP link to RTD is stable. remote-as 100 RTB is configured for route dampening with default parameters.10.250.252 router bgp 100 bgp dampening network interface Serial1 ip address 192.208.252 router bgp 300 network 192.5 S1 192.208.208.

external Dampinfo: penalty 910. version 25 Paths: (1 available.2 Page 77 Sam Halabi-cisco Systems . * valid.208.208. d damped.10.10.EGP.10.250. h history.208.10.6” on RTD. e .0.208.250. h history. d damped.2 Status codes: s suppressed.15.0. flapped 1 times in 0:02:03 The route has been given a penalty for flapping but the penalty is still below the “suppress limit” (default is 2000).incomplete Network Next Hop Metric LocPrf Weight Path h 192.0 BGP routing table entry for 192.2 Status codes: s suppressed.5 0 0 300 i *> 203.208.0 0.15. local router ID is 203.internal Origin codes: i .IGP. The route is not yet sup- pressed.10.0 0 32768 i The BGP entry for 192. Which means that we do not have a best path to the route but information about the route flapping still exists.0 192.5 (192.10.0 0.EGP.208. RTB’s BGP table will look like this: RTB#sh ip bgp BGP table version is 24.5 from 192.208. RTB#sh ip bgp 192.208. ? .255. ? . i . > best. I will do a “clear ip bgp 192.0 0 32768 i In order to simulate a route flap.0 192. local router ID is * valid.5 0 0 300 i *> 203.250.250. metric 0. i .10. If the route flaps few more times we will see the following: 1/26/96-Rev: A1.incomplete Network Next Hop Metric LocPrf Weight Path *> 192.208.internal Origin codes: i .0 255.174) Origin IGP.0 has been put in a “history” state.10.15.0. e . > best.255.RTB#sh ip bgp BGP table version is 24. no best path) 300 (history entry) 192.IGP.0.10.

208.D m. reuse in 0:27:00 The route has been dampened (suppressed).208. > best.255.The dampening information will be purged when the penalty becomes less than half of the reuse-limit. metric 0. in our case 750 (default). in our case (750/2=375).10. The Following are the commands used to show and clear flap statistics information: show ip bgp flap-statistics (displays flap statistics for all the paths) show ip bgp-flap-statistics regexp <regexp> (displays flap statistics for all paths that match the regexp) show ip bgp flap-statistics filter-list <list> (displays flap statistics for all paths that pass the filter) show ip bgp flap-statistics A.IGP.m. h history.m (clears flap statistics for a single entry) clear ip bgp A.0. valid.internal Origin codes: i . no best path) 300.B.C.m longer-prefixes (displays flap statistics for more specific entries) show ip bgp neighbor [dampened-routes] | [flap-statistics] (displays flap statistics for all paths from a neighbor) clear ip bgp flap-statistics (clears flap statistics for all routes) clear ip bgp flap-statistics regexp <regexp> (clears flap statistics for all the paths that match the regexp) clear ip bgp flap-statistics filter-list <list> (clears flap statistics for all the paths that pass the filter) clear ip bgp flap-statistics A.m.incomplete Network Next Hop Metric LocPrf Weight Path *d 192.C.15.m.m.D m.0 255.15. flapped 3 times in 0:05:18 .5 0 0 300 i *> 203. local router ID is 203. external Dampinfo: penalty 2615.208.10.0 from 192.174) Origin IGP. The route will be reused when the penalty reaches the “reuse value”.D m.208.250.2 Status codes: s suppressed.10.0.m.10.208. e .RTB#sh ip bgp BGP table version is 32.B. (suppressed due to dampening) 192. i .208. ? .m. * valid.0 BGP routing table entry for 192.0.B.10.C. version 32 Paths: (1 available.10.B. d damped.EGP.D flap-statistics (clears flap statistics for all paths from a neighbor) 1/26/96-Rev: A1.0 192.2 Page 78 Sam Halabi-cisco Systems .0 0 32768 i RTB#sh ip bgp 192.255.5 (192.m (displays flap statistics for a single entry) show ip bgp flap-statistics A.C.10.

Path selection is based on the following: 1-If NextHop is inaccessible do not consider it. 2-Prefer the largest Weight. We put that path in our routing table and we propagate it to our BGP neighbors. 10-Prefer the route with the lowest ip address value for BGP router ID. The ollowing is a design example thatis intended to show the configura- tion and routing tables as they actually appear on the C.0 How BGP selects a Path Now that we are familiar with the BGP attributes and terminology. 4-If same Local Preference prefer the route that the specified router has originated. 9-If IGP synchronization is disabled and only internal path remain prefer the path through the closest IGP neighbor. 8-If path is the same length prefer External path over Internal.2 Page 79 Sam Halabi-cisco Systems . 5-If no route was originated prefer the shorter AS path. 1/26/96-Rev: A1. 6-If all paths are external prefer the lowest origin code (IGP<EGP<INCOMPLETE).25. 7-If origin codes are the same prefer the path with the lowest MULTI_EXIT_DISC.isco routers. the following list indicates how BGP selects the best path for a particular destination. Remember that we only select one path as the best path. 3-If same weight prefer largest Local Preference. S1 192.63.X AS300 S0/0 Page 80 Sam Halabi-cisco Systems .10.1 AS500 RTG L0 195.X E0 AS400 S0 195.X.2 L0 203.63.10.X AS200 RTD S2/1 128.6 RTE L0 200.15.0 Practical design example: RTF 203.208.174 RTC L0 128.2 L0 S2/0 128.5 128.10.41 RTA E0 203.211.208.X.250.211.1 S0 203.250.2 S0/1 192.X.174 S1 S1 203.1 IBGP RTB S0 128.2 S1 AS100 192.X S0 1/26/96-Rev: A1.10.X.26.213.5 195.10.X.

1 255. Assuming that AS200 and AS300 are the two ISPs we are connected to.255.41 255.250. This is NOT the final configuration.0.2 update-source Loopback0 RTF# hostname RTF ip subnet-zero interface Ethernet0 ip address 203.252 router ospf 10 network it is always good to run IBGP within your AS in order to have a better control of your routes.213.1 255.0 interface Serial0 ip address 128.0 neighbor router ospf 10 network area 0 router bgp 100 network interface Serial1 ip address 203.1 255.2 Page 81 Sam Halabi-cisco Systems .0 0.2 remote-as 200 neighbor 203.250. In this example we will run IBGP inside AS100 between RTA and RTB and we will run OSPF as an IGP.255.255.255 area 0 1/26/96-Rev: A1. the following are the first run of configuration for all the routers.255.0. Whenver you have an AS that is connected to two ISPs via EBGP.2 remote-as 100 neighbor 203.We will build the above configuration step by step and see what can go wrong along the way.0 interface Ethernet0 ip address mask RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.

250.5 neighbor 192.2 Page 82 Sam Halabi-cisco Systems .63.0 neighbor 128.213.6 remote-as 400 1/26/96-Rev: A1.213.130 255.255.41 remote-as 100 RTC# hostname RTC ip subnet-zero interface Loopback0 ip address remote-as 100 neighbor ! interface Serial2/1 ip address 128.63.2 area 0 router bgp 100 network 203.RTB# hostname RTB ip subnet-zero interface Serial0 ip address interface Serial2/0 ip address 128.6 255.252 interface Serial1 ip address 192.0 router bgp 200 network router ospf 10 network 203.5 remote-as 300 neighbor 203.213.

10.255.252 ! interface Serial0/1 ip address router bgp 300 network 192.10.2 Page 83 Sam Halabi-cisco Systems .208.RTD# hostname RTD ip subnet-zero interface Loopback0 ip address interface Serial0/0 ip address 192.255.1 remote-as 500 neighbor 192.10.252 clockrate 1000000 router bgp 400 network 200.1 255.208.5 neighbor interface Serial0 ip address 195.5 remote-as 200 neighbor 255.252 interface Serial1 ip address 128.2 255.10.0 neighbor 255.6 remote-as 100 RTE# hostname RTE ip subnet-zero interface Loopback0 ip address remote-as 500 1/26/96-Rev: A1.

The path info is intuitive.0.15.2.IGP. RTB#sh ip bgp BGP table version is 4.0 203. For example network 128.2 remote-as 300 neighbor 195.incomplete Network Next Hop Metric LocPrf Weight Path *i128. as if the link between RTB and RTD does not exist.213.63.0 0 32768 i Let me go over the basic notations of the above table.2 Status codes: s suppressed.0 203.0.252 interface Serial1 ip address 195. e . throughtout this example I will only use the network command to inject networks into BGP.174 local router ID is 203.0 neighbor Page 84 Sam Halabi-cisco Systems . This is why.250.2 100 0 200 400 i *>i203. interface Serial0 ip address 192.10.0 0.2 100 0 200 400 500 300 i *i195.1 255.213.2 100 0 200 400 500 i *i200.211.15. Let us assume to start with that s1 on RTB is shutdown.213. * valid.0 has a nexthop 0. h history.63.255.1 255.255.10. d damped.RTG# hostname RTG ip subnet-zero interface Loopback0 ip address rather than redistribut- ing IGP into BGP.250. The following is RTB's BGP table.10.13. The “i” at the beginning means that the entry was learned via an internal BGP peer.internal Origin codes: i . 1/26/96-Rev: A1.0 remote-as 400 It is always better to use the network command or redistribute static entries into BGP to advertise networks.0.213.41 0 100 0 i *>203.252 router bgp 500 network 195.13. > best. ? .0 is learned via path 200 with nexthop of 128. i .0 128.2 0 100 0 200 i *i192.0. The “i” at the end indicates the ORIGIN of the path information to be IGP.41 0 100 0 i *>i203.0 128. Note that any locally generated entry such as

213.OSPF.14.0.0 via a nexthop of 128.250.IGRP.EIGRP external.252 is subnetted.255.0 is directly connected. We could also change the nexthop by using the bgp nexthopself command between RTA and RTB.255. 02:50:45.63. Notice the nexthop attribute.13. will install this path in the ip routing table and will advertise it to other bgp Serial0 O 203. L1 . Bgp will only pick one best Path to reach a destination.The > symbol indicates that BGP has chosen the best route based on the list of decision steps that I have gone through earlier in this document under "How BGP selects a Path".13.15. and this way RTB would know how to reach the nexthop 128. it doesn't look like any of the BGP entries has made it to the routing table.connected.41 [110/75] via 203. 1 subnets C 203. RTB has not learned about 128.250.OSPF external type 1. E .0 255.IS-IS level-1.63.250.RIP. default Gateway of last resort is not set 203. M .OSPF external type 2. Let us look at the IP routing table: RTB#sh ip rou Codes: C .255 is subnetted.2 Page 85 Sam Halabi-cisco Systems .1. B .BGP D .IS-IS.IS-IS level-2.0 via OSPF. We can run OSPF on RTA s0 and make it passive. 1 subnets O 203.250. L2 .EIGRP.0 [110/74] via 203.OSPF inter area E1 .2. There are two problems here: Problem 1: The Nexthop for these entries 128.2 is unreachable.63. RTA's configs would be: 1/26/96-Rev: A1.250. IA .250.0 255.1. Serial0 203. R .213. I . EX . S . O .EGP i .255.63.213. 02:50:46. E2 .2 which is the ebgp nexthop carried into IBGP.static.15. This is true because we do not have a way to reach that nexthop via our IGP (OSPF). Serial0 Well. * . RTB knows about 128.

IGP.250.15.2 Page 86 Sam Halabi-cisco Systems .incomplete Network Next Hop Metric LocPrf Weight Path *>i128.255.213.0 100 0 200 400 500 300 i *>i195. 255. h history. remote-as 100 neighbor 203.10.0 mask 255. e . 0.2 Status codes: s suppressed.255.255. i .internal Origin codes: i . local router ID is 203.2 update-source Loopback0 The new BGP table on RTB now looks like this: RTB#sh ip bgp BGP table version is 10. which means that BGP is ok with next hop.250.0 area 0 router bgp 100 network 203.255 area 0 network interface Serial0 ip address remote-as 200 neighbor 203.RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203. Let us look at the routing table now: 1/26/96-Rev: A1.0 0 32768 i Note that all the entries have >.250.0 128.2 100 0 200 400 i *>i203.250.0 router ospf 10 passive-interface Serial0 network 203.41 0 100 0 i *> 203.41 255.2 100 0 200 400 500 i *>i200.13.2 0 100 0 200 i *>i192.213.EGP.0 128.0 neighbor 203.1 * valid.63. ? . 203.0 interface Ethernet0 ip address 203. > best. d damped.63.15.41 0 100 0 i *>i203.0.

This is the synchronization issue. 00:04:46.IS-IS level-1.255.255 is subnetted.15.OSPF external type 2. Serial0 O 203. the only difference is that 128. 1 subnets O 203. 1 subnets O 128.41 [110/75] via 203. E .252 is subnetted.1.250. we will have the entries in the routing table. Serial0 Problem 2: We still do not see the BGP entries.63.252 is subnetted.connected.15. Note that RTF has no notion of networks 192. S .0 is directly connected.EGP i .EIGRP external.255.1.255. Serial0 128.IGRP.RIP.OSPF.208.250. R .0 255.15. In this IA . Serial0 203.211.213.OSPF inter area E1 . If you turn off synchronization on RTB this is what will happen: 1/26/96-Rev: A1. EX . 00:04:47.250. 1 subnets C 203.10.2 Page 87 Sam Halabi-cisco Systems .0 [110/74] via 203. O .BGP D . if we turn synchronization off.213. I .255.IS-IS.213.10.250. M .13.250.63. but connectivity would still be broken.15.250.0 [110/138] via 203.0.OSPF external type 1.static.0 is now reachable via OSPF.15.RTB#sh ip rou Codes: C . 00:04:46. L2 . L1 .IS-IS level-2.250. E2 .13.0 or because we have not redistributed BGP into OSPF yet.0 255.1.EIGRP.255. B .0 255. * - candidate default Gateway of last resort is not set 203. BGP is not putting these entries in the routing table and will not send them in BGP updates because it is not synchronized with the IGP.255.

14.OSPF.IS-IS. EX . Serial0 O 203.41 [110/74] via 203. M . 00:01:07 B 192.63.0 is variably subnetted.1. B .13.211.IS-IS level-1.connected.0 255. L1 . L1 .0.250.250. I . 1 subnets C 203. R .EIGRP.0 [200/0] via 128.0 [200/0] via 128.OSPF.BGP D .EIGRP external.1. but there is no way we can reach those networks because RTF in the middle does not know how to reach them: RTF#sh ip rou Codes: C . E2 .0 255.41 [110/11] via 203. E2 . is subnetted.0 255.IS-IS level-1.0 [200/0] via is subnetted.0.static.0.213. M . Serial1 C I .250.255. 2 subnets.213.255 [110/75] via 203. R .mobile.EIGRP.0 255. E . but we will need it for other issues later on. Serial0 [110/138] via is directly connected. 00:14:15.BGP D . 2 subnets. Serial0 The routing table looks fine. B .OSPF inter area E1 .255. * - candidate default Gateway of last resort is not set B 200. S .250.250.255. IA .250.EIGRP external.0.252 is subnetted. with a metric of 2000.213.0 is directly connected.10. O .IS-IS level-2.RTB#sh ip rou Codes: C .EGP i . Page 88 Sam Halabi-cisco Systems .250.10. Ethernet0 128. 00:12:37.13. Serial0 B 203.2. IA .255. Ethernet0 203. 00:01:07 203. 2 masks B 128.0 [200/0] via 128.0 [110/74] via 203.2.213. * - candidate default Gateway of last resort is not set 203.IS-IS. 2 masks O 203.41. 1/26/96-Rev: A1.213.0 [200/0] via 128.OSPF external type 1.250.15. 00:01:08 203.250.252 is subnetted.0 is directly connected. S .250.OSPF external type 2.250.0 255.OSPF external type 2.255.213.connected.IS-IS level-2. 1 subnets C 203. L2 .OSPF external type 1.RIP. 00:01:08 O 128.OSPF inter area E1 .static. 00:01:07 B is variably subnetted.250.RIP.1. Let’s redistribute OSPF into BGP on RTA . turning off synchronization in this situation did not help this particular issue.255.15. Ethernet0 So. O . 00:14:15.15.0 255.0 255.EGP i .255. 00:12:37. E . 1 subnets O 203.255.14.IGRP.2. L2 . EX .63.255. 00:12:37. 1 subnets O 128.

Serial0 O 203. Serial0 128.250.BGP D .0 I .41 255. E2 .255.Serial0 O 128.0 is variably subnetted.IS-IS level-2. O .213.255 area 0 network neighbor 128.15.0 is variably subnetted.1.255.2 remote-as 100 neighbor 203.250.OSPF inter area E1 .250.14. 00:00:15. E .200. Serial0 O E2 203.15.OSPF external type 2.255. 00:00:15.2 remote-as 200 neighbor 203.connected.15.213.15.EIGRP.0 is directly connected.250.IS-IS level-1.IGRP.2 update-source Loopback0 The routing table will look like this: RTB#sh ip rou Codes: C .10.0 [110/2000] via Serial0 O E2 192.0 [110/74] via 203.0.0 [110/2000] via 203.8 is directly connected.static.1.63. L1 .255.0 interface Serial0 ip address 128.15. IA . 255. EX .EIGRP external. 00:00: Serial0 1/26/96-Rev: A1. 00:00:14.255. M .15.255.1 255.0.EGP i .250.255.OSPF. Loopback1 C 203. 00:00: Serial0 203.255 [110/75] via 203. 2 masks O E2 hostname RTA ip subnet-zero interface Loopback0 ip address 203. Serial0 203.0. B .13.0 0.15. Serial0 O E2 [110/2000] via 2 subnets. 00:00:15.1.208. S . 2 subnets C 203.0 mask 255.255 area 0 router bgp 100 network 203.250.0 [110/2000] via 203. 2 masks O 203.0.252 [110/138] via 203.RIP.1 255.41 255.2 Page 89 Sam Halabi-cisco Systems .255. L2 .252 is subnetted.15.250.0 [110/2000] via 203.OSPF external type 1. 00:00:14. 2 * - candidate default Gateway of last resort is not set O E2 200.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203.0 0.255. R .1. 00:00: interface Ethernet0 ip address 203.13.

250.255.10.The BGP entries have disappeared because OSPF has a better distance (110) than internal bgp (200).255 area 0 network 128.255 area 0 router bgp 100 no synchronization network Otherwise some looping will occur because in order to get to nexthop 192.5 via IGP.0. I will also enable OSPF on serial 1 of RTB and make it passive in order for RTA to know about the nexthop 192. Let us bring RTB’s s1 up and see what all the routes will look like. because it will not sync up with OSPF due to the difference in masks. The updated configs of RTA and RTB follow: RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.0 neighbor 128.0.0 0.14.1 update-source Loopback0 1/26/96-Rev: A1.63.213.0 interface Ethernet0 ip address 203.0 for the same reason.2 remote-as 100 neighbor I will also turn sync off on RTA in order for it to advertise 255.0 interface Serial0 ip address remote-as 200 neighbor 203.250.5 we would have to go the other way via EBGP.255.41 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203.250.63. I will also keep sync off on RTB in order for it to advertise mask 255.0.2 Page 90 Sam Halabi-cisco Systems .255.0 0.250. Page 91 Sam Halabi-cisco Systems . interface Serial1 ip address 192.13.200. h history. local router ID is 203.0. i -internal Origin codes: i .0 203. * valid.255.5 0 100 0 300 i *>i195.0 192.0 0 200 400 i *> 203.5 100 0 300 500 i * 128. d damped.0 0 32768 i *>i203. e .63.EGP.6 ? .255.255.0 0.0.incomplete Network Next Hop Metric LocPrf Weight Path *> 128.213.2 0 0 200 i *>i192.10.250.0 0 200 400 500 i *> 200.2 0 100 0 i 1/26/96-Rev: A1.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network area 0 network 192.250.41 remote-as 100 And the BGP tables look like this: RTA#sh ip bgp BGP table version is 0.13.IGP.0.0 0 32768 i *> Status codes: s suppressed.255.RTB# hostname RTB ip subnet-zero interface Serial0 ip address 203.208.2 255.255.0 192.5 remote-as 300 neighbor 203.255 area 0 router bgp 100 no synchronization network neighbor 192.0. > best.0 0.

250. Potential asymmetry might occur if traffic going out from RTA comes back via RTB.250.10.5 0 0 300 i *> 195. You might find out that all incoming traffic to your AS is coming via one single point even though you have multiple points to the internet. I have chosen to receive partial routes from AS200 and only local routes from AS300.13.2 0 100 0 200 i * 192. Because of aggregation your whole AS might look as one whole entity to the outside world and entry points to your network could occur via RTA or RTB.5 0 300 500 400 200 i *> 192.10.0 0 32768 i There are multiple ways to design our network to talk to the two different ISPs AS200 and AS300.213. Both RTA and RTB are generating default routes into OSPF with RTB being more preferred (lower metric). ? . This might occur if you are using the same pool of IP addresses (same major net) when talking to the two ISPs.0 192.250. One way is to have a primary ISP and a backup ISP.211. In this example.213. if AS400 has somehow set its exit point to be via AS200 based on attributes such as local preference or metric or weight then there is nothing you can do.0 203.0.10 Status codes: s suppressed.41 0 100 0 i *>i203.incomplete Network Next Hop Metric LocPrf Weight Path *>i128. i -internal Origin codes: i .5 0 300 500 400 i *>i203. In our example. d damped.10.250.2 Page 92 Sam Halabi-cisco Systems .208. In our example. We could learn partial routes from one of the ISPs and default routes to both ISPs.250. local router ID is e .14.0 128.13. traffic from AS400 destined to your network will always come in via RTA because of the shorter path. One service provider might be closer to a certain destination than another. One other potential reason for asymmetry is the different advertised path length to reach your AS. You might try to affect that decision by prepending path numbers to your updates to make the path length look longer (set as-path prepend).13.0 192. I have chosen two different major nets when talking to the two ISPs.5 0 300 500 i *>i200. This way I could balance outgoing traffic between the two ISPs.0. But.63.200. h history.IGP.15.RTB#sh ip bgp BGP table version is 12.15.41 0 100 0 i *> 203.0.EGP. > best.0 203.250.0 0.2 100 0 200 400 i * 192. * valid. This is the final configuration for all of the routers: 1/26/96-Rev: A1.208.0 128.

the local preference for routes coming from AS200 is set to 200.0 to be the candidate default.0.0 neighbor 128.255. I have also picked network Also with IGRP/EIGRP we can redistribute a static route to 0.0.2 remote-as 100 neighbor 203.0 interface Ethernet0 ip address 203.1 255.255 area 0 network area 0 default-information originate metric 2000 router bgp 100 no synchronization network 203.255.0 is automatically redistributed into RIP without additional configuration.0.0.0 0.255.255.RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.1 255. The “default-information originate” command is used with OSPF to inject the default route inside the OSPF domain. update-source Loopback0 ip classless ip default-network 200. the default information is injected into the IGP domain after redistributing BGP into IGRP/EIGRP.0 route-map setlocalpref permit 10 set local-preference 200 On RTA. For RIP. into the IGP domain. For IGRP and EIGRP.250.200.213. 0.0 interface Serial0 ip address 128.250.0. This command is also used with ISIS and BGP.14.2 remote-as 200 neighbor 128.15.2 Page 93 Sam Halabi-cisco Systems . 1/26/96-Rev: A1.0 0.2 route-map setlocalpref in neighbor 203. using the “ip default-network” command.213.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203.200.0 network 255.

208.13.0 area 0 default-information originate metric 1000 ! router bgp 100 no synchronization network 203.2 Page 94 Sam Halabi-cisco Systems . interface Serial1 ip address 203.0.10 255.5 remote-as 300 neighbor 192.10.255 area 0 network 192.250.252 router ospf 10 network route-map localonly in neighbor 0.RTF# hostname RTF ip subnet-zero interface Ethernet0 ip address 203.0.252 interface Serial0 ip address router ospf 10 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network 203.250.0 ip as-path access-list 1 permit ^300$ route-map localonly permit 10 match as-path 1 set local-preference 300 1/26/96-Rev: A1.0 neighbor 192.255.6 0.255.6 remote-as 100 ! ip classless ip default-network 192.255.255 area 0 ip classless RTB# hostname RTB ip subnet-zero interface Loopback1 ip address ! interface Serial1 ip address 192.0 255.1 255.

? .63.252 router bgp 200 network 0 300 0 300 RTC# hostname RTC ip subnet-zero interface Loopback0 ip address 128.255 access-list 1 permit any On RTC.IGP.213.0.5 Status codes: s suppressed.0.208. Any path info that does not match ^300$ will be dropped.6 remote-as 400 ip classless access-list 1 deny 195.213.0/16 and indicated the specific routes to be injected into AS100.0 aggregate-address 128.0 255.255.0 summary-only neighbor 128.255.10. local router ID is interface Serial2/0 ip address 128.213.250. If the ISP refuses to do this task then you have to filter on the incoming end of AS100. e . Note that I have only advertised AS300’s local routes.252 ! interface Serial2/1 ip address I have aggregated 128. This way AS100 will pick RTB for AS300’s local routes. and this way RTA will be preferred.2 remote-as 100 neighbor 128.63. d damped.EGP.2 Page 95 Sam Halabi-cisco Systems .130 255.208.213. 1/26/96-Rev: A1.10. If you wanted to advertise the local routes and the neighbor routes (customers of the ISP) you can use the following: ^300_[0-9]* This is the output of the regular expression indicating AS300’s local routes: RTB#sh ip bgp regexp ^300$ BGP table version is 14.213. Any other routes on RTB (if they exist) will be sent internally with a local preference of 100 which is lower than 200 coming in from RTA. i - internal Origin codes: i .213.0 192.1 distribute-list 1 out neighbor 128.For RTB.213.incomplete Network Next Hop Metric LocPrf Weight Path *> 192. the local preference for updates coming in from AS300 is set to 300 which is higher than the IBGP updates coming in from RTA.255. h history.213. * valid. > best.0 0.63.63. interface Serial0 ip address 192.10.174 remote-as 100 RTG# hostname RTG ip subnet-zero interface Loopback0 ip address 195.255.0 router bgp 300 network 192.0 aggregate-address neighbor 0.1 255.1 255.211.2 255.2 send-community neighbor 192.252 ! interface Serial0/1 ip address 192.2 remote-as 300 neighbor host 255.2 Page 96 Sam Halabi-cisco Systems .0.RTD# hostname RTD ip subnet-zero interface Loopback0 ip address ! interface Serial0/0 ip address 192.0 summary-only neighbor 192.2 route-map setcommunity out neighbor 195.255.174 255.255 access-list 2 permit any access-list 101 permit ip 195.2 remote-as 400 ! ip classless access-list 1 permit 195.0 route-map setcommunity permit 20 match ip address 2 ! route-map setcommunity permit 10 match ip address 1 set community no-export 1/26/96-Rev: A1.252 router bgp 500 network interface Serial1 ip address 255.10.1 remote-as 500 neighbor 255.EGP.213. > best.14.2 Page 97 Sam Halabi-cisco Systems .250.255. local router ID is 203.10.1 remote-as 500 ip classless RTE is aggregating interface Serial1 ip address 128.2 200 0 200 400 i *> 203.252 router bgp 400 network 200.0. d damped.208.0.0 interface Serial0 ip address Network Next Hop Metric LocPrf Weight Path *> 128.IGP.0.6 255.2 0 200 0 200 i *>i192. 128.0 192.5 remote-as 200 neighbor 195.0 0.41 Status codes: s suppressed.255. e .255.0 summary-only neighbor 128.0. This way RTD will not export that route to RTB.255.0 255.0 128.0 updates towards RTD.1 255.0. It doesn’t matter in our case because RTB is not accepting these routes anyway. And following are the final bgp and routing tables for RTA. i - internal Origin codes: i .213.250. ? .15.255.0 0 32768 i *> 203.5 0 300 0 300 i *> 200.0 aggregate-address 0 100 0 i 1/26/96-Rev: A1.208. h history. 203.0 0. I have demonstrated the use of community filtering by adding a no-export community to 195.0/ 0 32768 i *>i203.On RTG.200. RTE# hostname RTE ip subnet-zero interface Loopback0 ip address 200.13.211. RTF and RTB: RTA#sh ip bgp BGP table version is 21.213. * valid.

255.63.15. 00:41:25. 2 subnets. is directly connected.0 [110/1000] via 203. 00:41:25. M .255.63.0 255. Serial0 B* [200/0] via 203.0 255.255.213.EGP i . Loopback0 203.BGP D .250.208. 00:02:38 1/26/96-Rev: A1. 2 masks B 128. 3 masks O 203. 3 subnets.255 [110/75] via 203. 2 subnets. I .255.IS-IS.0.0 external type 1.250. 00:41:25.15.0 is directly connected. Ethernet0 128.255.0 [20/0] via 128. E .255.0 level-1.EIGRP external.255. Ethernet0 O 203.OSPF inter area E1 .connected.200. IA .13. E2 . S .mobile.255.RIP.255.0 is variably subnetted. Ethernet0 B 203. Ethernet0 C 203.252 is directly connected. 2 masks O E2 192.0 255.15.IGRP.208. 00:41:25 C 203.250.RTA#sh ip rou Codes: C .213.2.OSPF.EIGRP. L1 . 00:41:26 C to network 200.static.0.0 255.250. 00:41: [110/138] via Ethernet0 O 192.252 [110/74] via 203.10. O .10 255.0 Page 98 Sam Halabi-cisco Systems .213.0 [20/0] via 128.14.OSPF external type 2.250. EX . B .0.IS-IS level-2.0 is variably subnetted. L2 . R . * - candidate default Gateway of last resort is 128.2.0 is variably subnetted.255.

Ethernet0 128.15.OSPF external type 1.255. The gateway of last resort is set to RTB.250. 00:45:01. Serial1 C 203.0.250.OSPF.0 [110/2000] via 203.252 [110/74] via 203. Serial1 O 192.static.0 is directly connected.0 [110/2000] via 203.255 [110/65] via 203. L1 .2.255.0 00:03:33. 2 masks O 203.1.250. 01:12:09.0 [110/1000] via 2 masks O E2 192. Other known networks such as 200.0.15. E2 .41 255.14. 00:03:47.4 255.255.250. Ethernet0 O*E2 0.208.OSPF external type 2. Serial1 C 203. 2 masks O 203.0 192. I .0 are to be reached via RTA. 1/26/96-Rev: A1.BGP D . IA .connected.63.IGRP.255.250. Ethernet0 O 128.0 255.OSPF inter area E1 .250.0 255. E . 01:12: Page 99 Sam Halabi-cisco Systems .13.2.0 is variably subnetted.0 255.IS-IS level-2.255.0 is variably subnetted.200.10.255.RTF#sh ip rou Codes: C .0 is variably subnetted.10 255. L2 . Serial1 Note RTF’s routing table which indicates that networks local to AS300 such as 2 subnets.2.255.IS-IS level-1. B . 00:48:50.0 is directly connected. EX .255.213.14. O .14.250.250. then the default advertised by RTA will kick in with a metric of 2000.255 [110/11] via 203. 01:12:09.0 255.1. Serial1 203.EIGRP external.255.255. 01:12:09.252 [110/128] via 203. In case something happens to the connection between RTB and RTD. S .RIP.EIGRP.0 [110/1000] via 203.0 [110/2000] via to network 0.IS-IS. 2 subnets. M .15.0 are to be reached via RTB.213. R . i .250.200.0 is variably subnetted. Ethernet0 203.0 2 subnets. Ethernet0 O E2 2 subnets.14. 2 masks O E2 128. * - candidate default Gateway of last resort is 203. Ethernet0 O E2 01:12:11.0.

63.0 is subnetted. S Status codes: s suppressed.0 192.250. IA . Serial0 128. I .250.8 is directly connected.250.13.0 [20/0] via 192.15.1.IGRP. 01:15:40. 01:20:33.250. * valid.0.10.5 0 300 0 300 i *>i200. 2 masks B* 192. R .41 255.252 is directly connected.255. 00:46: 255.0 is variably subnetted. 2 subnets.255.255. e .0 is directly connected. 2 masks O E2 128.14. i - internal Origin codes: i .EGP i .0 [110/2000] via 203. local router ID is 203.255.250. Serial1 203. M .OSPF inter area E1 .RTB#sh ip bgp BGP table version is 14.EIGRP.10.connected.255.255.2 Page 100 Sam Halabi-cisco Systems .250.41 0 100 0 i *> 203.15.OSPF external type ? .0 255. L2 .13.255.4 255.0 203.10. B . E .255.15. 2 subnets C 203.63.41 0 100 0 i *>i203.1. O . > best.0 128.252 [110/138] via 203.250.EIGRP external. 01:20:33. E2 . [110/75] via 203. * - candidate default Gateway of last resort is [110/2000] via 203.0 255. Serial0 1/26/96-Rev: A1. Loopback1 C 203. 00:05: Network Next Hop Metric LocPrf Weight Path *>i128.255.250.0 * 192.0/16 128. h history.IS-IS level- Serial0 O E2 200.208.250.IS-IS level-1. Serial0 O 128.1. 01:20: D .0 [110/2000] via 203. L1 .2 200 0 200 400 i *>i203.0 is variably subnetted. 2 subnets.0.1. Serial0 203.208.200. Serial0 O E2 203.63.0 [110/74] via 203.0 0 32768 i RTB#sh ip rou Codes: C .5 to network 192.RIP.2 0 200 0 200 i *> 255.0.static. EX .0 is variably subnetted. 2 subnets. 00:50:46 C 192. 2 masks O 203.EGP. Serial0 O 203.10. d damped.OSPF external type 2.0 255.14.0 0.OSPF.