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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allows you to create loop free interdomain routing between autonomous systems.2 Page 4 Sam Halabi-cisco Systems . 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). 1/26/96-Rev: A1. 1. Two BGP speaking routers form a TCP connection between one another (peer routers) and exchange messages to open and confirm the connection parameters. The version number will change whenever BGP updates the table due to some routing information changes. BGP routers will exchange network reachability information.3 Information exchange between peers BGP peers will initially exchange their full BGP routing tables. 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. 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. From then on incremental updates are sent as the routing table changes.0 Introduction The Border Gateway Protocol (BGP).1. defined in RFC 1771. An autonomous system is a set of routers under a single technical administration. 1. they are also called neighbors. BGP keeps a version number of the BGP table and it should be the same for all of its BGP peers.2 What are peers (neighbors) Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers. 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.

it could be used as a transit service for other ASs. 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.0 EBGP and IBGP If an Autonomous System has multiple BGP speakers. 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). AS200 is a transit autonomous system for AS100 and AS300. As far as this paper is concerned.2 Page 5 Sam Halabi-cisco Systems .2. It is necessary to ensure reachability for networks within an AS before sending the information to other external ASs. As you see below. EBGP AS100 IBGP AS300 AS200 1/26/96-Rev: A1.

The next step in the configuration process is to define BGP neighbors. 1/26/96-Rev: A1. 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.3.2 Page 6 Sam Halabi-cisco Systems . In the first example RTA and RTB are in different autonomous systems and in the second example both routers belong to the same AS.0 Enabling BGP routing Here are the steps needed to enable and configure BGP. The next section will introduce you to what is involved in forming a valid peer connection. Let us assume you want to have two routers RTA and RTB talk 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. One sure way to verify reachability is an extended ping between the two IP addresses. 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. 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.1 BGP Neighbors/Peers Two BGP routers become neighbors or peers once they establish a TCP connection between one another.3.A special case (EBGP multihop) will be discussed later when the external BGP peers are not directly connected.2 Page 7 Sam Halabi-cisco Systems .A special case for loopback interfaces is discussed later. 2. The TCP connection is essential in order for the two peer routers to start exchanging routing updates. After these values are confirmed and accepted the neighbor connection will be established. 1. The ip-address is the next hop directly connected address for EBGP1 and any IP address2 on the other router for IBGP. etc. It is essential that the two IP addresses used in the neighbor command of the peer routers be able to reach one another. 1/26/96-Rev: A1. the BGP router ID and the keepalive hold time. the BGP version they are running (version 3 or 4). 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.

1 RTB IBGP RTC 175.2 AS100 AS300 remote-as 200 1/26/96-Rev: A1. BGP sessions begin using BGP Version 4 and negotiating downward to earlier versions if necessary. 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 remote-as 100 neighbor 175.212.It is important to reset the neighbor connection in case any bgp configuration changes are made in order for the new parameters to take effect.213.2 Page 8 Sam Halabi-cisco Systems .1.1 remote-as 200 RTB# router bgp 200 neighbor 129.1.2 remote-as 200 RTC# router bgp 200 neighbor 175.220. To prevent negotiations and force the BGP version used to communicate with a neighbor. clear ip bgp address (where address is the neighbor address) clear ip bgp * (clear all neighbor connections) By default.1.213.2 AS200 RTA# router bgp 100 neighbor

1 BGP state = Established. external link BGP version 4. The difference between EBGP and IBGP is manifested by having the remote-as number pointing to either an external or an internal AS. pay special attention to the BGP state. keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 2828 messages. dropped 10 In the next section we will discuss special situations such as EBGP multihop and loopback addresses. 1/26/96-Rev: A1. #SH IP BGP N BGP neighbor is 129. Anything other than state established indicates that the peers are not up. RTB and RTC are run- ning IBGP.213. the table will increase the version and a version that keeps incrementing indicates that some route is flapping causing routes to keep getting updated). table version = 3.220. 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. up for 0:10:59 Last read 0:00:29. remote router ID 175. remote AS 200. 0 in queue Connections established 11. Any time new information comes in.2 Page 9 Sam Halabi-cisco Systems . Also. You should also note the BGP is version 4.In the above example RTA and RTB are running EBGP.1. 0 in queue Sent 2826 messages. as long as there is some IGP running that allows the two neighbors to reach one another.1.212. The following is an example of the information that the command “sh ip bgp neighbors” will show you. 0 notifications. hold time is 180. 0 notifications. the EBGP peers are directly connected and the IBGP peers are not. IBGP routers do not have to be directly connected.

0 BGP and Loopback interfaces Using a loopback interface to define neighbors is commonly used with IBGP rather than EBGP. RTA and RTB are running internal BGP inside autonomous system 100. 1/26/96-Rev: A1.1. some extra configuration needs to be done on the neighbor router.1 RTB RTA 190.212.225. In the case of EBGP. RTA will do so by adding the update-source int loopback configuration (neighbor 190. in this case RTA has to force BGP to use the loopback IP address as the source in the TCP neighbor connection.225. 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. RTB is using in its neighbor command the loopback interface of RTA (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 190.1 remote-as 100 In the above example. 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.1 remote-as 100 neighbor 190.1).1 AS100 RTA# router bgp 100 neighbor 190. If the IP address of a loopback interface is used in the neighbor com- mand.2 Page 10 Sam Halabi-cisco Systems .11. 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 update-source int loopback 1 RTB# router bgp 100 neighbor 150. Loopback Interface 1 150.225.1. most of the time the peer routers are directly connected and loopback does not apply.4.212.225.

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.213.2) and that is why it does not need the ebgp-multihop command.11. Some IGP or static routing should also be configured in order to allow the non directly connected neighbors to reach one another.3 180. The following example gives a better illustration of EBGP multihop.Note that RTA has used the physical interface IP address (190. RTA needs to indicate that it will be using ebgp-multihop.225.2 remote-as 100 RTA is indicating an external neighbor that is not directly connected. there could be a requirement for EBGP speakers to be not directly connected. 1/26/96-Rev: A1.1.1 remote-as 300 neighbor Page 11 Sam Halabi-cisco Systems .1) of RTB as a neighbor and that is why RTB does not need to do any special configuration. The following example shows how to achieve load balancing with BGP in a particular case where we have EBGP over parallel lines. The multihop is used only for external BGP and not for internal BGP.2 AS100 AS300 RTA# router bgp 100 neighbor 180.11.0 EBGP Multihop In some special cases.11.1 ebgp-multihop RTB# router bgp 300 neighbor 129. RTB is indicating a neighbor that is directly connected (129.1. RTA RTB 129. On the other hand. 5.

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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255.3 remote-as 300 network 150. If RTA wants to redistribute to RTB routes about 170. 3.4 AS 100 2. RTA and RTC are running BGP. network 150.0 0.0.2 Page 15 Sam Halabi-cisco Systems .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 AS 300 Example 1: Assume RTA and RTB are running rip. 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 passive-interface Serial0 redistribute bgp 100 route-map SETMETRIC router bgp 100 neighbor 2.255 1/26/96-Rev: A1. RTA is getting updates via BGP and redistributing them to rip.0.2 RTC 2.0 network RTA RTB 3.2. it will have a metric of 2 and then we break out of the route map list. 1/26/96-Rev: A1.0.255 access-list 1 permit 0. we have to use an outbound route map on RTC: RTC# router bgp 300 network 170.0. It is always very important to ask the question.255. let’s look at how to start exchanging network information.255. what will happen to routes that do not match any of the match statements because they will be dropped by default.10. Example 2: Suppose in the above example we did not want AS100 to accept updates about 170.255. If there is no match then we go down the route map list which says.In the above example if a route matches the IP address 170.2 Page 16 Sam Halabi-cisco Systems .10. Since route maps cannot be applied on the inbound when matching based on an ip address.2 remote-as 100 neighbor 2. set everything else to metric 5.0 neighbor I will go through these methods one by one.255 Now that you feel more comfortable with how to start BGP and how to define a neighbor.2. There are multiple ways to send network information using BGP.2.2 route-map STOPUPDATES out route-map STOPUPDATES permit 10 match ip address 1 access-list 1 deny 170.0 0.

1/26/96-Rev: A1.0. The network command will work if the network you are trying to advertise is known to the router.0. This is a different concept from what you are used to configuring with IGRP and RIP.0 ip route 192. whether connected. An example of the network command follows: RTA# router bgp 1 network 192.255.213. static or learned dynamically.213.7.0. A maximum of 200 entries of the network command are accepted.213.0 mask 255. With this command we are not trying to run BGP on a certain interface.0 null 0 The above example indicates that router A.0/16.0 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.0 255.2 Page 17 Sam Halabi-cisco Systems .255. Note that we need the static route to get the router to generate 192.0.0. rather we are trying to indicate to BGP what networks it should originate from this box.0. will generate a network entry for 192. 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).0 because the static route will put a matching entry in the routing table. The mask portion is used because BGP4 can handle subnetting and supernetting.213.

0 and RTC is announcing 175.1 1.) into BGP.0.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor Redistribution The network command is one way to advertise your networks via BGP. Let us look at the example below. EIGRP.7.0. This sounds scary because now you are dumping all of your internal routes into BGP.1. RTA is announcing 129. some of these routes might have been learned via BGP and you do not need to send them out again.2 Page 18 Sam Halabi-cisco Systems .0 (this will limit the networks originated by your AS to RTB RTA RTC AS100 175.1.220. Another way is to redistribute your IGP (IGRP.0 mask 255. Look at RTC’s configuration: RTD AS300 1.0. etc. OSPF. AS200 If you use a network command you will have: RTC# router eigrp 10 network 175.0 129.0) If you use redistribution instead you will have: 1/26/96-Rev: A1. remote-as 300 network 175.213. 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.1.220. RIP.220.

213.1 remote-as 300 neighbor redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1.0 0.1.0 again into BGP) This will cause 129.0 but AS100 is. 1/26/96-Rev: A1.0 to be originated by your AS.255.1.0.RTC# router eigrp 10 network 175.1 distribute-list 1 out redistribute eigrp 10 access-list 1 permit The access-list is used to control what networks are to be originated from AS200.220.2 Page 19 Sam Halabi-cisco Systems .213.0.1. This is misleading because you are not the source of 129.1 remote-as 300 redistribute eigrp 10 (eigrp will inject 129.1.1. So you would have to use filters to prevent that network from being sourced out by your AS.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1.1. The correct configuration would be: RTC# router eigrp 10 network 175.0.213.

2 Page 20 Sam Halabi-cisco Systems . Please remember that these routes are generated in addition to other BGP routes that BGP has learned via neighbors (internal or external). or redistribution or static. Injecting BGP into IGP is always done by redistribution.0 null0 The null 0 interface means to disregard the packet.1 remote-as 300 redistribute static ip route 175.0 255.0 redistribute bgp 200 default-metric 1000 100 250 100 1500 router bgp 200 neighbor 1. BGP passes on information that it learns from one peer to other peers.0.255. So if I get the packet and there is a more specific match than 175.2 Static routes and redistribution You could always use static routes to originate a network or a subnet.220.1. In the above example the same could have been accomplished by doing: RTC# router eigrp 10 network 175. The only difference is that BGP will consider these routes as having an origin of incomplete (unknown). We have discussed how we can use different methods to originate routes out of our autonomous system. (which exists of course) the router will send it to the specific match otherwise it will disregard it.0. This is a nice way to advertise a supernet.220.220. The difference is that routes generated by the network command. will indicate your AS as the origin for these networks.1. 1/26/96-Rev: A1.

10.1 AS 200 160.0. 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.1 150.2 RTC remote-as 100 neighbor 160.2 remote-as 300 network 160.0 or network 150.0 RTC# router bgp 300 neighbor Note that you do not need network 150.0 RTB# router bgp 200 neighbor 160.2 Page 21 Sam Halabi-cisco Systems .10.2 remote-as 300 network 150.0 RTB 150. This is to insure a loop free interdomain topology.0 AS 100 RTA 160.1 remote-as 200 network 170.10.2 in RTC unless you want RTC to also generate these networks on top of passing them on as they come in from AS100 and AS200.20.20.10. 1/26/96-Rev: A1.20.0 AS 300 RTA# router bgp 100 neighbor An important point to remember is that BGP will not accept updates that have originated from its own AS.

20.0.0 to AS100 with origin still AS100.40. RTA will generate a route You might ask.20.2 RTA AS 100 170. assume AS200 above had a direct BGP connection into AS100.10.0 Internal BGP IBGP is used if an AS wants to act as a transit system to other ASs.2 RTC 175.For example.0.10.1 remote-as 100 neighbor 170. IBGP 175.1 150.50.0 and will send it to AS300. 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).0 AS300 RTA# router bgp 100 neighbor 190.40.0 1/26/96-Rev: A1.2 remote-as 300 network RTB will pass 150.0.1 RTE 175. RTA will notice that the update has originated from its own AS and will ignore it.10. why can’t we do the same thing by learning via EBGP redistributing into IGP and then redistributing again into another AS? We can.1 170.0.0 AS500 AS400 but IBGP offers more flexibility and more efficient ways to exchange information within an AS.2 Page 22 Sam Halabi-cisco Systems .50. then RTC will pass this route to AS200 with the origin kept as AS100. RTD IBGP RTB 190.10.10.

That is why it is important to sustain a full mesh between the IBGP speakers within an AS. the receiving BGP speaker will not redistribute that information to other BGP speakers in its own AS. 9.10. the route origin. In the following section I will try to explain these attributes and show how they are used.0 RTC# router bgp 400 neighbor remote-as 100 network 175.2 Page 23 Sam Halabi-cisco Systems . We will start with the path attribute.0.10.10. local preference.30.RTB# router bgp 100 neighbor 150. In the above diagram. 1/26/96-Rev: A1. administrative weights.1 remote-as 100 neighbor 175. RTA and RTB are running IBGP and RTA and RTD are running IBGP also.40. This is why an IBGP peering should be made between RTB and RTD in order not to break the flow of the updates. BGP will always propagate the best path to its neighbors. metric and so on. The decision process is based on different attributes. origin code. The receiving BGP speaker will redistribute that information to other BGP speakers outside of its AS.10. is that when a BGP speaker receives an update from other BGP speakers in its own AS (IBGP).1 remote-as 400 network 190. path length. the protocol will have to decide which paths to choose in order to reach a specific destination. such as next hop. BGP will choose only a single path to reach a specific destination.50.0 The BGP decision algorithm After BGP receives updates about different destinations from different autonomous systems.0 An important point to remember. 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).

10. the AS number is prepended to that update.0 and path (100) in order to reach 170.0 AS 100 AS200 RTB RTA AS 300 RTC 100 200 180.10 300 Whenever a route update passes through an AS.0.10.e.10.10. An example of AS-SET is given later.0.0. RTC will have to traverse path (200) in order to reach 190. when that route traverses AS300 and RTC will append its own AS number to it. An AS-SET is an ordered mathematical set {} of all the ASs that have been traversed.0.0 and 180. In the above example. So when Page 24 Sam Halabi-cisco Systems . reaches RTA it will have two AS numbers attached to it: first 200 then 300. The AS_path attribute is actually the list of AS numbers that a route has traversed in order to reach a destination.0 is (300. 1/26/96-Rev: A1.0.200).0. So as far as RTA is concerned the path to reach 190.0. RTB will have to take path (300. network 190.10. traverse AS300 and then AS100 in order to reach 170.0.100) i.0 is advertised by RTB in AS200.0. The same applies for As_path Attribute 170.

This is indicated with an “e” in the BGP table.2 Page 25 Sam Halabi-cisco Systems . Example: RTA RTB RTE 170.2 170.1 150.0 AS300 1/26/96-Rev: A1.20. INCOMPLETE: NLRI is unknown or learned via some other means.0 Origin Attribute The origin is a mandatory attribute that defines the origin of the path information. This usually occurs when we redistribute a static route into BGP and the origin of the route will be incomplete.1 IBGP AS 100 170.40.1 175.10. This normally happens when we use the bgp network command or when IGP is redistributed into BGP.10. then the origin of the path info will be IGP. EGP: NLRI is learned via EGP (Exterior Gateway Protocol). This is indicated with an “?” in the BGP table.10.0. This is indicated with an “i” in the BGP table. The origin attribute can assume three values: IGP: Network Layer Reachability Information (NLRI) is interior to the originating AS.

0. RTE will reach 150.10.0 RTE# router bgp 300 neighbor 170.0 via: i (which means.0 via: 100 ? (the next AS is 100 and the origin is incomplete “?”.0 null0 RTB# router bgp 100 neighbor 150.10.RTA# router bgp 100 neighbor 190.50. 1/26/96-Rev: A1. Page 26 Sam Halabi-cisco Systems .1 remote-as 100 network remote-as 100 network 190.10. RTA will also reach 190.1 remote-as 100 neighbor 170.0 255.10. the entry is in the same AS and the origin is IGP). RTA will reach 170.50. RTE will also reach 190.0.0 via: 300 i (which means the next AS path is 300 and the origin of the route is IGP).30.10. coming from a static route).0 redistribute static ip route remote-as 300 network via: 100 i (the next AS is 100 and the origin is IGP).10.

0 RTA RTB 150.0.0 AS 300 The BGP nexthop attribute is the next hop IP address that is going to be used to reach a certain destination.2 and RTA will advertise 150.20. RTC will advertise 170.1 AS 100 170. the protocol states that the next hop advertised by EBGP should be carried into IBGP. This is described in the following sections.20.This is not true if the next hop is on a multiaccess media.10.2 RTC to its IBGP peer RTB with a next hop of 170.2 via IGP.0.20.0 to RTA with a next hop of 170.1 150. So according to RTB.0.0 is For example.0.0 to RTC with a next hop of 170.20. RTA will advertise 170. 1/26/96-Rev: A1.10. otherwise RTB will drop packets destined to 170. if RTB is running igrp you could also run IGRP on RTA network 170.10. In the above example.50.0 BGP Nexthop Attribute 150.2 and NOT in which case the nexthop will be the ip address of the router that is closest to the destination.10. the next hop is always the IP address of the neighbor specified in the neighbor command1.2.0 because the next hop address would be inaccessible. You would want to make IGRP passive on the link to RTC so BGP is only exchanged. Because of that rule.10. 1.30. You should make sure that RTB can reach 170.10. For IBGP.10.20. For EBGP.0.10. the next hop to reach 170.2 Page 27 Sam Halabi-cisco Systems . IBGP 170.


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

0 with a NextHop = a command called next-hop-self is created.We will discuss peer-group-names later on 1/26/96-Rev: A1.10.2 1.20.1 remote-as 100 neighbor 170.20. 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.2 Page 31 Sam Halabi-cisco Systems .1 next-hop-self RTC will advertise 180.12.10. In the previous example the following will solve our problem: RTC# router bgp 300 neighbor 170.3 Next-hop-self Because of certain situations with the nexthop as we saw in the previous example.0.

10.3. etc. BGP has the following distances.2. 90 for EIGRP and 110 for OSPF.2 Page 32 Sam Halabi-cisco Systems . Default distance is 120 for RIP.0 via two routing protocols: EBGP with a distance of 20 and IGP with a distance higher than 20. RTA and RTC are running EBGP and RTB and RTC are running EBGP. RTA and RTB are running some kind of IGP (RIP.2 3.13.3 AS 200 AS 100 2. 100 for IGRP. 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. By default.1 3.0 BGP Backdoor 150.0 via RTB (IGP). then we have two options: 1/26/96-Rev: A1. RTA will receive updates about AS 300 Consider the above diagram.0 If we want RTA to learn about 160.3. EBGP updates have a distance of 20 which is lower than the IGP distances.10.10.0.) By definition.0.1 RTC 170.0 RTA RTB IGP IGRP.

0 from RTB via EIGRP with distance 90. 1/26/96-Rev: A1.1.2. RTA will learn 160. but because of the backdoor command EIGRP will be preferred. and will also learn it from RTC via EBGP with distance 20. 2.0 router bgp 100 neighbor 2.Change EBGP’s external distance or IGP’s distance which is NOT recommended.2 Page 33 Sam Halabi-cisco Systems .1 remote-as 300 network 160.0. Example: RTA# router eigrp 10 network 160.0 backdoor Network 160.Use BGP backdoor BGP backdoor will make the IGP route. Normally EBGP will be preferred.0 will be treated as a local entry but will not be advertised as a normal network entry would. The configured network is the network that we would like to reach via IGP. the preferred route.0.10.10. For BGP this network will be treated as a locally assigned network except it will not be advertised in bgp updates.0. Use the following command: network address backdoor.

Synchronization states: If your autonomous system is passing traffic from another AS to a third AS.As far as the router is concerned.0.10.0. If RTB starts advertising to AS400 that he can reach 170.0.1 RTD RTC AS400 AS 300 Before we discuss synchronization let us look at the following scenario.10.10.1 (remember that the next hop is carried via IBGP).0 Synchronization RTA and RTB are running IBGP. Assume that RTA has not redistributed network BGP should not advertise a route before all routers in your AS have learned about the route via IGP1. RTB will have to send the traffic to RTE.2.2. RTC in AS300 is sending updates about 170.0 AS 100 170.14. 1. we will check to see if we have a route in the ip routing table.0 RTE Was 170.0 2. 1/26/96-Rev: A1.0. so RTB will get the update and will be able to reach 170. BGP will wait until IGP has propagated the route within the AS and then will advertise it to external peers. so at this point RTE has no idea that 170.0 propagated within IGP? RTA RTB IBGP will flow in and get dropped at RTE.2.2 Page 34 Sam Halabi-cisco Systems . This is called synchronization.0.2 170.0 even exists.0.10.0 into IGP.0.2.0 via next hop 2. In order to reach the next hop.10.0 then traffic coming from RTD to RTB with destination 170.0.10. This could be done by defining a static route.

In the above example. if you have all your routers in the AS running BGP and you are not running any IGP. router bgp 100 no synchronization.0.1 Disabling synchronization In some cases you do not need synchronization.0.0 via IGP before it starts sending the update to RTD. You have to disable synchronization manually in this case for routing to work correctly.10.0.10. We can fool RTB into thinking that IGP has propagated the information by adding a static route in RTB pointing to 170. RTB will wait to hear about 170.0. (Make sure you do a clear ip bgp address to reset the session) 1/26/96-Rev: A1. the router has no way of knowing that. 14.2 Page 35 Sam Halabi-cisco Systems . or if all routers in your AS will be running BGP. you can disable synchronization. and your router will be waiting forever for an IGP update about a certain route before sending it to external peers. Disabling synchronization is not automatic.10. If you will not be passing traffic from a different autonomous system through your AS.0 otherwise we will have a problem reaching that network. Disabling this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly. Care should be taken to make sure that other routers can reach 170.

0 170.2 remote-as 400 neighbor 2.Example: in its ip routing table and will advertise it to RTD even if it does not have an IGP path to 170.1 neighbor RTA# router bgp 100 network 150.1 2.0) RTD# router bgp 400 neighbor 1.10.4 remote-as 100 1/26/96-Rev: A1.0 Was 170.0 neighbor AS300 RTB# router bgp 100 network propagated within IGP? RTA RTB IBGP 3.0.2 RTC RTD AS400 175.2 Page 36 Sam Halabi-cisco Systems . AS100 3.0.1 remote-as 100 network remote-as 100 no synchronization (RTB will put 170.1.1.

10.1.0 and has to decide which way to go. RTB has also learned about network 175.15.0 RTA AS200 RTB AS4 2.0. A weight can be a number from 0 to 65535.0. Routes with a higher weight are preferred when multiple routes exist to the same destination. The weight is used for a best path selection process.10.0.0 Weight Attribute 170.10.0 190.10.10. The weight is assigned locally to the router.0.2. then we will force RTC to use RTA as a next hop to reach 175.10.0) w=200 RTC w= 100 AS300 The weight attribute is a Cisco defined attribute.0) (175. Paths that the router originates have a weight of 32768 by default and other paths have a weight of zero.2 1.0. 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.0.0.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: A1.0 AS100 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.10. This is achieved by using multiple methods: 1.1. RTC has now two ways for reaching 175.0.0 from AS4 and will propagate it to RTC.1 (175.2 Page 37 Sam Halabi-cisco Systems . RTA has learned about network 175. Let us study the above example.Using the neighbor command neighbor {ip-address|peer-group} weight weight 2.0 from AS4 and will propagate the update to RTC.

2.2 Page 38 Sam Halabi-cisco Systems .0 from RTA will have 200 weight) neighbor 2.1.2 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 remote-as 100 neighbor 1.1.2.e.0 from RTB will have 100 weight) *Routes with higher weight are preferred when multiple routes exist to the same destination. The same outcome can be achieved via ip as-path and filter lists.1 remote-as 100 neighbor remote-as 200 neighbor 2.2 remote-as 200 neighbor 2.2. RTC# router bgp 300 neighbor 1.1 filter-list 5 weight 200 neighbor i.2 weight 100 (route to 175. RTC# router bgp 300 neighbor 1.3-Using route-maps example: RTC# router bgp 300 neighbor remote-as 200 neighbor 2. packets from AS100.1.2.1 weight 200 (route to remote-as 100 neighbor 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.1 route-map setweightin in neighbor 2.2.10. would have weight 200) route-map setweightin permit 20 set weight 100 (anything else would have weight 100) 1/26/96-Rev: A1. RTA will be preferred as the next hop.1.

1. Local preference will help us determine which way to exit AS256 in order to reach that network.0 RTB RTA 3.2 AS34 RTC 1.1.2 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. A path with a higher local preference is more preferred.11. The default value for local preference is 100.0.213. Let us assume that RTD is the preferred exit point.3 128.2 Page 39 Sam Halabi-cisco Systems . In the above diagram.0 from two different sides of the organization.10.0 Local Preference Attribute 170. The following configuration will set the local preference for updates coming from AS300 to 200 and those coming from AS100 to 150.1 IBGP 128. Unlike the weight attribute which is only relevant to the local router.1 AS100 AS300 set local pref 200 set local pref 150 AS256 1. 1/26/96-Rev: A1.16.3.11. local preference is an attribute that is exchanged among routers in the same AS. 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.0.1.3. AS256 is receiving updates about 170.213.3. 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.2 Page 40 Sam Halabi-cisco Systems . In the above example.0.3. both RTC and RTD will realize that network 170. 1/26/96-Rev: A1. More flexibility is provided by using route maps. All traffic in AS256 addressed to that network will be sent to RTD as an exit point. Since local preference is exchanged within AS256.11.RTC# router bgp 256 neighbor 1. This means that updates coming from AS34 will also be tagged with the local preference of 200. all updates received by RTD will be tagged with local preference 200 when they reach RTD.4 setlocalin in neighbor 128.213. 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 3.11.4 remote-as 300 neighbor 128.1 remote-as 100 neighbor 128.11.1 remote-as 256 bgp default local-preference 200 In the above configuration RTC will set the local preference of all updates to This might not be needed.213. any update coming from AS300 will be set with a local preference of 200.3. Any other updates such as those coming from AS34 will be set with a value of 150. The same RTD will set the local preference of all updates to 200.2 remote-as 256 bgp default local-preference 150 RTD# router bgp 256 neighbor remote-as 300 neighbor 3.0 has a higher local preference when coming from AS300 rather than when coming from AS100.

Unless otherwise specified. BGP4) or Inter-As (BGP3) is a hint to external neighbors about the pre- ferred path into an AS.4.2. metric is exchanged between ASs. AS100 is getting information about network 180.10.0 AS100 RTA 180.3 AS400 set metric 200 set metric 120 2. The Metric default value is 0.4. When an update enters the AS with a certain metric. AS300 3.2 Page 41 Sam Halabi-cisco Systems . that metric is used for decision making inside the AS. Unlike local preference.0. In the above diagram. a router will compare metrics for paths from neighbors in the same AS.0 Metric Attribute metric=0 METRIC (MULTI_EXIT_DISC) (INTER_AS) 170.2 RTD 180.3. 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.3 RTC 1.2 set metric 50 4. A lower value of a metric is more preferred.3.4 RTB 2.0 The metric attribute which is also called Multi_exit_discriminator (MED.2.1 1.10. When the same update is passed on to a third AS. RTD and RTB.2. A metric is carried into an AS but does not leave the AS. RTC and 1/26/96-Rev: A1.1. 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. that metric will be set back to 0 as shown in the above diagram.4.2 via three different routers: RTC. remote-as 100 neighbor route-map setmetricout out route-map setmetricout permit 10 set metric 50 With the above configs.3 remote-as 300 neighbor 4. When RTA gets an update from RTB with metric 50. RTA will pick RTC as next hop. Assume that we have set the metric coming from RTC to 120. This is illustrated in the configs below: RTA# router bgp 100 neighbor 2.2 Page 42 Sam Halabi-cisco Systems .1 remote-as 300 neighbor 3. In order to force RTA to compare the metrics we have to add bgp always-compare-med to RTA. considering all other attributes are the same.1.2.3 remote-as 400 RTC# router bgp 300 neighbor 2.RTD are in AS300 and RTB is in AS400.2 remote-as 300 route-map setmetricout permit 10 set metric 120 RTD# router bgp 300 neighbor 3.4.1 remote-as 300 route-map setmetricout permit 10 set metric 200 RTB# router bgp 400 neighbor 4. he can not compare it to 120 because RTC and RTB are in different ASs (RTA has to choose based on some other attributes). route-map setmetricout out neighbor 1.1.4 remote-as 100 neighbor 4. the metric coming from RTD to 200 and the metric coming from RTB to 50. 1/26/96-Rev: A1.3.2. Given that by default a router compares metrics coming from neighbors in the same AS.3.2 remote-as 100 neighbor route-map setmetricout out neighbor 1.4. 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.

21 remote-as 300 neighbor 3.0.In order to have RTB included in the metric comparison.10. Metric can also be set while redistributing routes into BGP.0.10.0. we have to configure RTA as follows: RTA# router bgp 100 neighbor 2. 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 180.3 remote-as 400 bgp always-compare-med In this case RTA will pick RTB as the best next hop in order to reach network 180.0 255.0 with a metric of Page 43 Sam Halabi-cisco Systems .0.3 remote-as 300 neighbor 1/26/96-Rev: A1.0 null 0 will cause RTB to send out 180.2.

3. 200 will replace any old community that already exits.3.3.3 remote-as 300 neighbor 3. this attribute will not be sent to neighbors by default. 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 3.3 route-map setcommunity out 1/26/96-Rev: A1. prefer. We can use route maps to set the community attributes. redistribute.967.2 Page 44 Sam Halabi-cisco Systems . 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. etc. 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.18.) according to those communities. The community attribute is a way to group destinations in a certain community and apply routing decisions (accept.294.3. Even if we set the community attribute.3.3 send-community neighbor Community Attribute The community attribute is a transitive. if we use the keyword additive then the 200 will be added to the community. optional attribute in the range 0 to 4.

3. In order to achieve this. you can filter BGP based on routing updates to or from a particular neighbor.2.1 3. If RTC wanted to stop those updates from propagating to AS100. an access-list is defined and applied to the updates to or from a neighbor.10.0 and sending it to RTC. BGP updates can be filtered based on route information. 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.10.10. choosing one over the other depends on the spe- cific network configuration.3.3.10. All methods will achieve the same results.0 RTA RTB 2. RTB is originating network 160. 19.0 BGP Filtering Sending and receiving BGP updates can be controlled by using a number of different filtering methods.0 AS 300 In order to restrict the routing information that the router learns or advertises.3.0.1 Route Filtering 150.2. on path information or on communities.1 RTC 170.2. we would have to apply an access-list to filter those updates and apply it when talking to RTA: 1/26/96-Rev: A1.2 3.0.2 Page 45 Sam Halabi-cisco Systems .2.0.3 AS 200 AS 100 2.0 160.0.19. 255.0/8 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 160.0 0.0.RTC# router bgp 300 network 170.0) The following access list: access-list 1 permit 160.0.0/8 (this notation means that we are using 8 bits of subnet mask starting from the far left of the IP address. 255.255.2 Page 46 Sam Halabi-cisco Systems .10.255. is path filtering which is described in the next section.10.0.x) Using access-lists is a bit tricky when we are dealing with supernets that might cause some conflicts.0.2 distribute-list 1 out access-list 1 deny 160.0/8 only.0.0.255 Another type of filtering.X. 1/26/96-Rev: A1.0.255 (filter out all routing updates about 160.0/8.2 remote-as 100 neighbor 2.0 0.255.0 This list will permit 0.x. neighbor access-list 1 permit 0.2.0.X and our goal is to filter updates and advertise only 0.0. In order to restrict the update to only remote-as 200 neighbor 2.255 will permit 160.0.255. this is equivalent to 160.0.0/9 and so on. Assume in the above example that RTB has different subnets of 160.0.

0 RTC# router bgp 300 neighbor 3. This term will be discussed shortly 1/26/96-Rev: A1.10.0 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 AS100.2 remote-as 100 neighbor 3.2 Page 47 Sam Halabi-cisco Systems .2.3.2. 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 300 You can specify an access list on both incoming and outgoing updates based on the BGP autonomous system paths information.19.2 3.2.2 filter-list 1 out (the 1 is the access list number below) 1.2 Path Filtering 150.3.3. To do this use the following statements.1 RTC RTB RTA 2.10. In the above figure we can block updates about 160.2.0 AS400 160.3 remote-as 200 neighbor AS 200 AS 100 2.

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

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 . The regular expression is composed of the following: A.). 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. (Matches any single character) ^ (Matches the beginning of the input string) $ (Matches the end of the input string) \character (Matches the character) .2 Page 49 Sam Halabi-cisco Systems .2.1 AS-Regular Expression A regular expression is a pattern to match against an input string.Ranges: A range is a sequence of characters contained within left and right square brackets. Examples of regular expressions follow: a* any occurrence of the letter a. ex: [abcd] B.* (coming from AS100) ^$ (originated from this AS) 1/26/96-Rev: A1.Atoms An atom is a single character . the end of the input string. right brace (}). or a space. 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. left brace ({). the beginning of the input string. By building a regular expression we specify a string that input must match. In case of BGP we are specifying a string consisting of path information that an input should match.(Matches a comma (.19.Branch A branch is a 0 or more concatenated pieces.

150.255 1/26/96-Rev: A1.1 send-community neighbor 3.2.0 255.0.1 remote-as 300 neighbor and here are few examples of how we can use it.0.0 3.255.2 3.3.1 route-map setcommunity out route-map setcommunity match ip address 1 set community no-export access-list 1 permit 0.3.2 Page 50 Sam Halabi-cisco Systems .255.10. Another method is community filtering.1 RTC 170.10.0 RTA RTB 2.3 AS 200 AS 100 BGP Community Filtering We have already seen route filtering and as-path filtering.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. The no-export community attribute is used: RTB# router bgp 200 network neighbor Community has been discussed in section 19.

3.255. Example 2: RTB# router bgp 200 network 160. ip community-list community-list-number {permit|deny} community-number For example we can define the following route map.3. When RTC gets the updates with the attribute no-export. 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.255 In the above example.3.1 send-community neighbor 3.0 neighbor 3. it will not propagate them to its external peer RTA.3. RTB was sending updates to RTC with a community of 100 200.255.10.Note that we have used the route-map setcommunity in order to set the community to no-export.3.0 255.0. RTB has set the community attribute to 100 200 additive.1 route-map setcommunity out route-map setcommunity match ip address 2 set community 100 200 additive access-list 2 permit 0.1 remote-as 300 neighbor 3.0. Note also that we had to use the “neighbor send-community” command in order to send this attribute to RTC.2 Page 51 Sam Halabi-cisco Systems .3.0. The value 100 200 will be added to any existing community value before being sent to RTC. In example two above. 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. If RTC wants to set the weight based on those values we could do the following: 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.3. Any route that has only 200 as community will match list 2 and will have weight 20. The key- word exact states that community should consist of 200 only and nothing else.3.3. Remember that anything that does not match.3 remote-as 200 neighbor 3.2 Page 52 Sam Halabi-cisco Systems . 1/26/96-Rev: A1. The last community list is here to make sure that other updates are not dropped. any route that has 100 in its community attribute will match list 1 and will have the weight set to 20. will be dropped by default.RTC# router bgp 300 neighbor 3.3. The keyword internet means all routes because all routes are members of the internet community.

Example 1: RTC# router bgp 300 network neighbor 150. We can achieve this with a combination of neighbor and as-path access lists.1 3. 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.0.0 BGP Neighbors and Route maps 190.0.0 RTB RTC 170.2.0 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.2 Page 53 Sam Halabi-cisco Systems . remote-as 200 neighbor 3.2 AS 100 route-map stamp in 1/26/96-Rev: A1.3.0. we want to set the weight on the accepted routes to 20. Also. AS 200 2.0 AS600 AS400 RTA 160.10.

3. and will set a weight of 10 for updates that are behind AS400 and will drop updates coming from AS400.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.3.0 neighbor 3. Example 2: Assume that we want the following: 1. RTC# router bgp 300 network 170. 3.Other updates to have a weight of 10.2 Page 54 Sam Halabi-cisco Systems .0. 1/26/96-Rev: A1.Updates originating from AS200 to be accepted with weight 20.3. 2.3 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 .3.10.Updates originating from AS400 to be dropped.* The above statement will set a weight of 20 for updates that are local to AS200. Any other updates will be dropped.3 remote-as 200 neighbor 3.

the routers in AS600 will have network reachability information about 150. We can do this by prepending autonomous system numbers to the existing path info adver- tised to AS100.0. 300) which is longer than (400.0. 300.10..2 remote-as 100 neighbor 2.10.10. Suppose in the above diagram that RTC is advertising its own network 170. Assuming that all other attributes are the same. 300) and the second one is via AS400 with path (400. 1/26/96-Rev: A1.0. 300. When the information is propagated to AS600.0. A common practice is to repeat our own AS number using the following: RTC# router bgp 300 network 170.300).20. 200. AS600 will pick the shortest path and will choose the route via AS100. the first route is via AS100 with path (100.0 neighbor 2.2 route-map SETPATH out route-map SETPATH set as-path prepend 300 300 Because of the above configuration.0 via AS100 with a path information of: (100.2. AS300 will be getting all its traffic via AS100.2. The command that is used with a route map is: set as-path prepend <as-path#> <as-path#> .2 Page 55 Sam Halabi-cisco Systems . 300) received from AS100.10..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.2. 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 AS400.2. 200.0 to two different ASs: AS100 and AS200.0 via two different routes. AS600 will receive updates about 170.

Members of the peer group inherit all of the configuration options of the peer group.1.0 RTH RTA RTB 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 Page 56 Sam Halabi-cisco Systems .2 1.1 AS600 AS200 1.4.2 AS100 RTE 2.1 6.0.2 BGP Peer Groups 150.20. we define a peer group name and we assign these policies to the peer group. Instead of defining the same policies for each separate neighbor.6. Members can also be configured to override these options if these options do not affect outbound updates.5. 1/26/96-Rev: A1.1.3.0 RTG AS300 A BGP peer group.1 5. distribute-lists and filter-lists.10.2 2.2 6.2. you can only override options set on the inbound.5.3.2 170. etc.6. Update policies are usually set by route maps.2.1 RTF RTC 5.0.5. is a group of BGP neighbors with the same update policies.5.2.6.

2 remote-as 200 neighbor 1.2 by assigning filter-list 3. such as a route map SETMETRIC to set the metric to 5 and two different filter lists 1 and 2. 1/26/96-Rev: A1.1. we have defined a peer group named internalmap and we have defined some policies for that group.3.1.3. RTF and RTG.2 remote-as 100 neighbor 2.1. Note that we could only override options that affect inbound updates. In the same diagram we will configure RTC with a peer-group externalmap and we will apply it to external neighbors.2. We have defined a separate filter-list 3 for neighbor RTE.2 peer-group externalmap neighbor 4.3.2 peer-group externalmap neighbor 1.2 remote-as 600 neighbor Now.2.5.2. let us look at how we can use peer groups with external neighbors.2 peer-group internalmap neighbor 6. and this will override filter-list 2 inside the peer group.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.2 peer-group externalmap neighbor filter-list 3 in In the above configuration.5.2 Page 57 Sam Halabi-cisco Systems .4.4.2 peer-group internalmap neighbor 3.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 5.3. 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 peer-group internalmap neighbor 3. We have applied the peer group to all internal neighbors RTE.4. Also we did an override for the inbound updates of neighbor 1.

10.3. 1/26/96-Rev: A1.0.213.2 remote-as 100 network 170.0 RTA RTB 2.0.3 remote-as 200 neighbor 2.0. For example.10.0 aggregate-address 160.0.2 Page 58 Sam Halabi-cisco Systems . CIDR or supernetting is a new way of looking at IP addresses.0 RTC# router bgp 300 neighbor 3.0/16 where the 16 is the number of bits in the subnet mask counting from the far left of the IP address.1 remote-as 300 network 160.0.0 255.0.0. RTB# router bgp 200 neighbor 3. In the example below.0 AS 300 One of the main enhancements of BGP4 over BGP3 is CIDR (Classless Interdomain Routing). AS 200 AS 100 255.0 to RTA.0.2 3.0 to RTA. There is no notion of classes anymore (class A. which used to be an illegal class C network is now a legal supernet represented by 192.0. Aggregates are used to minimize the size of routing tables.1 3. We will configure RTC to propagate a supernet of that route 160.2.0 CIDR and Aggregate Addresses 160.10. Aggregation is the process of combining the characteristics of several different routes in such a way that a single route can be advertised.2.0. B or C). RTB is generating network RTC 170.0. This is similar to 192. network 192.0 RTC will propagate the aggregate address

The upcoming CIDR example discusses this situation.2 Page 59 Sam Halabi-cisco Systems .0 have been propagated to RTA.0 in its BGP table.0.10.0 on RTB) then the network entry is always injected into BGP updates even though we are using the “aggregate summary-only” command. For example. and all of the more specific routes.0.0). all the more specific routes are suppressed.0.0. This is what we mean by advertising the prefix and the more specific route.0.0.0 from being also propagated to RTA.0.0 and It is important to understand how each one works in order to have the desired aggregation behavior.0.0 but will not prevent 160.0 255. 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.0.0.0 and will suppress the more specific route 160.1 Aggregate Commands There is a wide range of aggregate commands.10.0.0 summary-only will propagate network 160.0 will propagate an addi- tional network 160. 1/26/96-Rev: A1.0. The command aggregate 160. In case we would like RTC to propagate network 160. The first command is the one used in the previous example: aggregate-address address mask This will advertise the prefix route. RTB can not generate an aggregate for 160.0.21. from redistributing an IGP or static into BGP or via the network command (network 160.0 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-address 160.0.10. The more specific route could have been injected into the BGP table via incoming updates from other ASs.0.0.0. Please note that if we are aggregating a network that is injected into our BGP via the network statement (ex: network 160.0 if it does not have a more specific entry of 160. The outcome of this is that both networks 160. ex: aggregate 129.0.0 255.0.0. 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.10. This will be discussed in an example by itself in the following sections.0 aggregate-address 160.255 Then we apply the route-map to the aggregate statement.0 255.0.2 remote-as 100 network 170. 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.2 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.255.0 to be propagated. etc.0 attribute-map SETORIGIN 1/26/96-Rev: A1.0.0.2. we can use the following route map: route-map CHECK permit 10 match ip address 1 access-list 1 deny route-map SETMETRIC set origin igp aggregate-address 160.0.0.) when aggregates are sent out.2. This will allow us to be selective about which more specific routes to suppress.0 255.10.255 access-list 1 permit 0.3.0 as-set.0 0. if we would like to aggregate 160.0.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.0.20. In the previous diagram. 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 and suppress the more specific route and allow remote-as 200 neighbor 2. RTC# router bgp 300 neighbor 3. and suppress all the more specific routes.1 RTC 170.0 without generating an entry for 160.2.2 Page 61 Sam Halabi-cisco Systems .0 AS 300 Request: Allow RTB to advertise the prefix 160.0. RTB# router bgp 200 neighbor 3.0.3 AS 100 AS 200 2.10. The outcome is that RTB will advertise the aggregate with an origin of incomplete (?).0.10.2. AS200 is the originator of 160.1 remote-as 300 redistribute static (This will generate an update for 3.0.0 even if you use the “aggregate summary-only” command because RTB is the originator of 160.2.0 RTA RTB 2. You cannot have RTB generate a prefix for with the origin path as *incomplete*) ip route 255.e. Solution 1: The first solution is to use a static route and redistribute it into BGP.0 null0 1/26/96-Rev: A1.10.10.2 is local to AS200 i. The problem here is that network 160.2 CIDR example 1 150.

0.1 remote-as 300 redistribute static ip route mask 255.2 Page 62 Sam Halabi-cisco Systems .0.0.0 255. RTB# router bgp 200 network 160. this will have the same effect except that the origin of the update will be set to IGP.0 null0 1/26/96-Rev: A1. 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 3.0.

10.3.0 from RTA and updates about 160.10. The as-set aggregate command is used in situations were aggregation of information causes loss of information regarding the path attribute. neighbor 2.1 and send it to RTD.4.1 remote-as 300 1/26/96-Rev: A1.0 neighbor 3.2 Page 63 Sam Halabi-cisco Systems .3. 160.0.3. All the path information is included in that set irrespective of which path came first.1 RTD RTC RTD would not know what the origin of that route is. By adding the aggregate as-set statement we force RTC to generate path information in the form of a set {}.0 RTB RTA 2.0 160.0 RTB# router bgp 200 network 160.3.2 3.0 from RTB.10.20. Suppose RTC wants to aggregate network 160.21.4 AS300 AS400 170. regardless of how many times it may have appeared in multiple paths that were aggregated.4. In the following example RTC is getting updates about 160.4.3 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. AS200 AS 100 remote-as 300 RTA# router bgp 100 network 160.

4.0 255.4 remote-as 400 aggregate 160.0.2 Page 64 Sam Halabi-cisco Systems .0.0 as-set (causes RTC to send RTD updates about summary-only aggregate 160.0.3. this may create loops if RT4 has an entry back into AS100 Case 2: RTC# router bgp 300 neighbor remote-as 100 neighbor 4.3 remote-as 200 neighbor 2.3.0 255.0.Case 1: RTC does not have an as-set statement.0. RTC will send an update 160.0.0. RTC# router bgp 300 neighbor 3.3 remote-as 200 neighbor RTD with path information (300) as if the route has originated from AS300.3.2 remote-as 100 neighbor 4.0/8 with no indication that 160.0.4 remote-as 400 aggregate summary-only (this causes RTC to send RTD updates about 160.0.0 is actually coming from two different autonomous systems. with an indication that 160.4.0 belongs to a set {100 200}) 1/26/96-Rev: A1.

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

6 5. 1/26/96-Rev: A1.14.1 AS70 AS50 129.210.30. If you want to make a full IBGP mesh inside AS500 then you would need nine peer connections for each router.6. The outside world will see only one AS500.5.2 Page 66 Sam Halabi-cisco Systems . By using confederation we can divide AS500 into multiple ASs: AS50.30. Note that RTA has no knowledge of ASs 50.4 RTC RTD AS60 128.213. We give the AS a confederation identifier of 500.2 135.1 128.20.212. but we are only interested in the BGP speakers that have EBGP connections to other ASs). RTD and RTA. I will show a sample configuration of routers RTC.1 128. 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.1 129. For each AS50. AS60 and AS70.5 AS600 AS 100 5.11.Example: RTA 6.5.10. 60 or 70.5.1 AS 500 Let us assume that you have an autonomous system 500 consisting of nine BGP speakers (other non BGP speakers exist also.213. 8 IBGP peers and one EBGP peer to external ASs. RTA has only knowledge of AS500.

30.1 remote-as 50 (IBGP connection within AS50) neighbor 129.213.1 remote-as 50 (IBGP connection within AS50) neighbor 128.213.6 remote-as 600 (EBGP connection to external AS600) RTA# router bgp 100 neighbor 5.2 Page 67 Sam Halabi-cisco Systems .RTC# router bgp 50 bgp confederation identifier 500 bgp confederation peers 60 70 neighbor remote-as 100 (EBGP connection to external AS100) RTD# router bgp 60 bgp confederation identifier 500 bgp confederation peers 50 70 neighbor remote-as 50(BGP connection with confederation peer 50) neighbor 135.1 remote-as 70 (BGP connection with confederation peer 70) neighbor 6.212.1 remote-as 60 (BGP connection with confederation peer 60) neighbor 135.4 remote-as 500 (EBGP connection to confederation 500) 1/26/96-Rev: A1.2 remote-as 60 (IBGP connection within AS60) neighbor 128.1 remote-as 70 (BGP connection with confederation peer 70) neighbor

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

3.12 AS300 An autonomous system can have more than one route reflector.8.8. RTA and RTB are clients and anything else is a non-client.8 AS 100 RTG (RR) 7. it will do the following depending on the peer type: 1/26/96-Rev: A1. the AS could be divided into multiple clusters.4. Other RRs could belong to the same cluster (client group) or to other clusters. each RR will be configured with other RRs as non-client peers in a fully meshed topol- ogy.Example: AS200 8. Consider the above diagram.2 1.6 5.5.1. According to RTC. Remember that clients of an RR are pointed at using the “neighbor <ip-address> route-reflector-client” command.4 3.12.7 In a simple con- figuration. a RR would treat other RRs just like any other IBGP speaker. Clients should not peer with IBGP speakers outside their cluster.1 RTE RTA RTB RTF 12. When a route is received by a RR.1. RTG is a RR in a third cluster.6.6.5. RTA.5 2. The same RTD is the RR for its clients RTE and RTF.7.12.2 Page 69 Sam Halabi-cisco Systems .4.3 RTD (RR) RTC (RR) 6. RTB and RTC form a single cluster with RTC being the RR.2. RTC and RTG are fully meshed but routers within a cluster are not. Note that RTD.

7 remote-as 100 neighbor 4.5.6 route-reflector-client neighbor 5. 1/26/96-Rev: A1.2.8. remote-as 200 RTB# router bgp 100 neighbor 3.12.Route from an EBGP peer: send the update to all client and non-client peers.1.12 remote-as 300 RTD# router bgp 100 neighbor 6.7. remote-as 100 neighbor 5. The following is the relative BGP configuration of routers RTC. 2.7. RTD and RTB: RTC# router bgp 100 neighbor 2.2 Page 70 Sam Halabi-cisco Systems .Route from a client peer: reflect to all the non-client peers and also to the client peers.6.5 route-reflector-client neighbor 7.5.Originator-id: this is an optional.3 remote-as 100 As the IBGP learned routes are reflected. if the routing information comes back to the originator.2.12. it is possible to have the routing information loop.2. The Route-Reflector scheme has a few methods to avoid this: 1.8. Thus.Cluster-list: this will be discussed in the next section.2 route-reflector-client neighbor 1.6.1.Route from a non-client peer: reflect to all the clients within the cluster.5.3. it will be ignored.7 remote-as 100 neighbor remote-as 100 neighbor 1.6 remote-as 100 neighbor due to poor configuration. This attribute will carry the router-id (RID) of the originator of the route in the local AS.1.3 remote-as 100 neighbor 12.1 route-reflector-client neighbor 7. non transitive BGP attribute that is four bytes long and is created by a RR.2 remote-as 100 neighbor 2.4 remote-as 100 neighbor 8.4.

4 RTH 1.5 RTB RTF AS500 13. RTD. In case RTD goes down.6 (RR) 2. When a RR reflects a route from its clients to non-clients outside of the cluster.6.10.9. the advertisement will be ignored.8.5.2 10.4. If the local cluster-id is found in the cluster-list.23.13 Usually.11 8.13. The following are the configuration of RTH. RTF and RTC: 1/26/96-Rev: A1. RTH will take its place. RTF and RTH belong to one cluster with both RTD and RTH being RRs for the same cluster. it will append the local cluster-id to the cluster-list. a RR can identify if the routing information is looped back to the same cluster due to poor configuration. Using this attribute. If this update has an empty cluster-list the RR will create one.7.9 AS200 AS300 11.2 Page 71 Sam Halabi-cisco Systems .4.1 RTE RTA 5. A cluster-list is a sequence of cluster-ids that the route has passed.8 AS 100 RTG (RR) 7. In this case. In the above diagram. the cluster will be identified by the router-id of the RR. Note the redundancy in that RTH has a fully meshed peering with all the RRs. RTD.9.3 RTD (RR) RTC (RR) In order to increase redundancy and avoid single points of failure.5.11.8. a cluster of clients will have a single RR.1. a cluster might have more than one RR.7 4.1 Multiple RRs within a cluster AS400 9. RTE. 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.

1/26/96-Rev: A1.1.4.2 route-reflector-client neighbor 4.7 remote-as 100 neighbor 10.3.7 remote-as 100 neighbor 3.5.1 route-reflector-client neighbor 2.6.7 remote-as 100 neighbor remote-as 200 Note that we did not need the cluster command for RTC because only one RR exists in that cluster.5.8.2 remote-as 100 neighbor 2.4 remote-as 100 neighbor remote-as 100 neighbor remote-as 100 neighbor 5.6.5 remote-as 100 neighbor route-reflector-client neighbor 7.RTH# router bgp 100 neighbor remote-as 100 neighbor remote-as 100 neighbor remote-as 100 neighbor 4.10 remote-as 100 neighbor 8.5 remote-as 100 neighbor Page 72 Sam Halabi-cisco Systems .5.6 remote-as 100 neighbor 6.4 remote-as 100 neighbor 13.5.5 route-reflector-client neighbor 6.4.9 remote-as 300 bgp route-reflector 10 (This is the cluster-id) RTD# router bgp 100 neighbor remote-as 400 bgp route-reflector 10 (This is the cluster-id) RTF# router bgp 100 neighbor 10.6 route-reflector-client neighbor 7.1 remote-as 100 neighbor 1.13 remote-as 500 RTC# router bgp 100 neighbor 1.5 route-reflector-client neighbor 6.11.6 remote-as 100 neighbor 6.6.1.

Then more clusters could be cre- ated gradually.1.5 2. One could start creating clusters by configuring a single router as RR and making other RRs and their clients normal IBGP peers.1. If the clients inside a cluster do not have direct IBGP peers among one another and they exchange updates through the RR.3 RTD RTC (RR) 6.13 8.5.2 Page 73 Sam Halabi-cisco Systems .3.1 RTE RTA RTB RTF AS400 14. Example: AS300 AS200 13.If BGP client-to-client reflection were turned off on the RR and redundant BGP peering was made between the clients.2.An important thing to note. If peer groups were to be configured.14 1/26/96-Rev: A1.6 5.5. These routers could be either members of a client group or a non-client group.4.4 3. This would allow easy and gradual migration from the current IBGP model to the route reflector model.2 RR and conventional BGP speakers It is normal in an AS to have BGP speakers that do not understand the concept of route reflectors. The route reflector scheme will allow such conventional BGP speakers to coexist. 23.2.6. then using peer groups would be alright. is that peer-groups were not used in the above configuration. We will call these routers conventional BGP speakers. The router sub-command bgp client-to-client reflection is enabled by default on the RR. 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. peer-goups should not be used. 1.8 AS 100 4.3.

13.6 remote-as 100 neighbor 6. Another means of controlling loops is to put more restrictions on the set clause of out-bound route-maps.5.13 remote-as 300 RTC# router bgp 100 neighbor 4.3 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.1 remote-as 100 neighbor 14. 23. Normal IBGP mesh could be done between these rout- ers and RTD.6. Clients do not have to understand the route reflection scheme. we would remove the IBGP full mesh and have RTA and RTB become clients of RTC. RTE and RTF have the concept of route reflec- tion. RTA and RTB are what we call conventional routers and cannot be configured as RRs. The set clause for out-bound route-maps does not affect routes reflected to IBGP peers. The following is the configuration of RTD and RTC: RTD# router bgp 100 neighbor 6.5. 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. RTC.14 remote-as 400 When we are ready to upgrade RTC and make it a RR.1.1 remote-as 100 neighbor 13.3 remote-as 100 neighbor 2.2. More restrictions are also put on the nexthop-self which is a per neigh- bor configuration option. RTC could be made a RR with clients RTA and RTB.1. it is only the RRs that would have to be upgraded. Later on.2 Page 74 Sam Halabi-cisco Systems .6.2. when we are ready to upgrade. 1/26/96-Rev: A1.2 remote-as 100 neighbor 1.In the above diagram.5 route-reflector-client neighbor 3.6.2 remote-as 100 neighbor 1.5 remote-as 100 neighbor remote-as 100 neighbor route-reflector-client neighbor 5. RTD.4.

default is 750.will turn on dampening. no bgp dampening . 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. external to an AS. criteria are defined to identify poorly behaved routes.0) is a mechanism to minimize the instability caused by route flapping and oscillation over the network. range is 1-255.0 Route Flap Dampening Route dampening (introduced in Cisco IOS version 11. To accomplish this. the advertisement of the route will be suppressed.24. default is 4 times half-life-time. Once the penalty decreases below a predefined “reuse-limit”. the route advertisement will be un-suppressed. dampening will be off by default. learned via IBGP will not be dampened. at that point the information is purged from the router. Initially. <reuse-value> range is 1-20000. <max-suppress-time> maximum duration a route can be suppressed.will turn off dampening. 1/26/96-Rev: A1. Routes. <suppress-value> range is 1-20000. A route which is flapping gets a pen- alty for each flap (1000).2 Page 75 Sam Halabi-cisco Systems . bgp dampening <half-life-time> . The penalty will be exponentially decayed based on a preconfigured “half-time”. current default is 15 min. As soon as the cumulative penalty reaches a predefined “sup- press-limit”. The following are the commands used to control route damp- ening: bgp dampening . 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. This is to avoid the IBGP peers having higher penalty for routes external to the AS. default is 2000.change the half-life time. This might change if there is a need to have this feature enabled by default. The dampening information is kept until the pen- alty becomes less than half of “reuse-limit”.

6 255.0 neighbor router bgp 300 network remote-as 300 RTD# hostname RTD interface Loopback0 ip address RTB# hostname RTB interface Serial0 ip address 203.2 255.208.252 interface Serial1 ip address 192.174 RTB’s BGP table would look like this: 1/26/96-Rev: A1.10.0 neighbor 192.15.252 router bgp 100 bgp dampening network 203.2 Page 76 Sam Halabi-cisco Systems .250.5 S1 192.10.Example: S0 203. Assuming the EBGP link to RTD is stable.2 RTB S0/0 RTD AS100 AS300 L0 192.6 remote-as 100 RTB is configured for route dampening with default parameters.208.192 interface Serial0/0 ip address 192.

0.10.250. local router ID is 203.0 0 32768 i The BGP entry for * valid. i .208.10. * valid.IGP.0 0 32768 i In order to simulate a route flap.0 has been put in a “history” state. If the route flaps few more times we will see the following: 1/26/96-Rev: A1.250. e .10.10.250. > best.208.EGP.208.2 Status codes: s suppressed.208.6” on RTD.255.0 BGP routing table entry for 192.IGP. RTB#sh ip bgp 192. The route is not yet sup- pressed. > best.0.5 0 0 300 i *> 203.RTB#sh ip bgp BGP table version is 24.15.174) Origin IGP.5 (192.2 Page 77 Sam Halabi-cisco Systems . metric 0.10.15.incomplete Network Next Hop Metric LocPrf Weight Path *> 192.15. h history.208.internal Origin codes: i . e .5 from 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).0 192. d damped.10. version 25 Paths: (1 available.10.208. h history. RTB’s BGP table will look like this: RTB#sh ip bgp BGP table version is 24. i .208.0 255.10.0. ? .incomplete Network Next Hop Metric LocPrf Weight Path h 192. local router ID is 203.0.208. d damped.2 Status codes: s suppressed. Which means that we do not have a best path to the route but information about the route flapping still exists.255.208.0 192. I will do a “clear ip bgp 192.5 0 0 300 i *> 203. no best path) 300 (history entry) 192. ? . external Dampinfo: penalty 910.15.0.0 0.EGP.0 0.internal Origin codes: i .

> best. reuse in 0:27:00 The route has been dampened (suppressed).m.IGP.10.208. ? .C.208.0.m.15.m.0 0. in our case 750 (default).D m. d damped.incomplete Network Next Hop Metric LocPrf Weight Path *d 192.D flap-statistics (clears flap statistics for all paths from a neighbor) 1/26/96-Rev: A1.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.208.208.0.D m.C.0 192.internal Origin codes: i . version 32 Paths: (1 available.0 0 32768 i RTB#sh ip bgp 192.255.m (displays flap statistics for a single entry) show ip bgp flap-statistics A.The dampening information will be purged when the penalty becomes less than half of the reuse-limit. (suppressed due to dampening) 192. local router ID is 203.0. metric 0.5 (192.5 0 0 300 i *> 203.10.B.0 BGP routing table entry for 192. The route will be reused when the penalty reaches the “reuse value”.EGP.10.m.B.m.250.m.2 Page 78 Sam Halabi-cisco Systems .255. 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.174) Origin IGP.C.D m.RTB#sh ip bgp BGP table version is 32.10. i .208. flapped 3 times in 0:05:18 .B. h history. valid.10.m (clears flap statistics for a single entry) clear ip bgp A.C. external Dampinfo: penalty 2615.5 from 192.2 Status codes: s suppressed.250. in our case (750/2=375). * valid.10.B. e .0 no best path) 300.

isco routers. 4-If same Local Preference prefer the route that the specified router has originated. 10-Prefer the route with the lowest ip address value for BGP router ID. 1/26/96-Rev: A1. The ollowing is a design example thatis intended to show the configura- tion and routing tables as they actually appear on the C. Remember that we only select one path as the best path. 2-Prefer the largest Weight. 3-If same weight prefer largest Local Preference. We put that path in our routing table and we propagate it to our BGP neighbors. 6-If all paths are external prefer the lowest origin code (IGP<EGP<INCOMPLETE). 7-If origin codes are the same prefer the path with the lowest MULTI_EXIT_DISC.0 How BGP selects a Path Now that we are familiar with the BGP attributes and terminology. 8-If path is the same length prefer External path over Internal.2 Page 79 Sam Halabi-cisco Systems . Path selection is based on the following: 1-If NextHop is inaccessible do not consider it. 5-If no route was originated prefer the shorter AS path. the following list indicates how BGP selects the best path for a particular destination.25. 9-If IGP synchronization is disabled and only internal path remain prefer the path through the closest IGP neighbor. IBGP RTB S0 128.15.X AS200 RTD S2/1 128.213.1 200.14.2 Page 80 Sam Halabi-cisco Systems .2 L0 192.200.2 S1 203.10.211.X AS300 S0/0 S0 203.10.6 RTE L0 200.1 S1 S1 195.208.63.X S0 192.13.130 S2/0 128.X E0 203.213.1 1/26/96-Rev: A1.X. RTC L0 128.X.213.X.2 S0/1 192.X AS400 S0 195.X.X.6 AS100 Practical design example: RTF 203.208.2 S1 128.5 L0 203.1 AS500 RTG L0 RTA E0 203.

15.We will build the above configuration step by step and see what can go wrong along the way.255.250.1 255.252 router ospf 10 network neighbor 255. Whenver you have an AS that is connected to two ISPs via EBGP.14.2 update-source Loopback0 RTF# hostname RTF ip subnet-zero interface Ethernet0 ip address 203.250.255 area 0 1/26/96-Rev: A1. interface Ethernet0 ip address 203.255.0 mask 255.14.255 area 0 router bgp 100 network 203.0 interface Serial0 ip address 128. In this example we will run IBGP inside AS100 between RTA and RTB and we will run OSPF as an IGP.255.250.2 255. the following are the first run of configuration for all the routers.2 remote-as 200 neighbor 203. RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.255. This is NOT the final configuration.0 0.0. it is always good to run IBGP within your AS in order to have a better control of your routes. Assuming that AS200 and AS300 are the two ISPs we are connected to. 0.0.2 remote-as 100 neighbor 203.252 router ospf 10 network interface Serial1 ip address Page 81 Sam Halabi-cisco Systems .

213.0 router ospf 10 network area 0 router bgp 100 network 203.252 router bgp 200 network remote-as 300 neighbor 203.130 remote-as 100 neighbor 128.250.252 interface Serial1 ip address ! interface Serial2/1 ip address neighbor 192.213.5 neighbor 128.255.2 255.255.6 remote-as 400 1/26/96-Rev: A1. Page 82 Sam Halabi-cisco Systems .250.192 interface Serial2/0 ip address remote-as 100 RTC# hostname RTC ip subnet-zero interface Loopback0 ip address 128.6 255.RTB# hostname RTB ip subnet-zero interface Serial0 ip address 203.255.

10.255.6 remote-as 100 RTE# hostname RTE ip subnet-zero interface Loopback0 ip address neighbor 255.213.RTD# hostname RTD ip subnet-zero interface Loopback0 ip address 192.0 neighbor remote-as 500 neighbor clockrate 1000000 router bgp 400 network 200.208.192 interface Serial0/0 ip address interface Serial0 ip address 195.255.1 remote-as 500 1/26/96-Rev: A1.2 255.10.252 router bgp 300 network 192.252 ! interface Serial0/1 ip address 192.2 Page 83 Sam Halabi-cisco Systems .255.200.5 255.6 255.5 remote-as 200 neighbor interface Serial1 ip address 128.255.1 255.174 255.213.

2 Status codes: s suppressed. i . Let us assume to start with that s1 on RTB is shutdown.2 100 0 200 400 i *>i203. Note that any locally generated entry such as local router ID is 203. This is why.252 interface Serial1 ip address 195.RTG# hostname RTG ip subnet-zero interface Loopback0 ip address 195.14.213. e .13.2 0 100 0 200 i *i192.192 interface Serial0 ip address remote-as 400 It is always better to use the network command or redistribute static entries into BGP to advertise networks.10.211.10. rather than redistribut- ing IGP into BGP. ? . * valid.63.10.0 128. RTB#sh ip bgp BGP table version is 0.208.2 Page 84 Sam Halabi-cisco Systems . > best.250.255. The following is RTB's BGP table.63.213.0 is learned via path 200 with nexthop of 128.63. h history.250.10.2 remote-as 300 neighbor 255.0 neighbor 192.252 router bgp 500 network 195.EGP. 1/26/96-Rev: A1.0 128.255.250. The “i” at the end indicates the ORIGIN of the path information to be IGP.0 0 32768 i Let me go over the basic notations of the above table.0.0. The “i” at the beginning means that the entry was learned via an internal BGP peer.0 128.0 203.2 100 0 200 400 500 300 i *i195.208.255.2.internal Origin codes: i .15. For example network 128.0. d damped.41 0 100 0 i *> 255.0 has a nexthop 0.13.IGP.211.213. as if the link between RTB and RTD does not exist.255.41 0 100 0 i *>i203.15.250. throughtout this example I will only use the network command to inject networks into BGP.211.10.0.incomplete Network Next Hop Metric LocPrf Weight Path *i128.174 100 0 200 400 500 i *i200.213.0 203.250. The path info is intuitive.

0 255.250. This is true because we do not have a way to reach that nexthop via our IGP (OSPF).IGRP.213.0 via a nexthop of 128. E2 . There are two problems here: Problem 1: The Nexthop for these entries 128. it doesn't look like any of the BGP entries has made it to the routing table.15.250.1.OSPF inter area E1 . will install this path in the ip routing table and will advertise it to other bgp peers.OSPF external type 1.2 Page 85 Sam Halabi-cisco Systems .EIGRP. Let us look at the IP routing table: RTB#sh ip rou Codes: C .connected. 02:50:46. M .BGP D .15.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".0 [110/74] via 203.250. L2 .OSPF external type 2. Serial0 O 203. Notice the nexthop attribute. O .213. Serial0 203. Serial0 Well. We could also change the nexthop by using the bgp nexthopself command between RTA and RTB. 1 subnets C 203.255. E .13.63. and this way RTB would know how to reach the nexthop 128.250.63. IA .213. RTA's configs would be: 1/26/96-Rev: A1.1.63.0 is directly connected.IS-IS. I .2.0 255.250.IS-IS level-2.250. * .252 is subnetted.255 is subnetted. RTB knows about 128. EX .0 via OSPF.255.255. RTB has not learned about 128. 02:50:45.candidate default Gateway of last resort is not set 203.2 which is the ebgp nexthop carried into IBGP.0. R . 1 subnets O is unreachable.15.OSPF. B . [110/75] via 203.IS-IS level-1.15.63.EGP i .RIP. We can run OSPF on RTA s0 and make it passive. L1 .14.13.EIGRP external. S .static.250. Bgp will only pick one best Path to reach a destination.

13.0 128.1 255.0. i . * valid. area 0 network 0 100 0 i *>i203. e . h history.2 remote-as 100 neighbor 203.0 203.41 0 100 0 i *> 203.255.0 mask Let us look at the routing table now: 1/26/96-Rev: A1. 100 0 200 400 500 i *>i200.2 100 0 200 400 500 300 i *>i195.0 0.internal Origin codes: i .255.63.IGP.250.2 0 100 0 200 i *>i192. > best. d damped.0.250.213.incomplete Network Next Hop Metric LocPrf Weight Path *>i128. 255.0.2 remote-as 200 neighbor 203.208.0 interface Serial0 ip address 128.0 0 32768 i Note that all the entries have >.250.250.0. ? .0 area 0 router bgp 100 network 203.250.0 203.250.0 0.13.0 neighbor 128.0 128.255.EGP.213.0. which means that BGP is ok with next hop. hostname RTA ip subnet-zero interface Loopback0 ip address 203.2 Status codes: s suppressed.0.41 0.213.0 128.0 interface Ethernet0 ip address 203.2 Page 86 Sam Halabi-cisco Systems .0.252 router ospf 10 passive-interface Serial0 network 100 0 200 400 i *>i203.2 update-source Loopback0 The new BGP table on RTB now looks like this: RTB#sh ip bgp BGP table version is 10.250. local router ID is 203.

EIGRP.2 Page 87 Sam Halabi-cisco Systems .connected.0 or 195.15.1. 1 subnets O 203. Note that RTF has no notion of networks 192.IS-IS. E2 .255.255.255 is subnetted.208.EGP i .RTB#sh ip rou Codes: C . we will have the entries in the routing table.255.0 [110/74] via 203. If you turn off synchronization on RTB this is what will happen: 1/26/96-Rev: A1.BGP D .252 is subnetted.15. Serial0 203. I . This is the synchronization issue. S . [110/75] via 203. IA .static.213. 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.250. 00:04:46.15. Serial0 Problem 2: We still do not see the BGP entries.15.0 255.1. L2 . 00:04: [110/138] via 203.250. L1 . the only difference is that 128.250.IGRP.0 255. 1 subnets O 128.0 because we have not redistributed BGP into OSPF yet.OSPF external type 1.IS-IS but connectivity would still be broken. 00:04:47.OSPF inter area E1 .211.OSPF. R .252 is subnetted.0. if we turn synchronization off. Serial0 O O .13.10.63. Serial0 128. EX . * - candidate default Gateway of last resort is not set 203.63. B .13. 1 subnets C 203.0 is directly connected.255.213.IS-IS level-2.OSPF external type 2. In this scenario.RIP.250.255.0 255. E .14.EIGRP external. M .250.0 is now reachable via OSPF.

1 subnets C 203.1.208.OSPF.0 [200/0] via 128.EIGRP. L1 .IS-IS level-1. Ethernet0 So.213.0 [200/0] via is subnetted.0 255.252 is subnetted. IA . 2 masks O 203.EIGRP external.15.252 is subnetted.63. 1 subnets O 203. 2 subnets.0 255.250.255 [110/75] via 203. 1/26/96-Rev: A1.255. turning off synchronization in this situation did not help this particular issue.13. S . L1 . 1 subnets O 128.255. L2 . Ethernet0 203.213. E2 .250.15.EGP i .BGP D .0.1.0 is variably subnetted.IS-IS level- 00:12:37. 1 subnets C 203.41.OSPF external type 1.2.255. * - candidate default Gateway of last resort is not set 203.213.250.BGP D . ip rou Codes: C .0 is directly connected.0 is variably subnetted.1.OSPF. 00:01:07 B 195. I .15.10.0. 2 masks B 128. 00:01:07 B 192.0 255.RIP. 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 .0 is directly connected. IA .250. I .IS-IS.255.13.2. S .250. EX . 00:14:15.255.EIGRP external.255.0 255.OSPF inter area E1 . Ethernet0 128.0 255. E .OSPF external type 1. but we will need it for other issues later on.static. Let’s redistribute OSPF into BGP on RTA .0 [200/0] via is * - candidate default Gateway of last resort is not set B 200. E2 .mobile.2.0 255. 00:12:37.255. M . Serial0 O 203.IGRP. B .13.255. EX .250.14. O . 00:01:08 203. 2 subnets.OSPF external type 2. 00:01:07 203.0 [110/74] via 203. B .0. 00:01:08 O 128.0 is directly connected.0 255.15.0 [200/0] via 255.0 [200/0] via 128. with a metric of 2000.252 [110/138] via 203. 00:14:15. L2 . R . Serial0 128.250.IS-IS level-2.255. Serial0 B 203.41 [110/11] via 203.2 Page 88 Sam Halabi-cisco Systems . M .213.IS-IS level-1. Serial0 The routing table looks fine.IS-IS.connected. E .connected. R .IGRP. 00:12:37.200. Serial1 C 203. O .OSPF inter area E1 .RIP.255.15.211.EGP i .250.OSPF external type 2.0 [110/74] via 203.EIGRP.13.10.

15. Serial0 1/26/96-Rev: A1.IGRP.255. I .2 remote-as 100 neighbor 203.14. L1 . Serial0 O E2 192.41 remote-as 200 neighbor 203.10.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203.15.13. E2 .10.0.0 is variably subnetted. [110/2000] via 203. 00:00: [110/2000] via area 0 network 128.IS-IS level-2.250.0.static.0.250.41 255.0.0 [110/2000] via 203.250.0 [110/2000] via 203.13.250. S . external.EIGRP.255 area 0 router bgp 100 network 255.63.1. 2 subnets C 203.0.0 255.1. Serial0 O E2 203.IS-IS level-1.250.255. 2 masks O 203. 2 masks O E2 128.1 is directly connected.213.BGP D .63. B .0 interface Serial0 ip address Page 89 Sam Halabi-cisco Systems .OSPF external type 1.connected. 00:00:14.15.OSPF inter area E1 .OSPF external type 2.RIP.0 [110/74] via 203.EGP i .1. EX .0 [110/2000] via 203.2 update-source Loopback0 The routing table will look like this: RTB#sh ip rou Codes: C .8 is directly connected. 0.0 255.14.0. 00:00:15.0 is variably subnetted. 2 interface Ethernet0 ip address 203.255. Serial0 2 subnets. Loopback1 C 203. Serial0 128.IS-IS. R .255. Serial0 O 203.13.0 neighbor 128.250.0 0.15.252 is subnetted.15.13. 00:00:15.250.213. Serial0 O E2 195.Serial0 O 128.250. 00:00:15.15. 00:00:14.255. M .255. 00:00:14. * - candidate default Gateway of last resort is not set O E2 200. L2 . [110/138] via 203. Serial0 203. 00:00: 255.255. O .RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.255 [110/75] via IA .0 mask 255. E .250.OSPF. area 0 router bgp 100 no synchronization network remote-as 200 neighbor 203.250. The updated configs of RTA and RTB follow: RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203.41 255. Let us bring RTB’s s1 up and see what all the routes will look like.The BGP entries have disappeared because OSPF has a better distance (110) than internal bgp (200).1 255.255.0 neighbor 128.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203.0 0.13.0 mask we would have to go the other way via EBGP.250. because it will not sync up with OSPF due to the difference in masks.0. I will also turn sync off on RTA in order for it to advertise 203.1 via IGP. remote-as 100 neighbor 203.255 area 0 network interface Serial0 ip address 128.213.2 Page 90 Sam Halabi-cisco Systems .15.0 interface Ethernet0 ip address update-source Loopback0 1/26/96-Rev: A1. Otherwise some looping will occur because in order to get to nexthop 192.13. I will also enable OSPF on serial 1 of RTB and make it passive in order for RTA to know about the nexthop 192. I will also keep sync off on RTB in order for it to advertise 203.0 for the same reason.208.0.63.

208.255.0 0.0.0 e . ? .252 interface Serial1 ip address 192.15.RTB# hostname RTB ip subnet-zero interface Serial0 ip address 203.255 area 0 router bgp 100 no synchronization network 203.10.EGP.14.0 0 32768 i *> 203.IGP. router ospf 10 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network 203.0 128.250.2 0 100 0 i 1/26/96-Rev: A1.2 0 0 200 i *>i192.0 d damped.0 203.213.incomplete Network Next Hop Metric LocPrf Weight Path *> 128.5 remote-as 300 neighbor 203.0 0.0 neighbor 192. h history.0 0 32768 i *>i203.255 area 0 network 192.41 remote-as 100 And the BGP tables look like this: RTA#sh ip bgp BGP table version is 100 0 300 500 i * 0 200 400 500 i *> * valid.255. i -internal Origin codes: i .2 Status codes: s suppressed.213.6 255.15.0. local router ID is 203.2 Page 91 Sam Halabi-cisco Systems . 0 100 0 300 i *>i195. 0 200 400 i *> 203.0.0 0. > best.255.

One way is to have a primary ISP and a backup ISP. Potential asymmetry might occur if traffic going out from RTA comes back via RTB. > best.0 128.213.0 0. This way I could balance outgoing traffic between the two ISPs.250. One service provider might be closer to a certain destination than another.213.10. One other potential reason for asymmetry is the different advertised path length to reach your AS.0 0 32768 i There are multiple ways to design our network to talk to the two different ISPs AS200 and AS300. In this example. 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. * valid. local router ID is 203.208.5 0 300 500 400 200 i *> 192. This is the final configuration for all of the routers: 1/26/96-Rev: A1.13.15.0. ? .63. In our example.0 192.250.0 203.10.0. 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).5 0 0 300 i *> 195.0 128.10. I have chosen to receive partial routes from AS200 and only local routes from AS300. 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. In our example.10. e . d damped.2 100 0 200 400 i * 192. i -internal Origin codes: i .5 0 300 500 i *>i200.41 0 100 0 i *>i203.IGP. But. 203.208.RTB#sh ip bgp BGP table version is 12. I have chosen two different major nets when talking to the two ISPs.5 0 300 500 400 i *>i203. 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.211.250.2 0 100 0 200 i * 192.41 0 100 0 i *> 203.63.2 Page 92 Sam Halabi-cisco Systems .10 Status codes: s suppressed.200. h history. Both RTA and RTB are generating default routes into OSPF with RTB being more preferred (lower metric). traffic from AS400 destined to your network will always come in via RTA because of the shorter path. We could learn partial routes from one of the ISPs and default routes to both ISPs. This might occur if you are using the same pool of IP addresses (same major net) when talking to the two ISPs.incomplete Network Next Hop Metric LocPrf Weight Path *>i128.

the default information is injected into the IGP domain after redistributing BGP into IGRP/EIGRP.15.13.0 route-map setlocalpref permit 10 set local-preference 200 On RTA.250.213.0 0.41 255.0 network 203. I have also picked network area 0 default-information originate metric 2000 router bgp 100 no synchronization network 203.250.0 0.2 Page 93 Sam Halabi-cisco Systems .0 into the IGP domain.0. the local preference for routes coming from AS200 is set to update-source Loopback0 ip classless ip default-network interface Ethernet0 ip address 203. 0. For IGRP and EIGRP.0 neighbor 128.255. using the “ip default-network” command. to be the candidate default.255.2 remote-as 200 neighbor 128.255 area 0 network 1/26/96-Rev: A1.RTA# hostname RTA ip subnet-zero interface Loopback0 ip address 203. For RIP.255.1 255.0 interface Serial0 ip address 128.0.2 route-map setlocalpref in neighbor 203.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive-interface Serial0 network 203. This command is also used with ISIS and BGP. The “default-information originate” command is used with OSPF to inject the default route inside the OSPF domain. remote-as 100 neighbor 203.213.1 255.0 is automatically redistributed into RIP without additional configuration.250. Also with IGRP/EIGRP we can redistribute a static route to 0.

250.0 interface Serial1 ip address 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: A1.255 area 0 network 192.208.5 route-map localonly in neighbor 203.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive-interface Serial1 network 203.252 interface Serial0 ip address ! interface Serial1 ip address 192.0 neighbor 192.1 255.255 area 0 ip classless RTB# hostname RTB ip subnet-zero interface Loopback1 ip address 203.0 255.250.252 router ospf 10 network 203.0 area 0 default-information originate metric 1000 ! router bgp 100 no synchronization network 203.41 remote-as 100 ! ip classless ip default-network hostname RTF ip subnet-zero interface Ethernet0 ip address remote-as 300 neighbor 192.2 Page 94 Sam Halabi-cisco Systems .10.10 255.10.208.

0.6 remote-as 400 ip classless access-list 1 deny 195.0 the local preference for updates coming in from AS300 is set to 300 which is higher than the IBGP updates coming in from RTA. and indicated the specific routes to be injected into AS100. e . i - internal Origin codes: i . 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. 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.63. local router ID is 203. * valid. Any path info that does not match ^300$ will be dropped.63. 1/26/96-Rev: A1.213.For RTB. > best.63.255. d damped.213.10.2 Page 95 Sam Halabi-cisco Systems .255.255.130 255. Note that I have only advertised AS300’s local routes. 255.192 interface Serial2/0 ip address aggregate-address 128.255.IGP.1 remote-as 100 neighbor 128.252 router bgp 200 network 128.10 Status codes: s suppressed.15. I have aggregated 128. and this way RTA will be preferred.0.0.0 summary-only neighbor 128.0 access-list 1 permit any On RTC. If the ISP refuses to do this task then you have to filter on the incoming end of AS100.63.250.EGP.213.incomplete Network Next Hop Metric LocPrf Weight Path *> 192.208.1 distribute-list 1 out neighbor 0 300 0 300 RTC# hostname RTC ip subnet-zero interface Loopback0 ip address 128.2 255.252 ! interface Serial2/1 ip address 128. ? .5 255. This way AS100 will pick RTB for AS300’s local routes. h history.

2 remote-as 400 ! ip classless access-list 1 permit 195.2 255.1 send-community neighbor interface Serial0 ip address 255.255 host 255.2 remote-as 300 neighbor ! interface Serial0/0 ip address Page 96 Sam Halabi-cisco Systems . 255.255 access-list 2 permit any access-list 101 permit ip 195.0 summary-only neighbor 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: A1.255.252 router bgp 300 network 192.208.0 0.0 aggregate-address neighbor route-map setcommunity out neighbor 195.10.252 interface Serial1 ip address remote-as 100 RTG# hostname RTG ip subnet-zero interface Loopback0 ip address 195.RTD# hostname RTD ip subnet-zero interface Loopback0 ip address 192.255.1 remote-as 500 neighbor 192.5 router bgp 500 network ! interface Serial0/1 ip address 192.0 255.174 255.

0.2 200 0 200 400 i *> 203.13.0/16.0 203.0 0 32768 i *> 203.0 255. It doesn’t matter in our case because RTB is not accepting these routes anyway.41 Status codes: s suppressed. h history. ? .0 0 32768 i *>i203.63.2 Page 97 Sam Halabi-cisco Systems .0/16 128.0 0 300 0 300 i *> 200. e .255. local router ID is 203.0 0.252 router bgp 400 network 200.0.EGP. 0 100 0 i 1/26/96-Rev: A1.211.0 192.1 remote-as 500 ip classless RTE is aggregating 128.0 interface Serial0 ip address 195.0 summary-only neighbor And following are the final bgp and routing tables for RTA.15. RTF and RTB: RTA#sh ip bgp BGP table version is 21.208. * valid. Network Next Hop Metric LocPrf Weight Path *> 128.250.2 0 200 0 200 i *>i192.5 remote-as 200 neighbor 195.213.1 255.IGP. aggregate-address 200.255. i - internal Origin codes: i .200. > best.250.208.2 255.255.63.On RTG. I have demonstrated the use of community filtering by adding a no-export community to 195. d damped. 255.15.10. RTE# hostname RTE ip subnet-zero interface Loopback0 ip address 200.14.252 interface Serial1 ip address 128.13.0 updates towards RTD. This way RTD will not export that route to RTB. [110/1000] via 203.14.0 [200/0] via 203.63.13.OSPF external type 1. 00:41:26 C 255.10 255. 2 subnets.connected. O .0. 2 subnets. IA .255. 3 masks O 203.255.0 is directly connected.63.0 [20/0] via 128.250.14. M .255.213.208. * - candidate default Gateway of last resort is 128. 00:41:25.255.IS-IS level- Ethernet0 O 2 masks O E2 192. 00:41:25 C 203. 00:02:38 1/26/96-Rev: A1.63.15. L1 .0 255.BGP D .250.255 [110/75] via is directly connected. Ethernet0 O L2 .2. 00:41:25.0 255.0 255.15.static. E .2 Page 98 Sam Halabi-cisco Systems . [20/0] via 128.0 255. Serial0 B* Loopback0 203. EX .mobile.2.IS-IS level-2.252 [110/74] via 203.255.0 is variably subnetted.213.10. Ethernet0 C 203.EGP i .EIGRP.255.RIP. Ethernet0 B 203.250.OSPF. R .0 255.252 [110/138] via 203. 00:41:25.OSPF external type 2.OSPF inter area E1 .2 to network 200. 3 subnets.200.0.RTA#sh ip rou Codes: C . 2 masks B 128.2. I .EIGRP external.0 is variably subnetted.63.2.252 is directly connected. S .0 is variably subnetted.200.0 192. E2 . Ethernet0 128. B .10.250. 00:41:25.208.IGRP.

R .213.OSPF external type 1. L2 .10.252 [110/128] via 203. Ethernet0 128.0.0 is variably subnetted.213.10 2 masks O E2 to network 0.0 [110/2000] via 203.0 255.0 [110/2000] via 203.250. Serial1 Note RTF’s routing table which indicates that networks local to AS300 such as 192. Ethernet0 O E2 200.0 255.0 is variably subnetted.200.15. Ethernet0 O E2 203.0.14. M .13. Ethernet0 O 128.0.0 is variably subnetted. 00:45:01.0.0. Serial1 C 203. then the default advertised by RTA will kick in with a metric of IA .255.0 [110/1000] via 203.0.0 is directly connected.RTF#sh ip rou Codes: C . level-2. E2 . In case something happens to the connection between RTB and RTD.4 is directly connected.0 [110/2000] via 203. E . 01:12:09.0 255. 00:03:47.OSPF external type 2. 01:12:09.BGP D . 1/26/96-Rev: A1.15.IS-IS level-1.255. 2 subnets.OSPF inter area E1 .250.2 Page 99 Sam Halabi-cisco Systems . Ethernet0 O*E2 0.0 are to be reached via RTB.255 [110/11] via 203. 00:03: 01:12:09.250. Ethernet0 203.1.14. 2 subnets.14. L1 .14. The gateway of last resort is set to RTB.EIGRP external. 2 subnets.0 is variably subnetted. Other known networks such as 200.41 255. 2 masks O E2 128. EX . 01:12:11.14.0 0.250.0 192.250.250.OSPF. O . Serial1 C 203. Serial1 203.EIGRP.255 [110/65] via 203.static.0. 01:12:09.0 I . 2 subnets.255.1.255.EGP i .IGRP. B .1. S .0 are to be reached via RTA. 2 masks O 203.0. * - candidate default Gateway of last resort is 203.2. 2 masks O 203.connected. Serial1 O [110/74] via 203.0 255.RIP.10. 00:48: 255.0 [110/1000] via 203.

BGP D . E . 255.0 is variably subnetted.208.250.250.EIGRP external.0 255.2 200 0 200 400 i *>i203.250.213. * valid. 2 masks O E2 128.255. I .208.250.255 [110/75] via 203.250. 01:20:34. i - internal Origin codes: i .IS-IS level-2. Serial0 O 0 200 0 200 i *> 192.255. L1 .OSPF. 255.0 128.41 0 100 0 i *> 128. Serial0 128.255.0 [110/2000] via 203.IGRP.14.15. Serial0 O is directly connected.0 255. 00:05: L2 .13.0 [20/0] via 192.15.IS-IS.200. * - candidate default Gateway of last resort is 203. E2 .0.13. 01:15:40.0.0 0 32768 i RTB#sh ip rou Codes: C .8 is directly connected. EX .250.0 [110/2000] via 203. M . > best. 2 subnets.5 to network 192.0. 01:20:33.15. 2 masks O 203. O .0 255.250.0 [110/2000] via Serial0 1/26/96-Rev: A1.10.IS-IS level-1.0 [110/74] via 203.208.0. 00:50:46 C 192.0. R .41 255.255. h history. B .OSPF external type 1. IA .13.252 [110/138] via 203.0 203. S .255. 2 subnets C 203.OSPF inter area E1 .255.5 0 300 0 300 i *>i200.208. e .RTB#sh ip bgp BGP table version is 14.incomplete Network Next Hop Metric LocPrf Weight Path *>i128.250.1.10 Status codes: s suppressed.RIP.5.15. Serial0 O E2 203.0 is variably subnetted.255.0 255.255. Serial0 O E2 200.static. 01:20:33.250.13.EGP i .250. 2 masks B* 192. 2 subnets.0 192.252 is subnetted.255.15. 00:46: ? . Serial0 Loopback1 C 203.255.250.connected. d damped. 0 100 0 i *>i203.OSPF external type 2.208. local router ID is 203.4 255. Serial1 203.213.EIGRP.0 * 192.1.0 is variably subnetted.2 Page 100 Sam Halabi-cisco Systems .1. 2 subnets.0 is directly connected.