BGP Case Studies

Document ID: 26634

Introduction
Prerequisites
Requirements
Components Used
Conventions
BGP Case Studies 1
How Does BGP Work?
eBGP and iBGP
Enable BGP Routing
Form BGP Neighbors
BGP and Loopback Interfaces
eBGP Multihop
eBGP Multihop (Load Balancing)
Route Maps
match and set Configuration Commands
network Command
Redistribution
Static Routes and Redistribution
iBGP
The BGP Decision Algorithm
BGP Case Studies 2
AS_PATH Attribute
Origin Attribute
BGP Next Hop Attribute
BGP Backdoor
Synchronization
Weight Attribute
Local Preference Attribute
Metric Attribute
Community Attribute
BGP Case Studies 3
BGP Filtering
AS Regular Expression
BGP Neighbors and Route Maps
BGP Case Studies 4
CIDR and Aggregate Addresses
BGP Confederation
Route Reflectors
Route Flap Dampening
How BGP Selects a Path
BGP Case Studies 5
Practical Design Example
NetPro Discussion Forums − Featured Conversations
Related Information

Introduction
This document contains five Border Gateway Protocol (BGP) case studies.

Prerequisites
Requirements
There are no specific requirements for this document.

Components Used
This document is not restricted to specific software and hardware versions.

Conventions
Refer to Cisco Technical Tips Conventions for more information on document conventions.

BGP Case Studies 1
The BGP, which RFC 1771 defines, allows you to create loop−free interdomain routing between autonomous
systems (ASs). An AS is a set of routers under a single technical administration. Routers in an AS can use
multiple Interior Gateway Protocols (IGPs) to exchange routing information inside the AS. The routers can
use an exterior gateway protocol to route packets outside the AS.

How Does BGP Work?
BGP uses TCP as the transport protocol, on port 179. Two BGP routers form a TCP connection between one
another. These routers are peer routers. The peer routers exchange messages to open and confirm the
connection parameters.

BGP routers exchange network reachability information. This information is mainly an indication of the full
paths that a route must take in order to reach the destination network. The paths are BGP AS numbers. This
information helps in the construction of a graph of ASs that are loop−free. The graph also shows where to
apply routing policies in order to enforce some restrictions on the routing behavior.

Any two routers that form a TCP connection in order to exchange BGP routing information are "peers" or
"neighbors". BGP peers initially exchange the full BGP routing tables. After this exchange, the peers send
incremental updates as the routing table changes. BGP keeps a version number of the BGP table. The version
number is the same for all the BGP peers. The version number changes whenever BGP updates the table with
routing information changes. The send of keepalive packets ensures that the connection between the BGP
peers is alive. Notification packets go out in response to errors or special conditions.

eBGP and iBGP
If an AS has multiple BGP speakers, the AS can serve as a transit service for other ASs. As the diagram in this
section shows, AS200 is a transit AS for AS100 and AS300.

In order to send the information to external ASs, there must be an assurance of the reachability for networks.
In order to assure network reachability, these processes take place:

• Internal BGP (iBGP) peering between routers inside an AS
• Redistribution of BGP information to IGPs that run in the AS

When BGP runs between routers that belong to two different ASs, this is called exterior BGP (eBGP). When
BGP runs between routers in the same AS, this is called iBGP.

Enable BGP Routing
Complete these steps in order to enable and configure BGP.

Assume that you want to have two routers, RTA and RTB, talk via BGP. In the first example, RTA and RTB
are in different ASs. In the second example, both routers belong to the same AS.

1. Define the router process and the AS number to which the routers belong.

Issue this command to enable BGP on a router:

router bgp autonomous−system

RTA#
router bgp 100

RTB#
router bgp 200

These statements indicate that RTA runs BGP and belongs to AS100. RTB runs BGP and belongs to
AS200.
2. Define BGP neighbors.

The BGP neighbor formation indicates the routers that attempt to talk via BGP. The section Form
BGP Neighbors explains this process.

Form BGP Neighbors
Two BGP routers become neighbors after the routers establish a TCP connection between each other. The
TCP connection is essential in order for the two peer routers to start the exchange of routing updates.

After the TCP connection is up, the routers send open messages in order to exchange values. The values that
the routers exchange include the AS number, the BGP version that the routers run, the BGP router ID, and the
keepalive hold time. After the confirmation and acceptance of these values, establishment of the neighbor

• clear ip bgp address Note: The address is the neighbor address. You can prevent negotiations and force the BGP version that the routers use to communicate with a neighbor. • clear ip bgp * This command clears all neighbor connections. If there are any BGP configuration changes. Issue this command in router configuration mode: neighbor {ip address | peer−group−name} version value Here is an example of the neighbor command configuration: . The extended ping forces the pinging router to use as source the IP address that the neighbor command specifies.connection occurs. For iBGP. The two IP addresses that you use in the neighbor command of the peer routers must be able to reach one another. BGP sessions begin with the use of BGP version 4 and negotiate downward to earlier versions. ip−address is any IP address on the other router. if necessary. By default. The router must use this address rather than the IP address of the interface from which the packet goes. you must reset the neighbor connection to allow the new parameters to take effect. Issue this neighbor command to establish a TCP connection: neighbor ip−address remote−as number The number in the command is the AS number of the router to which you want to connect with BGP. The ip−address is the next hop address with direct connection for eBGP. One way to verify reachability is an extended ping between the two IP addresses. Any state other than Established is an indication that the two routers did not become neighbors and that the routers cannot exchange BGP updates.

notice these items: • The BGP version. if existent. A version that continues to increment indicates that there is some route flap that causes the continuous update of routes.212.213. peer routers frequently have direct connection. dropped 10 BGP and Loopback Interfaces The use of a loopback interface to define neighbors is common with iBGP.1 remote−as 200 RTB# router bgp 200 neighbor 129. Normally.2 remote−as 100 neighbor 175.1. hold time is 180.1 remote−as 200 In this example. remote AS 200. If you use the IP address of a loopback interface in the neighbor command. remote router ID 175.1. The remote AS number points to either an external or an internal AS. Also.1 BGP state = Established. # show ip bgp neighbors BGP neighbor is 129. which is 4 • The remote router ID This number is the highest IP address on the router or the highest loopback interface. 0 in queue Connections established 11. iBGP routers do not need to have direct connection.213. keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 2828 messages. • The table version The table version provides the state of the table. the table increases the version. you use the loopback interface to make sure that the IP address of the neighbor stays up and is independent of hardware that functions properly. but is not common with eBGP.220.1. which indicates either eBGP or iBGP.220. RTA and RTB run eBGP.12.1. Any time that new information comes in. 0 notifications. 0 notifications.2 remote−as 200 RTC# router bgp 200 neighbor 175.220. 0 in queue Sent 2826 messages. you need some extra configuration on the neighbor router.213. there must be some IGP that runs and allows the two neighbors to reach one another. the eBGP peers have direct connection. This section provides an example of the information that the show ip bgp neighbors command displays. issue this command: neighbor ip−address update−source interface . Note: Pay special attention to the BGP state. up for 0:10:59 Last read 0:00:29. But. but the iBGP peers do not have direct connection.1. table version = 3. and loopback does not apply. RTB and RTC run iBGP. Anything other than the state Established indicates that the peers are not up. Note: Also. RTA# router bgp 100 neighbor 129. external link BGP version 4. In the case of eBGP. The neighbor router needs to inform BGP of the use of a loopback interface rather than a physical interface to initiate the BGP neighbor TCP connection. In order to indicate a loopback interface.

1.11.11. In the neighbor command.1 remote−as 100 In this example. Refer to Sample Configuration for iBGP and eBGP With or Without a Loopback Address for a complete network scenario sample configuration.1.11.225.This example illustrates the use of this command: RTA# router bgp 100 neighbor 190. RTB uses the loopback interface of RTA. RTA must force BGP to use the loopback IP address as the source in the TCP neighbor connection.1. RTA adds update−source interface−type interface−number so that the command is neighbor 190. In this case.1. In order to force this action.11. The eBGP multihop allows a neighbor connection between two external peers that do not have direct connection. Use of this IP address is why RTB does not need any special configuration. 150.225. you can use eBGP multihop. 190.225. This statement forces BGP to use the IP address of the loopback interface when BGP talks to neighbor 190. as a neighbor. This example illustrates eBGP multihop: RTA# .11. a Cisco router can run eBGP with a third−party router that does not allow direct connection of the two external peers.212.1. eBGP Multihop In some cases.1 update−source loopback 1.1 update−source loopback 1 RTB# router bgp 100 neighbor 150.225. RTA and RTB run iBGP inside AS100. The multihop is only for eBGP and not for iBGP. Note: RTA has used the physical interface IP address of RTB.212.225.1 remote−as 100 neighbor 190. To achieve the connection.

1 ebgp−multihop neighbor 160.255.1 ebgp−multihop RTB# router bgp 300 neighbor 129. Because of this direct connection.1: one path via 1. RTB indicates a neighbor that has direct connection.2.10. BGP picks one of the lines on which to send packets.10.2.255. RTA has two choices to reach next hop 160.1.1.213.2.1 update−source loopback 0 network 150.2 RTB# int loopback 0 ip address 160.1.1.2. With the introduction of loopback interfaces.2 ip route 160.225.10.0 255. On the other hand.2.0.2 remote−as 100 RTA indicates an external neighbor that does not have direct connection.2.1 ebgp−multihop network 160.1 remote−as 300 neighbor 180. and ebgp−multihop. The example is a workaround in order to achieve load balancing between two eBGP speakers over parallel serial lines.0 1.10.2.1.10.225.1. the next hop for eBGP is the loopback interface. router bgp 100 neighbor 180.0.1.1 This example illustrates the use of loopback interfaces.0 255.255.0 255. The example in the eBGP Multihop (Load Balancing) section shows how to achieve load balancing with BGP in a case where you have eBGP over parallel lines.1.0 router bgp 200 neighbor 150.10. RTA needs to indicate its use of the ebgp−multihop command.1.0.10.0. .0 router bgp 100 neighbor 160.10.2 and the other path via 2.0 ip route 160. update−source.255.1.0 1.10. or an IGP.11.0.213. You should also configure an IGP or static routing to allow the neighbors without connection to reach each other.10. eBGP Multihop (Load Balancing) RTA# int loopback 0 ip address 150.1.0 255. You use static routes.10.255.10. and load balancing does not happen.255.1 remote−as 100 neighbor 150.0 2.0 2.0.1.10.11.1 255.0.0.0. RTB does not need the ebgp−multihop command.2.255.10.255.1 ip route 150.0. which is 129.1. In normal situations.0 ip route 150. RTB has the same choices.1 remote−as 200 neighbor 160.1.1.1. to introduce two equal−cost paths to reach the destination.1 update−source loopback 0 neighbor 150.1 255.1.10.

1.) When you apply route map MYMAP to incoming or outgoing routes.1. This next−instance check continues until you either break out or finish all the instances of the route map. Or the control of routing information can occur at injection in and out of BGP. when you use route maps to filter BGP updates rather than redistribute between protocols.Route Maps There is heavy use of route maps with BGP.1. If the match criteria are met and you have a deny. as the set action specifies. • route−map MYMAP permit 10 (The first set of conditions goes here. In the BGP context.2 and later releases do not have this restriction. and set specifies a set action if the criteria that the match command enforces are met.2. you proceed to a higher instance of the route map. If the match criteria are not met and you have a permit or deny. If you finish the list without a match.1. with the name MYMAP. In this example. The control and modification of routing information occurs through the definition of conditions for route redistribution from one routing protocol to another.1. or the same name tag. and the second has a sequence number of 20. For example. The match specifies a match criteria. Cisco IOS Software Release 11. For example. You break out of the list. You break out of the list. If there is a match for IP address 1. You can define multiple instances of the same route map. These commands illustrate the example: match ip address 1. the first set of conditions are applied via instance 10. The format of the route map follows: route−map map−tag [[permit | deny] | [sequence−number]] The map tag is simply a name that you give to the route map.) • route−map MYMAP permit 20 (The second set of conditions goes here. A filter on the outbound is acceptable. instance 20 is checked. you cannot filter on the inbound when you use a match command on the IP address. The sequence number is simply an indication of the position that a new route map is to have in the list of route maps that you have already configured with the same name. the route map is a method to control and modify routing information. there is no redistribution or control of the route. you can define a route map that checks outgoing updates. The related commands for match are: • match as−path • match community . match and set Configuration Commands Each route map consists of a list of match and set configuration commands. The first instance has a sequence number of 10. In Cisco IOS® Software releases earlier than Cisco IOS Software Release 11. if the match criteria are met and you have a permit.1 set metric 5 Now. the next instance of the route map is checked. there is a redistribution or control of the routes. the metric for that update is set to 5. there are two instances of the route map defined. the route is not accepted nor forwarded. If the first set of conditions is not met.

• 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 clns • set automatic−tag • set community • set interface • set default interface • set ip default next−hop • set level • set local−preference • set metric • set metric−type • set next−hop • set origin • set tag • set weight Look at some route map examples: .

0 passive−interface Serial0 redistribute bgp 100 route−map SETMETRIC router bgp 100 neighbor 2.2.0. RTA gets updates via BGP and redistributes the updates to RIP.0.2.0.10. the route has a metric of 2.0.0.10. Therefore.0.2 route−map STOPUPDATES out route−map STOPUPDATES permit 10 match ip address 1 access−list 1 deny 170.2.0.255.0 255.0 0.2.255.255. you can use this configuration: RTA# router rip network 3. If there is no match.0.10. which indicates setting everything else to metric 5.0.0.0. look at how to start the exchange of network information. and RTA and RTC run BGP.0 network 2.Example 1 Assume that RTA and RTB run Routing Information Protocol (RIP). Then. in Example 1. Note: Always ask the question "What happens to routes that do not match any of the match statements?" These routes drop.0.0. Suppose that RTA wants to redistribute to RTB routes about 170.2.2.10. by default.10.0 neighbor 2.10. you do not want AS100 to accept updates about 170. There are multiple ways to send network information with use of BGP.0. In this case.0.0.255 access−list 1 permit 0.10.255. you must use an outbound route map on RTC: RTC# router bgp 300 network 170.0 with a metric of 2 and all other routes with a metric of 5.0 network 150.0.255 In this example. you break out of the route map list.0 0.10. if a route matches the IP address 170.3 remote−as 300 network 150.255 Now that you feel more comfortable with how to start BGP and how to define a neighbor.0 route−map SETMETRIC permit 10 match ip−address 1 set metric 2 route−map SETMETRIC permit 20 set metric 5 access−list 1 permit 170. You cannot apply route maps on the inbound when you match with an IP address as the basis. Example 2 Suppose that.0.2 remote−as 100 neighbor 2. you proceed down the route map list. These sections go through the methods one by one: .

With this command.213.255. Open Shortest Path First (OSPF) protocol. or learned dynamically. you try to indicate to BGP what networks BGP should originate from this box. An example of the network command is: RTA# router bgp 1 network 192.0.255. Look at the RTC configuration: .213.213. Redistribution The network command is one way to advertise your networks via BGP.0 255.0 mask 255. The command uses a mask portion because BGP version 4 (BGP4) can handle subnetting and supernetting.0. RIP. The network command works if the router knows the network that you attempt to advertise.0.213.0. Instead.0/16. static.1. A maximum of 200 entries of the network command are acceptable. • network Command • Redistribution • Static Routes and Redistribution network Command The format of the network command is: network network−number [mask network−mask] The network command controls the networks that originate from this box. The /16 indicates that you use a supernet of the class C address and you advertise the first two octets. Note: You need the static route to get the router to generate 192.0 because the static route puts a matching entry in the routing table. Here is an example: RTA announces 129.0 ip route 192.0.0 and RTC announces 175. some of these routes can have been learned via BGP and you do not need to send them out again.220. Your IGP can be IGRP.213.0.0 null 0 This example indicates that router A generates a network entry for 192. you do not try to run BGP on a certain interface. Apply careful filtering to make sure that you send to the Internet−only routes that you want to advertise and not to all the routes that you have. or first 16 bits. This redistribution can seem scary because now you dump all your internal routes into BGP.0. Enhanced Interior Gateway Routing Protocol (EIGRP). Another way is to redistribute your IGP into BGP. whether connected.0. or another protocol. This concept is different than the familiar configuration with Interior Gateway Routing Protocol (IGRP) and RIP.

220.0.0 mask 255.If you issue the network command.1 remote−as 300 redistribute eigrp 10 !−−− EIGRP injects 129.220.0.0.220. AS100 is the source. If you use redistribution instead.255.213.0.1.220.0. you have: RTC# router eigrp 10 network 175. You are not the source of 129.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 .0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1.0 again into BGP.0 !−−− This limits the networks that your AS originates to 175.0.1.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1. This redistribution causes the origination of 129.0. The correct configuration is: RTC# router eigrp 10 network 175. you have: RTC# router eigrp 10 network 175.1.1.213.1.220.1.0 by your AS. So you have to use filters to prevent the source out of that network by your AS.1 remote−as 300 network 175.0.213.1.

0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1. the router sends the packet to the specific match. So if you get the packet and there is a more specific match than 175.0. Specific keywords such as internal. Remember that these routes are generated in addition to other BGP routes that BGP has learned via neighbors. external. The difference is that routes that generate from the network command.0. Static Routes and Redistribution You can always use static routes to originate a network or a subnet.. router bgp 200 neighbor 1. or static indicate your AS as the origin of these networks.220. You can accomplish the same result that the example in the Redistribution section accomplished with this: RTC# router eigrp 10 network 175. either internal or external. This document has discussed how you can use different methods to originate routes out of your AS.1.255. redistribution. This method is a nice way to advertise a supernet. the router disregards the packet. which exists..0 0. and nssa−external are necessary to redistribute respective routes..0.0. Otherwise.1 remote−as 300 neighbor 1. The null0 interface means disregard the packet. BGP passes on information that BGP learns from one peer to other peers.1.1.. Refer to Understanding Redistribution of OSPF Routes into BGP for more details.0. ip route 175.1. Redistribution of OSPF into BGP is slightly different than redistribution for other IGPs. or unknown.1 remote−as 300 redistribute static ..255 You use the access−list command to control the networks that originate from AS200.220.0 null0 .1.255.220. The only difference is that BGP considers these routes to have an origin that is incomplete.1 distribute−list 1 out redistribute eigrp 10 access−list 1 permit 175.0 255.1.220.255.0. The simple issue of redistribute ospf 1 under router bgp does not work. Here is an example: . Redistribution is always the method for injection of BGP into IGP.

10. which indicates that AS300 is also an origin for these routes.20.0 and sends the route to AS300. .10. Again. For example. For example. has a direct BGP connection into AS100.0 RTC# router bgp 300 neighbor 150.0.20. iBGP provides ways to control the best exit point out of the AS with use of local preference. redistributing into IGP.1 remote−as 200 network 170. RTC passes this route to AS200 and keeps the origin as AS100. assume that AS200. the difference is that the network command adds an extra advertisement for these same networks.0 to AS100 with the origin still AS100.20.10.10. This refusal ensures a loop−free interdomain topology. Then.10. RTA generates a route 150. Note: Remember that BGP does not accept updates that have originated from its own AS. RTA notices that the update has originated from its own AS and ignores the update.10. iBGP You use iBGP if an AS wants to act as a transit system to other ASs.0 in RTC unless you want RTC to generate these networks as well as pass on these networks as they come in from AS100 and AS200.1 remote−as 100 neighbor 160.0.10. but iBGP offers more flexibility and more efficient ways to exchange information within an AS. RTA# router bgp 100 neighbor 150.0.0. from the example in this section.10. The section Local Preference Attribute provides more information about local preference.0 or network 160.0. Is it true that you can do the same thing by learning via eBGP.10.0.0 RTB# router bgp 200 neighbor 160.10. and then redistributing again into another AS? Yes.20.2 remote−as 300 network 150.10.2 remote−as 300 network 160. RTB passes 150.00 Note: You do not need network 150.

1 remote−as 100 neighbor 175. BGP bases the decision on different attributes. The updates do not transmit to RTD. which is inside the AS.10. the BGP speaker that receives the update does not redistribute that information to other BGP speakers in its own AS. origin code.10. The BGP updates that come from RTB to RTA transmit to RTE.10.2 remote−as 100 network 175.10. BGP chooses only a single path to reach a specific destination. the protocol must choose paths to reach a specific destination. In the diagram in this section.20. metric.10. Therefore.10. and other attributes.40. make an iBGP peering between RTB and RTD in order to not break the flow of the updates. RTA and RTD also run iBGP. route origin. which is outside the AS.30.0 Note: Remember that when a BGP speaker receives an update from other BGP speakers in its own AS (iBGP). Therefore.50. RTA# router bgp 100 neighbor 190.0.1 remote−as 100 neighbor 170. such as next hop. local preference.0 RTC# router bgp 400 neighbor 175.10. administrative weights. The BGP Decision Algorithm After BGP receives updates about different destinations from different autonomous systems.10.0. RTA and RTB run iBGP.40.1 remote−as 400 network 190. . The BGP speaker that receives the update redistributes the information to other BGP speakers outside of its AS.0 RTB# router bgp 100 neighbor 150. sustain a full mesh between the iBGP speakers within an AS.2 remote−as 300 network 150. path length.50.

the AS number is prepended to that update.10. INCOMPLETE usually occurs when you redistribute routes from other routing protocols into BGP and the origin of the route is incomplete. the network has two AS numbers attached: first 200.0.0 is (300. 200).10. For RTA. RTB advertises network 190.0.10. RTB traverses AS300 and then AS100 in order to reach 170.0. • EGPNLRI is learned via exterior gateway protocol (EGP).10. An i in the BGP table indicates IGP. The CIDR Example 2 (as−set) section of this document provides an example of AS_SET.0 in AS200. then 300.0. The same process applies to 170.10.0.BGP always propagates the best path to the neighbors. An AS_SET is an ordered mathematical set {} of all the ASs that have been traversed. This normally happens when you issue the bgp network command .0. The origin attribute can assume three values: • IGPNetwork Layer Reachability Information (NLRI) is interior to the AS of origination. An ? in the BGP table indicates INCOMPLETE.0. RTB has to take path (300. An e in the BGP table indicates EGP. . When that route traverses AS300.0. BGP Case Studies 2 AS_PATH Attribute Whenever a route update passes through an AS. Refer to BGP Best Path Selection Algorithm for more information. 100).10.0 and 180.0. In the example in this section. RTC appends its own AS number to the network. Origin Attribute The origin is a mandatory attribute that defines the origin of the path information.10.0 reaches RTA. the path to reach 190. The AS_PATH attribute is actually the list of AS numbers that a route has traversed in order to reach a destination. The section BGP Case Studies 2 explains these attributes and their use.0 and path (100) in order to reach 170.10.0. So when 190. RTC has to traverse path (200) in order to reach 190. • INCOMPLETENLRI is unknown or learned via some other means.0.

10. BGP Next Hop Attribute .0.0.0.0. The "100 ?" means that the next AS is 100 and that the origin is incomplete and comes from a static route.1 remote−as 100 neighbor 170.255. RTA# router bgp 100 neighbor 190.20.0 redistribute static ip route 190. The "100 i" means that the next AS is 100 and the origin is IGP. This "i" means that the entry is in the same AS and the origin is IGP.0 RTA reaches 170. RTE also reaches 190.0.1 remote−as 100 network 190.10.10.50.10.0.10.0 255. RTE reaches 150.0 via i. The "300 i" means that the next AS path is 300 and the origin of the route is IGP.10.50.0 via 100 ?.10. RTA also reaches 190.10.50.20.30.2 remote−as 300 network 150.0 via 100 i.0 RTE# router bgp 300 neighbor 170.0 via 300 i.0.10.10.1 remote−as 100 network 170.10.0 null0 RTB# router bgp 100 neighbor 150.10.

0.10. RTA advertises 170.10. Otherwise.30.0.1 remote−as 100 RTC# router bgp 300 neighbor 170.10. For example.0.10.10. In the example in this section.2 via IGP. Note: RTA advertises 170.10.10.0 to RTB with a next hop equal to 170. RTA advertises 150. For eBGP. according to RTB.2.10.10.20.1. the next hop is always the IP address of the neighbor that the neighbor command specifies.10. You want to make iGRP passive on the link to RTC so that BGP is only exchanged.10.2. For iBGP.20.0. you can also run iGRP on RTA network 170. Make sure that RTB can reach 170.10.10.20.0 Note: RTC advertises 170.30.0 to RTA with a next hop of 170. the next hop to reach 170. the protocol states that the next hop that eBGP advertises should be carried into iBGP.0. RTB drops packets with the destination of 170.The BGP next hop attribute is the next hop IP address to use in order to reach a certain destination. if RTB runs iGRP. . RTA# router bgp 100 neighbor 170. Because of this rule.0 to RTC with a next hop of 170.2 remote−as 300 neighbor 150.10.20.1 remote−as 100 network 170.0 to its iBGP peer RTB with a next hop of 170.0 is 170.20.0 RTB# router bgp 100 neighbor 150.0.1 remote−as 100 network 150.0.2.0.10.0.2.10.10.10.10.10.10.0.10.20.0 because the next hop address is inaccessible. The eBGP next hop is carried in iBGP.20.0. RTC advertises 170.50.20.1.20.2 and not 150.0 to RTA with a next hop equal to 170. So.

Assume that RTC and RTD in AS300 run OSPF.20. The RTA use of RTD as a next hop to reach 180.20.20.0.10.2.0.20.10. RTC uses as next hop 170. RTC. and RTD is a multiaccess network.10. RTC can reach network 180. RTC does not use its own IP address. BGP Next Hop (Multiaccess Networks) This example shows how the next hop behaves on a multiaccess network such as Ethernet. 170. Note: RTC advertises 180.Take special care when you deal with multiaccess and nonbroadcast multiaccess (NBMA) networks.20.0 is more sensible than the extra hop via RTC.0.0 via 170.0. The sections BGP Next Hop (Multiaccess Networks) and BGP Next Hop (NBMA) provide more details.20.3. RTC runs BGP with RTA. further complications occur. RTC.3. RTC uses this address because the network between RTA. and RTD is not multiaccess.20.20. BGP Next Hop (NBMA) .0. If the common medium to RTA.0 to RTA with a next hop 170.10.3. but NBMA. When RTC sends a BGP update to RTA with regard to 180.

you can use the next−hop−self command.20.20.2. as in the BGP Next Hop (NBMA) example. For the BGP Next Hop (NBMA) example.10.10. next−hop−self Command For situations with the next hop. In this case. The syntax is: neighbor {ip−address | peer−group−name} next−hop−self The next−hop−self command allows you to force BGP to use a specific IP address as the next hop.20.3.0. this configuration solves the problem: RTC# router bgp 300 neighbor 170.20.0 with a next hop equal to 170.1 next−hop−self RTC advertises 180.1 remote−as 100 neighbor 170. routing fails. RTC advertises 180.20. . If the common medium is a frame relay or any NBMA cloud.10.0 to RTA with a next hop of 170. the exact behavior is as if you have connection via Ethernet. The next−hop−self command remedies this situation.The common medium appears as a cloud in the diagram.20. The problem is that RTA does not have a direct permanent virtual circuit (PVC) to RTD and cannot reach the next hop.0.10.

IGRP. then you have two options: • Change the external distance of eBGP or the IGP distance. BGP has these distances: • External distance¢0 • Internal distance¢00 • Local distance¢00 But you can use the distance command to change the default distances: distance bgp external−distance internal−distance local−distance RTA picks eBGP via RTC because of the shorter distance.0 via RTB (IGP). By definition.10. RTA and RTC run eBGP.BGP Backdoor In this diagram. either RIP. . or another protocol. RTA and RTB run some kind of IGP. which is less than the IGP distances. • Use BGP backdoor.0. eBGP updates have a distance of 20. The default distances are: • 120 for RIP • 100 for IGRP • 90 for EIGRP • 110 for OSPF RTA receives updates about 160.10. Note: This change is not recommended. If you want RTA to learn about 160.0 via two routing protocols: • eBGP with a distance of 20 • IGP with a distance that is greater than 20 By default. RTB and RTC run eBGP.0.

but is not advertised as a normal network entry.2. Normally eBGP is the preference.2.0. The configured network is the network that you want to reach via IGP.0. RTA also learns the address from RTC via eBGP with distance 20.0.10.10.10. EIGRP is the preference.1 remote−as 300 network 160.10. RTC in AS300 sends updates about 170.0 router bgp 100 neighbor 2. RTA and RTB run iBGP. but because of the backdoor command.2.0. In order to reach the next hop.0 backdoor Network 160. . RTB must send the traffic to RTE. look at this scenario.10. this network gets the same treatment as a locally assigned network.0. Issue the network address backdoor command.0 is treated as a local entry.0. RTA# router eigrp 10 network 150.0 from RTB via EIGRP with distance 90.0.0 via next hop 2. For BGP.1.10.2. so RTB gets the update and is able to reach 170. RTA learns 160.BGP backdoor makes the IGP route the preferred route. except BGP updates do not advertise this network. Synchronization Before the discussion of synchronization. Remember that the next hop is carried via iBGP.

0 into IGP.0. The disablement of synchronization is not automatic. Then. Then. BGP waits until IGP has propagated the route within the AS. You can also disable synchronization if all routers in your AS run BGP. RTB waits to hear about 170. RTE has no idea that 170. RTB starts to send the update to RTD. If RTB starts to advertise to AS400 that RTB can reach 170.10. you can disable synchronization. If you do not pass traffic from a different AS through your AS.0.0 flows in and drops at RTE. BGP advertises the route to external peers. Disable Synchronization In some cases.0.10.10.0. In the example in this section. Make sure that other routers can reach 170. The disablement of this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly.10.0. BGP should not advertise a route before all the routers in your AS have learned about the route via IGP.10. If all your routers in the AS run BGP and you do not run IGP at all.0 via IGP. the router has no way to know.0 even exists. You can make RTB think that IGP has propagated the information if you add a static route in RTB that points to 170. You have to disable synchronization manually in this case so that routing can work correctly: router bgp 100 no synchronization Note: Make sure that you issue the clear ip bgp address command to reset the session.0. Synchronization states that.0. At this point. if your AS passes traffic from another AS to a third AS.10.10. traffic that comes from RTD to RTB with destination 170. .0. you do not need synchronization.0.0.Assume that RTA has not redistributed network 170. Your router waits indefinitely for an IGP update about a certain route before the router sends the route to external peers.

0.3 remote−as 100 no synchronization !−−− RTB puts 170.1 remote−as 100 network 175.1.10. even if RTB does not have an IGP path to 170.0 neighbor 1.2 remote−as 400 neighbor 3.0 in its IP routing table and advertises the network !−−− to RTD.0. RTB# router bgp 100 network 150.4 remote−as 100 Weight Attribute .1. RTD# router bgp 400 neighbor 1.0.0.1.3.0.10.3.0 RTA# router bgp 100 network 150.3.3.1.10.0 neighbor 3.10.0.10.

2.768 by default. ♦ neighbor {ip−address | peer−group} weight weight • Use AS_PATH access lists. RTA.2.1.2 remote−as 200 neighbor 2.10. has preference as the next hop.0.1.10. you force RTC to use RTA as a next hop to reach 175.1.2 weight 100 !−−− The route to 175.The weight attribute is a Cisco−defined attribute. RTB has also learned about network 175.10.2. You can achieve the same outcome with IP AS_PATH and filter lists. If you set the weight of the updates on RTC that come from RTA so that the weight is greater than the weight of updates that come from RTB. RTA has learned about network 175. RTB propagates the update to RTC. and other paths have a weight of 0. The value is not propagated or carried through any of the route updates. Routes with a higher weight value have preference when multiple routes to the same destination exist.0.1 weight 200 !−−− The route to 175.0 and has to decide which way to go.10.0 from RTB has a 100 weight. The weight is assigned locally to the router. Look at the example in this section.2.0. RTC now has two ways to reach 175. neighbor 2.0. RTC# router bgp 300 neighbor 1.10.0. . Multiple methods achieve this weight set: • Use the neighbor command.10. Paths that the router originates have a weight of 32.0 from AS4. ♦ ip as−path access−list access−list−number {permit | deny} as−regular−expression neighbor ip−address filter−list access−list−number weight weight • Use route maps. The value only makes sense to the specific router.1. which has a higher weight value.0 from AS4. This attribute uses weight to select a best path. RTA propagates the update to RTC. A weight can be a number from 0 to 65.0.0.1 remote−as 100 neighbor 1.535.0 from RTA has a 200 weight.

1.1..2.. route−map setweightin permit 20 set weight 100 !−−− Anything else has weight 100. ip as−path access−list 6 permit ^200$ .1. RTC# router bgp 300 neighbor 1. ip as−path access−list 5 permit ^100$ .2. RTC# router bgp 300 neighbor 1.1 remote−as 100 neighbor 1.2 remote−as 200 neighbor 2..2..1 remote−as 100 neighbor 1. You also can achieve the same outcome with the use of route maps.2.2. route−map setweightin permit 10 match as−path 5 set weight 200 !−−− Anything that applies to access list 5.1. Local Preference Attribute .... such as packets from AS100.1.1.2 remote−as 200 neighbor 2. has weight 200.2 route−map setweightin in ..2.1 filter−list 5 weight 200 neighbor 2.2.1 route−map setweightin in neighbor 2. ip as−path access−list 5 permit ^100$ !−−− This only permits path 100.1.2 filter−list 6 weight 100 .2.1.

Assume that RTD is the exit point preference. Unlike the weight attribute.11.0 from two different sides of the organization.11.1 remote−as 256 bgp default local−preference 200 In this configuration. as the example in this section demonstrates: The bgp default local−preference command sets the local preference on the updates out of the router that go to peers in the same AS. local preference is an attribute that routers exchange in the same AS.10. The same RTD sets the local preference of all updates to 200.3. Local preference helps you determine which way to exit AS256 in order to reach that network. In the diagram in this section.1 remote−as 100 neighbor 128.0.Local preference is an indication to the AS about which path has preference to exit the AS in order to reach a certain network.0 has a higher local preference when updates come from AS300 rather than from AS100. The default value for local preference is 100.213. both RTC and RTD realize that network 170.10.1.213. You can also set local preference with route maps.1. which is only relevant to the local router.0. RTC sets the local preference of all updates to 150. This configuration sets the local preference for updates that come from AS300 to 200 and for updates that come from AS100 to 150: RTC# router bgp 256 neighbor 1. A path with a higher local preference is preferred more. Therefore.3.4 remote−as 300 neighbor 128. There is an exchange of local preference within AS256. All traffic in AS256 that has that network as a destination transmits with RTD as an exit . You set local preference with the issue of the bgp default local−preference value command.2 remote−as 256 bgp default local−preference 150 RTD# router bgp 256 neighbor 3. AS256 receives updates about 170.

Any other updates. Updates that come from AS34 also are tagged with the local preference of 200.. The use of route maps provides more flexibility. In the example in this section.3.point. such as updates that come from AS34.. For this reason. Metric Attribute . This tag can be unnecessary. ip as−path access−list 7 permit ^300$ .1 remote−as 256 . have a value of 150.11. all updates that RTD receives are tagged with local preference 200 when the updates reach RTD.. you can use route maps to specify the specific updates that need to be tagged with a specific local preference.3.. any update that comes from AS300 has a local preference of 200.4 remote−as 300 neighbor 3.4 route−map setlocalin in neighbor 128.213. Here is an example: RTD# router bgp 256 neighbor 3.3..3. route−map setlocalin permit 10 match as−path 7 set local−preference 200 route−map setlocalin permit 20 set local−preference 150 With this configuration.

0. A lower metric value is preferred more. MED (BGP4).1.4. RTA chooses RTC as the best next hop because 120 is less than 200.2 route−map setmetricout out neighbor 1.2 route−map setmetricout out neighbor 1. you need to issue the special configuration command bgp always−compare−med on the router.3. The attribute is a hint to external neighbors about the path preference into an AS. the router compares metrics for paths from neighbors in the same AS. When an update enters the AS with a certain metric.. RTD. Refer to How the bgp deterministic−med Command Differs from the bgp always−compare−med Command to understand how these commands influence BGP path selection. When RTA gets an update from RTB with metric 50. RTA cannot compare the metric to 120 because RTC and RTB are in different ASs.2. In order to force RTA to compare the metrics. The attribute provides a dynamic way to influence another AS in the way to reach a certain route when there are multiple entry points into that AS.3.The metric attribute also has the name MULTI_EXIT_DISCRIMINATOR.2 remote−as 100 neighbor 3. that metric returns to 0. Unlike local preference. RTA can only compare the metric that comes from RTC to the metric that comes from RTD.3 remote−as 400 .3. and RTB.10. A metric is carried into an AS but does not leave the AS.4. The commands are the bgp deterministic−med command and the bgp always−compare−med command.3. or INTER_AS (BGP3). RTC# router bgp 300 neighbor 2. In order for the router to compare metrics from neighbors that come from different ASs. An issue of the bgp always−compare−med command ensures the comparison of the MED for paths from neighbors in different ASs.2.2.1.. By default.2 remote−as 100 neighbor 2. These configurations illustrate this process: RTA# router bgp 100 neighbor 2. and the metric that comes from RTB to 50.1 remote−as 300 .0 via three different routers: RTC. you must issue the bgp always−compare−med command on RTA..3. The bgp always−compare−med command is useful when multiple service providers or enterprises agree on a uniform policy for how to set MED. a router compares metrics that come from neighbors in the same AS. The metric default value is 0.3.1.1. RTA must choose based on some other attributes. Note: There are two BGP configuration commands that can influence the multi−exit discriminator (MED)−based path selection.2. metric is exchanged between ASs. In the diagram in this section.1 remote−as 300 neighbor 3. Therefore. RTC and RTD are in AS300. AS100 gets information about network 180.3 remote−as 300 neighbor 4. Assume that you have set the metric that comes from RTC to 120. the metric that comes from RTD to 200. that metric is used to make decisions inside the AS. An issue of the bgp deterministic−med command ensures the comparison of the MED variable at route choice when different peers advertise in the same AS. and RTB is in AS400.2. When the same update passes on to a third AS. The diagram in this section shows the set of metric. Unless a router receives other directions.2.2 remote−as 300 route−map setmetricout permit 10 set metric 120 RTD# router bgp 300 neighbor 3.

RTB injects a network via static into AS100.4. optional attribute in the range of 0 to 4.4.0.0. • no−advertiseDo not advertise this route to any peer. Any router belongs to this community.3. in the example in this section. well known communities for use in this command are: • no−exportDo not advertise to eBGP peers.10.0 255.3 remote−as 400 bgp always−compare−med In this case.200. You can use route maps to set the community attributes. Community Attribute The community attribute is a transitive. Keep this route within an AS. • internetAdvertise this route to the Internet community. prefer.4 route−map setmetricout out route−map setmetricout permit 10 set metric 50 With these configurations. Here is the configuration: RTB# router bgp 400 redistribute static default−metric 50 ip route 180. and redistribute. The community attribute is a way to group destinations in a certain community and apply routing decisions according to those communities. Assume that.4 remote−as 100 neighbor 4.21 remote−as 300 neighbor 3.10.4.255.0 with a metric of 50. with consideration of the fact that all other attributes are the same.967.0. The route map set command has this syntax: set community community−number [additive] [well−known−community] A few predefined. The routing decisions are accept. You can also set metric during the redistribution of routes into BGP if you issue the default−metric number command.4.0.3. you must configure RTA in this way: RTA# router bgp 100 neighbor 2.4. among others.4. RTA picks RTC as next hop. route−map setmetricout permit 10 set metric 200 RTB# router bgp 400 neighbor 4.10.3 remote−as 300 neighbor 4. RTA picks RTB as the best next hop in order to reach network 180. • local−asUse in confederation scenarios to prevent the transmit of packets outside the local AS.0 null 0 !−−− This causes RTB to send out 180. In order to include RTB in the metric comparison. Here are two examples of route maps that set the community: .2.0.294. internal or external.

localpref 100.3. Router# configure terminal Enter configuration commands.10.3.0.0. Cisco IOS Software uses the older decimal format. valid.0 command displays the community attribute value in decimal format.0.3 route−map setcommunity out In Cisco IOS Software Release 12.10. version 9 Paths: (1 available.1 (200. issue the ip bgp−community new−format command globally on this router. Router# show ip bgp 6.3 send−community neighbor 3. • route−map communitymap match ip address 1 set community no−advertise or • route−map setcommunity match as−path 1 set community 200 additive If you do not set the additive keyword.0. an issue of the show ip bgp 6.200. hexadecimal. external. Router(config)# ip bgp−community new−format Router(config)# exit With the ip bgp−community new−format global configuration command. this attribute does not transmit to neighbors by default.3. In order to configure and display in AA:NN.0 and later. the community value displays in AA:NN format. In order to send the attribute to a neighbor. By default. you must use this command: neighbor {ip−address | peer−group−name} send−community Here is an example: RTA# router bgp 100 neighbor 3. version 7 Paths: (1 available. The first part of AA:NN represents the AS number.0/8.3. In this example. If you use the keyword additive. Even if you set the community attribute.0.0 BGP routing table entry for 6.0. table Default−IP−Routing−Table) Not advertised to any peer 1 10. best Community: 6553620 Now.0. metric 0. best #1.10.0/8. an addition of 200 to the community occurs.0. Here is an example: Without the ip bgp−community new−format command in global configuration.0.1) Origin IGP.0 BGP routing table entry for 6.0. the community attribute value appears as 6553620. best #1.3 remote−as 300 neighbor 3.200. and AA:NN. The value appears as 100:20 in the output of the show ip bgp 6. table Default−IP−Routing−Table) Not advertised to any peer .1 from 10.0 command in this example: Router# show ip bgp 6.0. End with CNTL/Z.10. and the second part represents a 2−byte number. issue the ip bgp−community new−format global configuration command. 200 replaces any old community that already exits. one per line.0. you can configure communities in three different formats: decimal.3.3.

255.1) Origin IGP. You can filter BGP updates with route information as a basis. external.1 (200.10.1 from 10.0 and sends the update to RTC.10. Route Filtering In order to restrict the routing information that the router learns or advertises. you must define an access list to filter those updates and apply the access list during communication with RTA: RTC# router bgp 300 network 170. You define an access list and apply the access list to the updates to or from a neighbor.3. best Community: 100:20 BGP Case Studies 3 BGP Filtering A number of different filter methods allow you to control the send and receive of BGP updates.2.2 distribute−list 1 out access−list 1 deny 160.0 neighbor 3. or with path information or communities as a basis.200. Issue this command in the router configuration mode: neighbor {ip−address | peer−group−name} distribute−list access−list−number {in | out} In this example. All methods achieve the same results.255 access−list 1 permit 0.0.10.255 .10.10.0.0.0 0.0.200. The choice of one method over another method depends on the specific network configuration.10.0. metric 0. If RTC wants to stop the propagation of the updates to AS100. 1 10. valid.2.3. localpref 100.10.255.0.2.2.3 remote−as 200 neighbor 2. you can filter BGP with the use of routing updates to or from a particular neighbor. RTB originates network 160.0 255.255.2 remote−as 100 neighbor 2.

Assume that.0. !−−− Filter out all routing updates about 160.x. The command access−list 1 permit 160.0/9.10.0/8 only. To block the updates.0.0 so that they do not go to AS100.0. You can specify an access list on both incoming and outgoing updates with use of the BGP AS paths information.0 255.0.0.0 0.0 0.0.255. This list permits 160. In the diagram in this section.0/8.0. as well as prefix list filtering.10.255.0.255 255.0.0 0.x.0.0.0/8. The use of access lists is a bit tricky when you deal with supernets that can cause some conflicts.255.0. Your goal is to filter updates and advertise only 160.255 permits 160.0. Refer to How to Block One or More Networks From a BGP Peer for sample configurations on how to filter networks from BGP peers.0.0. Path Filtering Another type of filtering is path filtering.0.0. RTB has different subnets of 160.0. which start from the far left of the IP address.0/8.0.0.10. in the example in this section.255.0. you must use an extended access list of this format: access−list 101 permit ip 160. Issue these commands: . This address is equivalent to 160. Note: The /8 notation means that you use 8 bits of subnet mask.0. The method uses the distribute−list command with standard and extended access control lists (ACLs).0.0. you can block updates about 160. In order to restrict the update to only 160. 160.0.x. define an access list on RTC that prevents the transmit to AS100 of any updates that have originated from AS200.x. and so on.

means "any character" and the * means "the repetition of that character". instead of the use of ^200$. ip as−path access−list access−list−number {permit | deny} as−regular−expression neighbor {ip−address | peer−group−name} filter−list access−list−number {in | out} This example stops the RTC send of updates about 160.0 with path information that starts with 200 and ends with 200. ip as−path access−list 1 deny ^200$ ip as−path access−list 1 permit . in which ^ means "starts with" and $ means "ends with".10. What happens if.2 remote−as 100 neighbor 2. 200 is first and 400 is last.10. This command shows all the paths that have matched the regular expression configuration. The access list prevents the transmission of these updates to RTA.0 to RTA: RTC# router bgp 300 neighbor 3.2. Here are some examples: . you use ^200? With an AS400. When you build a regular expression.0. which is necessary to permit the transmission of all other updates.2. An example is [abcd]. you specify a string that input must match. updates that AS400 originates have path information of the form (200. • Atom An atom is a single character. AS Regular Expression This section explains the creation of a regular expression.* is another regular expression in which the . the updates match the access list. You wanted path information that comes inside updates to match the string in order to make a decision. In order to check if you have implemented the correct regular expression.* The access−list 1 command in this example forces the denial of any updates with path information that starts with 200 and ends with 200. you specify a string that consists of path information that an input must match. Since RTB sends updates about 160. In the case of BGP.2 filter−list 1 out !−−− The 1 is the access list number below. In the example in the section Path Filtering.3. as in the diagram in this section.3. 400). These updates match the access list ^200 because the path information starts with 200.2.0. issue the show ip bgp regexp regular−expression command. A regular expression comprises: • Range A range is a sequence of characters within left and right square brackets. you specified the string ^200$. In this path information.2.3 remote−as 200 neighbor 2.* represents any path information. The ^200$ in the command is a "regular expression". The access list denies these updates. So . which is not the requirement. The . A regular expression is a pattern to match against an input string.

• Piece A piece is one of these symbols. _100$ • This expression indicates an origin of AS100. which includes none. matches any single character. the start of the input string. left brace ({). Here are some examples of regular expressions: a* • This expression indicates any occurrence of the letter "a". • Branch A branch is 0 or more concatenated pieces. $ ♦ The $ matches the end of the input string. ^ ♦ The ^ matches the start of the input string. ♦ The . ab?a • This expression matches "aa" or "aba".).* . or a space. . ? ♦ The ? matches the atom or the null string. \ ♦ The \ matches the character. − ♦ The _ matches a comma (. a+ • This expression indicates that at least one occurrence of the letter "a" must be present. _100_ • This expression means via AS100. ^100 . which follows an atom: * ♦ The * matches 0 or more sequences of the atom. right brace (}). the end of the input string. + ♦ The + matches 1 or more sequences of the atom.

1 route−map setcommunity out route−map setcommunity match ip address 1 set community no−export access−list 1 permit 0.3. Note: The neighbor send−community command is necessary in order to send this attribute to RTC.10.0.1 send−community neighbor 3.3.0.1 remote−as 300 neighbor 3.255 Note: This example uses the route−map setcommunity command in order to set the community to no−export. RTC does not propagate the updates to external peer RTA. and this section provides a few examples of how to use community. BGP Community Filtering This document has covered route filtering and AS−path filtering.0.0 255.3.3.0 neighbor 3. . ^$ • This expression indicates origination from this AS.3. Refer to Using Regular Expressions in BGP for sample configurations of regular expression filtering. Another method is community filtering. The section Community Attribute discusses community. In this example.255.3. When RTC gets the updates with the attribute NO_EXPORT. you want RTB to set the community attribute to the BGP routes that RTB advertises such that RTC does not propagate these routes to the external peers. Use the no−export community attribute.255. RTB# router bgp 200 network 160. • This expression indicates transmission from AS100.

10.3.0 neighbor 3.3.3 remote−as 200 neighbor 3.255.1 remote−as 300 neighbor 3.0. RTB has set the community attribute to 100 200 additive. The weight of this route is set to 20.1 route−map setcommunity out route−map setcommunity match ip address 2 set community 100 200 additive access−list 2 permit 0. ip community−list community−list−number {permit | deny} community−number For example.0.0.1 send−community neighbor 3.3 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 this example. in certain updates with the community value as a basis.3. match−on−community: route−map match−on−community match community 10 !−−− The community list number is 10. The keyword exact states that the community consists of 200 only and nothing else.255 A community list is a group of communities that you use in a match clause of a route map. This action adds the value 100 200 to any existing community value before transmission to RTC. like weight and metric. The last community list is here to make sure . RTB sent updates to RTC with a community of 100 200.3. If RTC wants to set the weight with those values as a basis. set weight 20 ip community−list 10 permit 200 300 !−−− The community number is 200 300.In this example. RTB# router bgp 200 network 160. Any route that has only 200 as community matches list 2 and has a weight of 20. you can do this: RTC# router bgp 300 neighbor 3. You can use the community list in order to filter or set certain parameters.3. you can define this route map.3.255.3.3. In the second example in this section. The community list allows you to filter or set attributes with different lists of community numbers as a basis. any route that has 100 in the community attribute matches list 1.3.0 255.3.

0 neighbor 3. Route maps associated with the neighbor statement have no effect on incoming updates when you match based on the IP address: neighbor ip−address route−map route−map−name Assume that.3 route−map stamp in route−map stamp match as−path 1 set weight 20 .that other updates do not drop. in the diagram in this section.0. by default. Use a combination of neighbor and as−path access lists: RTC# router bgp 300 network 170. BGP Neighbors and Route Maps You can use the neighbor command in conjunction with route maps to either filter or set parameters on incoming and outgoing updates.3 remote−as 200 neighbor 3.3. The keyword internet indicates all routes because all routes are members of the Internet community. Also. Remember that anything that does not match drops.3. Refer to Using BGP Community Values to Control Routing Policy in an Upstream Provider Network for more information.10. you want to set the weight on the accepted routes to 20. you want RTC to learn from AS200 about networks that are local to AS200 and nothing else.3.3.

The statement also sets a weight of 10 for updates that are behind AS400.2. If you want to influence this decision from the AS300 end.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 .2.10. These updates are permitted. and the second one is via AS400 with path (400. AS600 picks the shortest path and chooses the route via AS100. AS100 and AS200. you must manipulate the path information in order to manipulate the BGP decision process.2 remote−as 100 neighbor 2.0 to two different ASs. Use of set as−path prepend Command In some situations.2 route−map SETPATH out route−map SETPATH set as−path prepend 300 300 .10. 200. in the diagram in the section BGP Neighbors and Route Maps. When the information is propagated to AS600. You can do this if you prepend AS numbers to the existing path information that is advertised to AS100. 300).3 remote−as 200 neighbor 3.3.10.2.3.3.2.0. the routers in AS600 have network reachability information about 150. ip as−path access−list 1 permit ^200$ Any updates that originate from AS200 have path information that starts with 200 and ends with 200.0 via two different routes.0. If all other attributes are the same.* This statement sets a weight of 20 for updates that are local to AS200. The first route is via AS100 with path (100. AS300 gets all traffic via AS100. Any other updates drop. 300).3.0. you can make the path through AS100 appear to be longer than the path that goes through AS400. Assume that you want: • An acceptance of updates that originate from AS200 and have a weight of 20 • The drop of updates that originate from AS400 • A weight of 10 for other updates RTC# router bgp 300 network 170. A common practice is to repeat your own AS number in this way: RTC# router bgp 300 network 170. and drops updates that come from AS400. The command that you use with a route map is: set as−path prepend as−path# as−path# Suppose that. RTC advertises its own network 170.0 neighbor 2.0 neighbor 3.0.10.

distribute lists.6. Members of the peer group inherit all the configuration options of the peer group. 300) that AS600 received from AS400. 1 and 2.0. The configuration defines some policies for the group. RTE.6. Also. 300. instead. you define a peer group name and assign these policies to the peer group. 300). 300. 200. BGP Peer Groups A BGP peer group is a group of BGP neighbors with the same update policies.2 peer−group internalmap neighbor 3.5.3. You can also configure members to override these options if the options do not affect outbound updates. Route maps. the . issue this command: neighbor peer−group−name peer−group This example applies peer groups to internal and external BGP neighbors: 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. AS600 receives updates about 170. and RTG.3.2 peer−group internalmap neighbor 3.3.Because of this configuration. You can only override options that are set on the inbound. such as a route map SETMETRIC to set the metric to 5 and two different filter lists. You do not define the same policies for each separate neighbor.10. and filter lists typically set update policies. RTF. This path information is longer than the (400.2 peer−group internalmap neighbor 5.5. In order to define a peer group.2 filter−list 3 in This configuration defines a peer group with the name internalmap. The configuration applies the peer group to all internal neighbors.0 via AS100 with path information of: (100.3.

you define the remote−as statements outside of the peer group because you must define different external ASs.2.1.configuration defines a separate filter list 3 for neighbor RTE. BGP Case Studies 4 CIDR and Aggregate Addresses .4. Cisco introduced the BGP Dynamic Update Peer Groups feature. you configure RTC with a peer group externalmap and apply the peer group to external neighbors. This separation improves the convergence time and the flexibility of neighbor configuration.2.2 remote−as 200 neighbor 1.4. Note: You can only override options that affect inbound updates.2 peer−group externalmap neighbor 1. refer to BGP Peer Groups. The feature introduces a new algorithm that dynamically calculates and optimizes update groups of neighbors that share the same outbound policies. Note: In Cisco IOS Software Release 12. These neighbors can share the same update messages.1. look at how you can use peer groups with external neighbors.2 peer−group externalmap neighbor 1. In earlier releases of Cisco IOS Software. Now. This method to group updates limited outbound policies and specific session configurations. Also. The BGP Dynamic Update Peer Group feature separates update group replication from peer group configuration. This filter list overrides filter list 2 inside the peer group.2. With the same diagram in this section. The feature is available in later Cisco IOS Software releases as well.4.1.2 peer−group externalmap neighbor 4. the group of BGP update messages was on the basis of peer group configurations.2 filter−list 3 in Note: In these configurations.1.2 with the assignment of filter list 3. you override the inbound updates of neighbor 1.2.1.0(24)S.1.1.4.2 remote−as 100 neighbor 2.1. Refer to BGP Dynamic Update Peer−Groups for more details. RTC# router bgp 300 neighbor externalmap peer−group neighbor externalmap route−map SETMETRIC neighbor externalmap filter−list 1 out neighbor externalmap filter−list 2 in neighbor 2. For more information on peer groups.2 remote−as 600 neighbor 4.

3.3.0.0. In this example.0.0.10. or C.0. The "16" represents the number of bits in the subnet mask.0. The command aggregate−address 160.0 to RTA. You use aggregates in order to minimize the size of routing tables. Aggregate Commands There is a wide range of aggregate commands.0.0. such as class A.0 #RTC router bgp 300 neighbor 3. Aggregation is the process that combines the characteristics of several different routes in such a way that advertisement of a single route is possible. The first command is the one from the example in the section CIDR and Aggregate Addresses: aggregate−address address−mask This command advertises the prefix route and all the more−specific routes.0.10.0. the network is a legal supernet.0 to RTA: RTB# router bgp 200 neighbor 3. Now. B.10. there is no notion of classes.0 was once an illegal class C network.0. RTB generates network 160.0 propagates an additional network 160.0.2 remote−as 100 network 170.213.0. This representation is similar to 192.0.0 but does not prevent the propagation of 160.0.0.0.2.213.3. You configure RTC to propagate a supernet of that route 160. when you count from the far left of the IP address.3 remote−as 200 neighbor 2.0.One of the main enhancements of BGP4 over BGP3 is classless interdomain routing (CIDR).255.2. 192.0. With CIDR.0 RTC propagates the aggregate address 160.1 remote−as 300 network 160.0 to .0 255. You must understand how each one works in order to have the aggregation behavior that you desire.0 255. network 192.10. For example.0.0 aggregate−address 160.3.0.0.0/16. CIDR or supernetting is a new way to look at IP addresses.213.

20. Note: If you aggregate a network that injected into your BGP via the network statement.0.10.0 as−set The section CIDR Example 2 (as−set) discusses this command.255.10. The route injection can occur via: • Incoming updates from other ASs • Redistribution of an IGP or static into BGP • The network command.10.10.0. The command suppresses all the more−specific routes.0. the network entry always injects into BGP updates.0 255.255 access−list 1 deny 0. The action allows you to be selective about which more−specific routes to suppress.0 in the BGP table. But the command suppresses advertisement with a route map basis.0 summary−only propagates network 160.0.0.0. RTB cannot generate an aggregate for 160.0 255. aggregate−address address−mask as−set This command advertises the prefix and the more−specific routes. An injection of the more−specific route into the BGP table is possible.0.0 255.255 By definition of the suppress−map. you want to aggregate 160.0.0 and 160.0. suppress the more−specific route 160.0. aggregate 129.0 to RTA. For example. The command aggregate 160. .0 0. This injection occurs even though you use the aggregate summary−only command.0 If you want RTC to propagate network 160.0.255.255.0.0.0.0 and suppresses the more−specific route 160.0.0. Use this route map: route−map CHECK permit 10 match ip address 1 access−list 1 permit 160.0.0. The outcome is the propagation of both networks 160.0.0. network 160.0. aggregate−address address−mask suppress−map map−name This command advertises the prefix and the more−specific routes.0. define a route map and apply the route map to the aggregates.0. with the diagram in the section CIDR and Aggregate Addresses.0.0 only and not the more−specific route. Note: You cannot aggregate an address if you do not have a more−specific route of that address in the BGP routing table. If you want to suppress more−specific routes when you do the aggregation.0.0.0. which is the advertisement of both the prefix and the more−specific route. But the command includes as−set information in the path information of the routing updates.0.20. there is a suppression from the updates of any packets that the access list permits.0.RTA.0.0. Suppose that.0 if RTB does not have a more−specific entry of 160.0. for example. issue this command: aggregate−address address mask summary−only This command advertises the prefix only. and allow the propagation of 160.0. The example in the section CIDR Example 1 discusses this situation.

The first solution is to use a static route and redistribute into BGP. RTC# router bgp 300 neighbor 3.2 remote−as 100 network 170.0. CIDR Example 1 Request: Allow RTB to advertise the prefix 160.0 attribute−map SETORIGIN For more information. RTB# router bgp 200 .2.Then.0.10. RTB generates both networks because RTB is the originator of 160.0. apply this route map to the aggregate attribute−map command: route−map SETMETRIC set origin igp aggregate−address 160.0. which means that AS200 is the originator of 160.10. The outcome is that RTB advertises the aggregate with an origin of incomplete (?).0.2.10. There are two solutions to this problem.0.0.0. refer to Understanding Route Aggregation in BGP.10.0.0 255.0.0 aggregate−address 160.10.0.0. The problem with this request is that network 160. at the time of the send of aggregates.3. In order to set the origin of the aggregates to IGP. even if you use the aggregate summary−only command.0. apply the route map to the aggregate statement. such as metric.3 remote−as 200 neighbor 2.0 255.0 and suppress all the more−specific routes.0.0 without the generation of an entry for 160.0 suppress−map CHECK Here is another variation: aggregate−address address−mask attribute−map map−name This command allows you to set the attributes.0.2.0.0.0 is local to AS200.0.0.0.2.3.2 remote−as 100 neighbor 2. You cannot have RTB generate a prefix for 160.

1 remote−as 300 RTA# router bgp 100 network 160.0.0.0 255. You use the aggregate as−set command in situations in which the aggregation of information causes loss of information with regard to the path attribute.0. the AS number is listed only once.1 remote−as 300 redistribute static !−−− This generates an update for 160.0.0.3. neighbor 3. in addition to the static route. you add an entry for the network command.20. regardless of how many times the AS number appeared in multiple paths that were aggregated. RTB# router bgp 200 network 160.3. except that the entry sets the origin of the update to IGP.0.0 !−−− This entry marks the update with origin IGP.20. neighbor 3. RTC gets updates about 160.0.0. RTD does not know the origin of that route. RTB# router bgp 200 network 160.0 neighbor 3. Suppose that RTC wants to aggregate network 160.0.3.0 !−−− with the origin path as "incomplete".0 from RTB.0 .0.0 mask 255.0 null0 In the second solution.0.0/8 and send the network to RTD.0. you force RTC to generate path information in the form of a set {}.0 null0 CIDR Example 2 (as−set) You use the statement as−set in aggregation to reduce the size of the path information.10.0 from RTA and updates about 160.0. In this example.0.0. ip route 160. If you add the aggregate as−set statement. This entry has the same effect.0.0.3.3. irrespective of which path came first.0.10.0.3. With as−set.1 remote−as 300 redistribute static ip route 160. That set includes all the path information.0 255.0.

0 belongs to a set {100 200}. Case 2: RTC# router bgp 300 neighbor 3.0/8 !−−− with an indication that 160.0.2.0 255.0.0.0.4.1 remote−as 300 Case 1: RTC does not have an as−set statement. RTC sends an update 160.4.2.2 remote−as 100 neighbor 4.0. To the outside world.0. The trick is to divide an AS into multiple ASs and assign the whole group to a single confederation.0. RTC# router bgp 300 neighbor 3.0.0. neighbor 2.4 remote−as 400 aggregate 160.0.0.4.0. issue this command: bgp confederation identifier autonomous−system The confederation identifier is the AS number of the confederation group.0 actually comes from two different ASs.0.0.0.0 summary−only aggregate 160.0.3. In order to configure a BGP confederation. metric.0 as−set !−−− This command causes RTC to send RTD updates about 160. In this way.0.0 255.4 remote−as 400 aggregate 160. the confederation appears to be a single AS. !−−− This may create loops if RTD has an entry back into AS100 or AS200.2 remote−as 100 neighbor 4.0. as if the route originated from AS300.3.3 remote−as 200 neighbor 2. BGP Confederation and Route Reflectors. The issue of this command performs peering between multiple ASs within the confederation: bgp confederation peers autonomous−system [autonomous−system] Here is an example of confederation: .2.0.0/8 !−−− with no indication that 160.3 remote−as 200 neighbor 2.0.2. the confederation preserves next hop.3. are for Internet service providers (ISPs) that want further control of the explosion of iBGP peering inside their ASs. Even though these ASs have eBGP peers to ASs within the confederation. BGP Confederation The implementation of BGP confederation reduces the iBGP mesh inside an AS.2.0.0 255. Each AS alone has iBGP fully meshed and has connections to other ASs inside the confederation.3.4.0/8 to RTD with path information (300).0. The next two subjects.0 summary−only !−−− This command causes RTC to send RTD updates about 160.2. the ASs exchange routing as if they used iBGP. and local preference information.

You need eight iBGP peers and one eBGP peer to external ASs. and you define the list of confederation peers with the bgp confederation peers command. Other non−BGP speakers exist also.210. AS60.1 remote−as 50 (IBGP connection within AS50) neighbor 129.5. but you only have interest in the BGP speakers that have eBGP connections to other ASs. You give the AS a confederation identifier of 500. AS60.1 remote−as 50 (IBGP connection within AS50) neighbor 128.213.212.Assume that you have an AS500 that consists of nine BGP speakers. The outside world sees only one AS.1 remote−as 60 (BGP connection with confederation peer 60) neighbor 135. you can divide AS500 into multiple ASs: AS50.20.1 remote−as 70 (BGP connection with confederation peer 70) neighbor 5. If you use confederation.213. you define a full mesh of iBGP peers. If you want to make a full iBGP mesh inside AS500.14.5. RTC# router bgp 50 bgp confederation identifier 500 bgp confederation peers 60 70 neighbor 128. AS60. AS500. For each of AS50. and AS70. or AS70.10. and RTA: Note: RTA has no knowledge of AS50.11. you need nine peer connections for each router. and AS70. RTA has only knowledge of AS500. Here is a sample configuration of routers RTC.5 remote−as 100 (EBGP connection to external AS100) RTD# . RTD.

You can relax this restriction a bit and provide additional control.5. and the neighbors at which the command points are the clients of that RR. or reflect. and RTC within AS100.1 remote−as 70 (BGP connection with confederation peer 70) neighbor 6. RTC has a partial iBGP peering with RTA and RTB.2 remote−as 60 (IBGP connection within AS60) neighbor 128.212. RTA. In this example.14. RTC can be elected as an RR.1 remote−as 50(BGP connection with confederation peer 50) neighbor 135. As the iBGP section demonstrates.6.213. maintain a full iBGP mesh between RTA. neighbor route−reflector−client The router with this command is the RR. This route reflection reduces the number of iBGP peers within an AS. In this way. Peering between RTA and RTB is not necessary because RTC is an RR for the updates that come from RTA and RTB.6 remote−as 600 (EBGP connection to external AS600) RTA# router bgp 100 neighbor 5.5. iBGP learned routes to other iBGP speakers.6.30.210. In the example. which allows a router to advertise. the RTC configuration has the neighbor route−reflector−client command that points at the RTA and RTB IP addresses. If you utilize the RR concept. Other iBGP peers of the RR that are not clients are "nonclients". In normal cases. The combination of the RR and the clients is a "cluster". and RTC form a cluster with a single RR within AS100. . RTB. router bgp 60 bgp confederation identifier 500 bgp confederation peers 50 70 neighbor 129.30.4 remote−as 500 (EBGP connection to confederation 500) Route Reflectors Another solution for the explosion of iBGP peering within an AS is Route Reflectors (RRs). RTB. a BGP speaker does not advertise a route that the BGP speaker learned via another iBGP speaker to a third iBGP speaker.

An AS can have more than one RR. In this situation, an RR treats other RRs just like any other iBGP speaker.
Other RRs can belong to the same cluster (client group) or to other clusters. In a simple configuration, you can
divide the AS into multiple clusters. You configure each RR with other RRs as nonclient peers in a fully
meshed topology. Clients should not peer with iBGP speakers outside the client cluster.

Consider this diagram. RTA, RTB, and RTC form a single cluster. RTC is the RR. For RTC, RTA and RTB
are clients and anything else is a nonclient. Remember that the neighbor route−reflector−client command
points at clients of an RR. The same RTD is the RR for clients RTE and RTF. RTG is an RR in a third cluster.

Note: RTD, RTC, and RTG are fully meshed, but routers within a cluster are not. When an RR receives a
route, the RR routes as this list shows. However, this activity depends on the peer type:

1. Routes from a nonclient peerReflects to all the clients within the cluster.
2. Routes from a client peerReflects to all the nonclient peers and also to the client peers.
3. Routes from an eBGP peerSends the update to all client and nonclient peers.

Here is the relative BGP configuration of routers RTC, RTD, and RTB:

RTC#

router bgp 100
neighbor 2.2.2.2 remote−as 100
neighbor 2.2.2.2 route−reflector−client
neighbor 1.1.1.1 remote−as 100
neighbor 1.1.1.1 route−reflector−client
neighbor 7.7.7.7 remote−as 100
neighbor 4.4.4.4 remote−as 100

neighbor 8.8.8.8 remote−as 200

RTB#

router bgp 100
neighbor 3.3.3.3 remote−as 100
neighbor 12.12.12.12 remote−as 300

RTD#

router bgp 100
neighbor 6.6.6.6 remote−as 100
neighbor 6.6.6.6 route−reflector−client
neighbor 5.5.5.5 remote−as 100
neighbor 5.5.5.5 route−reflector−client
neighbor 7.7.7.7 remote−as 100
neighbor 3.3.3.3 remote−as 100

Because there is a reflection of the iBGP learned routes, there can be a routing information loop. The RR
scheme has a few methods to avoid this loop:

• originator−idThis is an optional, nontransitive BGP attribute that is 4 bytes long. An RR creates
this attribute. The attribute carries the router ID (RID) of the originator of the route in the local AS. If,
due to poor configuration, the routing information comes back to the originator, the information is
ignored.
• cluster−listThe section Multiple RRs within a Cluster covers cluster list.

Multiple RRs within a Cluster

Usually, a cluster of clients has a single RR. In this case, the router ID of the RR identifies the cluster. In order
to increase redundancy and avoid single points of failure, a cluster can have more than one RR. You need to
configure all RRs in the same cluster with a 4−byte cluster ID so that an RR can recognize updates from RRs
in the same cluster.

A cluster list is a sequence of cluster IDs that the route has passed. When an RR reflects a route from the RR
clients to nonclients outside of the cluster, the RR appends the local cluster ID to the cluster list. If this update
has an empty cluster list, the RR creates one. With this attribute, an RR can identify if the routing information
has looped back to the same cluster due to poor configuration. If the local cluster ID is found in the cluster
list, the advertisement is ignored.

In the diagram in this section, RTD, RTE, RTF, and RTH belong to one cluster. Both RTD and RTH are RRs
for the same cluster.

Note: There is redundancy because RTH has fully meshed peering with all the RRs. If RTD goes down, RTH
takes the place of RTD.

Here is the configuration of RTH, RTD, RTF, and RTC:

RTH#

router bgp 100
neighbor 4.4.4.4 remote−as 100
neighbor 5.5.5.5 remote−as 100
neighbor 5.5.5.5 route−reflector−client
neighbor 6.6.6.6 remote−as 100

7.3 remote−as 100 neighbor 11.8.4.4 remote−as 100 neighbor 13. RR and Conventional BGP Speakers An AS can have BGP speakers that do not understand the concept of RRs.2. If you turn off BGP client−to−client reflection on the RR and you make redundant BGP peering between the clients.6.10.10 remote−as 100 neighbor 5.11. These routers can be either members of a client group or a nonclient group.2.7.2 route−reflector−client neighbor 4. Then.10 remote−as 100 neighbor 8. Important Note: This configuration does not use peer groups.5.9 remote−as 300 bgp cluster−id 10 RTD# router bgp 100 neighbor 10.6.10 remote−as 100 neighbor 4.10.1.3.7.9.3.6. neighbor 6.13.6 route−reflector−client neighbor 7.9.6.10. a potential withdrawal to the source of a route on the RR transmits to all clients inside the cluster.7.4. Do not use peer groups if the clients inside a cluster do not have direct iBGP peers among one another and the clients exchange updates through the RR.1 remote−as 100 neighbor 1.3.3.5 remote−as 100 neighbor 5. The RR scheme allows such conventional BGP speakers to coexist.1.5.13 remote−as 500 RTC# router bgp 100 neighbor 1. This transmission can cause problems.2.2.13.8.4. The existence of these routers allows easy and gradual migration from the current iBGP model to the RR model.11 remote−as 400 bgp cluster−id 10 RTF# router bgp 100 neighbor 10.7 remote−as 100 neighbor 3. .6.5.1.8 remote−as 200 Note: You do not need the bgp cluster−id command for RTC because only one RR exists in that cluster.1. This document calls these routers conventional BGP speakers.7. The router subcommand bgp client−to−client reflection is enabled by default on the RR.6.1 route−reflector−client neighbor 2.3 remote−as 100 neighbor 9. If you configure peer groups.7 remote−as 100 neighbor 3.10.11. you can create more clusters gradually.6 route−reflector−client neighbor 7.4.5 route−reflector−client neighbor 6.4 remote−as 100 neighbor 7.2 remote−as 100 neighbor 2.7 remote−as 100 neighbor 10.6 remote−as 100 neighbor 6.10. You can start to create clusters if you configure a single router as an RR and make other RRs and RR clients normal iBGP peers.10.7. you can safely use peer groups.5.

1.2.14 remote−as 400 When you are ready to upgrade RTC and make RTC an RR. Later on.1 remote−as 100 neighbor 13.5. Here is the configuration of RTD and RTC: RTD# router bgp 100 neighbor 6.1.1. only the RRs require the upgrade.5.2. RTE.3.2. RTC. You can do normal iBGP mesh between these routers and RTD.1.6.5 remote−as 100 neighbor 5.14. RTA. You cannot configure these routers as RRs.5.2.13. when you are ready to upgrade. RTD. .4.1 remote−as 100 neighbor 14.5 route−reflector−client neighbor 3.6 remote−as 100 neighbor 6.4 remote−as 100 neighbor 2.3.In this diagram. you can make RTC an RR with clients RTA and RTB. and RTB are "conventional" routers.2 remote−as 100 neighbor 1.3 remote−as 100 neighbor 2.13 remote−as 300 RTC# router bgp 100 neighbor 4. and RTF have the concept of route reflection.2 remote−as 100 neighbor 1.6.6 route−reflector−client neighbor 5.4.5. Clients do not have to understand the route reflection scheme. remove the iBGP full mesh and have RTA and RTB become clients of RTC.6.14.6.13.

suppression of the route advertisement occurs. which is a per−neighbor configuration option. The penalty decays at a granularity of 5 seconds. dampening is off by default.000. When you use nexthop−self on RRs.0 introduced route dampening. and the default is 750. this document has mentioned two attributes that you can use to prevent potential information looping: originator−id and cluster−list. In this way.Avoid Loop of Routing Information So far. . The router keeps the dampening information until the penalty becomes less than half of the "reuse limit". • suppress−value The range is 1š0. The set clause for outbound route maps does not affect routes that reflect to iBGP peers. and the default is 4 times the half−life time. If there is a need. • no bgp dampeningTurns off dampening. this feature may be given default enablement in the future. • reuse−value The range is 1š0. unsuppression of the route advertisement occurs. the router purges the information. At that point. A route that flaps gets a penalty of 1000 for each flap. Route dampening also reduces oscillation over the network. Route Flap Dampening Cisco IOS Software Release 11. and the current default is 15 minutes. • max−suppress−time This is the maximum duration for the suppression of a route. These commands control route dampening: • bgp dampeningTurns on dampening. Initially. Route dampening is a mechanism to minimize the instability that route flapping causes. You define criteria to identify poorly behaved routes. A command that sets all parameters at the same time is: • bgp dampening half−life−time reuse suppress maximum−suppress−time This list details the syntax: • half−life−time The range is 1œ5 minutes. • bgp dampening half−life−time Changes the half−life time. Once the penalty decreases below a predefined "reuse limit". route dampening avoids a higher penalty for the iBGP peers for routes external to the AS. the clause only affects the next hop of eBGP learned routes because the next hop of reflected routes should not be changed. You can also put more restrictions on nexthop−self. The penalty decays exponentially based on a preconfigured "half−life time".000. and the default is 2000. Unsuppression of the routes is at a granularity of 10 seconds. As soon as the cumulative penalty reaches a predefined "suppress limit". Route dampening does not apply to routes that are external to an AS and learned via iBGP. The range is 1š55 minutes. Another means to control loops is to put more restrictions on the set clause of outbound route maps.

208. d damped. i − internal Origin codes: i − IGP.192 interface Serial0/0 ip address 192.15.208.15.0 neighbor 192.5 255.250.0.10.2 255.10. h history.0 192.15. * valid.255.0 0 32768 i In order to simulate a route flap. > best. If you assume that the eBGP link to RTD is stable. e − EGP.5 0 0 300 i *> 203.252 router bgp 100 bgp dampening network 203.255. * valid.208.208. RTB# hostname RTB interface Serial0 ip address 203. ? − incomplete Network Next Hop Metric LocPrf Weight Path .255. local router ID is 203.208.255.208.250. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 192.208.255.10.208. local router ID is 203. > best.6 255.250.15.0 0.252 interface Serial1 ip address 192.0.10.10.6 remote−as 100 The configuration of RTB is for route dampening with default parameters.252 router bgp 300 network 192.10. e − EGP.2 Status codes: s suppressed.10.250.6 command on RTD.174 255.0 neighbor 192.10. The RTB BGP table looks like this: RTB# show ip bgp BGP table version is 24.2 Status codes: s suppressed. h history. d damped. issue the clear ip bgp 192.5 remote−as 300 RTD# hostname RTD interface Loopback0 ip address 192. the RTB BGP table looks like this: RTB# show ip bgp BGP table version is 24.250.255.10.208.255.255. i − internal Origin codes: i − IGP.15.

This placement means that you do not have a best path to the route.10.10. e − EGP. flapped 1 times in 0:02:03 The route has received a penalty for flapping.10.174) Origin IGP.0 255.10.D m.5 from 192.208. In this case.208.C. Route suppression has not yet occurred.0 192.m.10. • show ip bgp neighbor [dampened−routes] | [flap−statistics] Displays flap statistics for all paths from a neighbor.208.m.0 255.208.10.D m.15.10.0. 750.B.0 BGP routing table entry for 192.0. ? − incomplete Network Next Hop Metric LocPrf Weight Path *d 192. • clear ip bgp flap−statistics A. * valid.B. • show ip bgp flap−statistics regexp regular−expression Displays flap statistics for all paths that match the regular expression. the purge occurs when the penalty becomes 375 (750/2=375).174) Origin IGP. The default is 2000.255.208.208. These commands show and clear flap statistics information: • show ip bgp flap−statisticsDisplays flap statistics for all the paths.B.250.m. no best path) 300 (history entry) 192.10.0.208. external Dampinfo: penalty 2615. i − internal Origin codes: i − IGP.10.2 Status codes: s suppressed. or suppressed.C. no best path) 300. you see: RTB# show ip bgp BGP table version is 32.10.m Displays flap statistics for a single entry.250.0.m.15.255. the reuse value is the default. • show ip bgp flap−statistics A. flapped 3 times in 0:05:18 .5 0 0 300 i *> 203.0 BGP routing table entry for 192.C. • clear ip bgp flap−statistics filter−list list Clears flap statistics for all the paths that pass the filter.5 (192. • clear ip bgp flap−statistics regexp regular−expression Clears flap statistics for all the paths that match the regular expression.208. d damped.208.0.10.10.C. The route is reused when the penalty reaches the "reuse value". • clear ip bgp A.208.B. reuse in 0:27:00 The route has been dampened.m longer−prefixDisplays flap statistics for more specific entries.m.D m. metric 0. but information about the route flapping still exists. RTB# show ip bgp 192.0 0 32768 i RTB# show ip bgp 192. (suppressed due to dampening) 192. version 32 Paths: (1 available. > best.0 0. If the route flaps a few more times.0. external Dampinfo: penalty 910. • show ip bgp flap−statistics filter−list list Displays flap statistics for all paths that pass the filter.208.250.0 0. local router ID is 203. version 25 Paths: (1 available.5 from 192.208. • show ip bgp flap−statistics A. h history.255.208. The dampening information is purged when the penalty becomes less than half of the reuse limit.255.0 192. How BGP Selects a Path Now that you are familiar with the BGP attributes and terminology. refer to BGP Best Path Selection .m. valid. but the penalty is still below the "suppress limit". h 192. • clear ip bgp flap−statistics Clears flap statistics for all routes.10.m Clears flap statistics for a single entry.208. metric 0.10.5 0 0 300 i *> 203.D flap−statisticsClears flap statistics for all paths from a neighbor.5 (192. In this case.208.15.0 is in a history state.0 0 32768 i The BGP entry for 192.10.

BGP Case Studies 5 Practical Design Example This section contains a design example that shows the configuration and routing tables as the tables actually appear on Cisco routers. In this example. This section shows how to build this configuration step by step and what can go wrong along the way. This is the first run of the configurations for all the routers: Note: These configurations are not the final configurations. AS200 and AS300. always run iBGP within your AS in order to have better control of your routes. Whenever you have an AS that connects to two ISPs via eBGP. and OSPF runs as an IGP. iBGP runs inside AS100 between RTA and RTB.Algorithm. Assume that you connect to two ISPs. RTA# hostname RTA .

255 area 0 router bgp 100 network 203.10.252 router ospf 10 network 203.250.255.255.0.0 0.ip subnet−zero interface Loopback0 ip address 203.213.1 255.14.15.130 255.0 0.41 255.0 interface Serial1 ip address 203.250.255.15.2 255.250.255.2 remote−as 200 neighbor 203.213.10.250.2 255.255.41 remote−as 100 RTC# hostname RTC ip subnet−zero interface Loopback0 ip address 128.0 interface Ethernet0 ip address 203.255.0 neighbor 192.255.2 update−source Loopback0 RTF# hostname RTF ip subnet−zero interface Ethernet0 ip address 203.0.250.255.255 area 0 router bgp 100 network 203.13.6 255.250.213.0.13.0.252 interface Serial1 ip address 192.63.1 255.255.252 router ospf 10 network 203.0 interface Serial0 ip address 128.15.13.255.255.250.63.0 neighbor 128.250.15.250.250.252 ! .255.5 255.213.252 router ospf 10 network 203.255.63.0.250.63.255.2 remote−as 100 neighbor 203.14.255.255.255.5 remote−as 300 neighbor 203.250.208.255.0 network 203.255.0.250.14.0 0.250.255 area 0 RTB# hostname RTB ip subnet−zero interface Serial0 ip address 203.1 255.192 interface Serial2/0 ip address 128.255.255.15.208.

208.255.6 255.255.63.10.1 255.255.1 remote−as 100 neighbor 128.211.255.208.252 ! interface Serial0/1 ip address 192.5 255.255.10.10.10.208.63.1 255.interface Serial2/1 ip address 128.10.10.211.211.1 remote−as 500 RTG# hostname RTG ip subnet−zero interface Loopback0 ip address 195.10.63.200.192 interface Serial0 ip address 192.208.255.2 remote−as 400 .252 router bgp 200 network 128.255.208.10.255.2 255.255.213.0 neighbor 128.1 255.10.2 255.174 255.255.10.2 remote−as 300 neighbor 195.255.6 remote−as 400 RTD# hostname RTD ip subnet−zero interface Loopback0 ip address 192.6 remote−as 100 RTE# hostname RTE ip subnet−zero interface Loopback0 ip address 200.213.255.10.0 neighbor 128.255.252 router bgp 300 network 192.10.255.255.63.255.1 remote−as 500 neighbor 192.208.174 255.211.2 255.252 interface Serial1 ip address 128.255.252 interface Serial1 ip address 195.10.0 interface Serial0 ip address 195.0.63.213.5 remote−as 200 neighbor 195.208.252 clockrate 1000000 router bgp 400 network 200.0 neighbor 192.208.200.213.0 neighbor 192.10.255.213.213.10.211.10.252 router bgp 500 network 195.255.255.192 interface Serial0/0 ip address 192.211.

250. RTB knows how to reach the next hop 128.213. such as 203. i − internal Origin codes: i − IGP. There is no way to reach that next hop via this IGP. L2 − IS−IS level−2.0. you start with the s1 interface on RTB shutdown.0.250.13.0 203.250.13. ? − incomplete Network Next Hop Metric LocPrf Weight Path *i128.13. BGP picks one best path to reach a destination. Note: Notice the Next Hop attribute.213.0. * valid. 1 subnets C 203.10.15. RTB has not learned about 128.0 0 32768 i In this table.255 is subnetted.2 100 0 200 400 500 i *i200.15.250.15. Serial0 O 203. network 128. > best. This RTA configuration appears here: . E − EGP i − IS−IS.0.250.211. d damped.0 [110/74] via 203.13.1.213.15.63. E2 − OSPF external type 2.15. L1 − IS−IS level−1.250.255. EX − EIGRP external.0 128.208.213.213.10.255.213.213.0 128.63. S − static. This is the RTB BGP table: RTB# show ip bgp BGP table version is 4.63.63. local router ID is 203.0.41 [110/75] via 203.63. You can run OSPF on the RTA s0 interface and make it passive. • An i at the endIndicates that the origin of the path information is IGP.15. installs the path in the IP routing table. This example uses the network command to inject networks into BGP. h history.0 via a next hop of 128. Serial0 203.0 0.63.250.2 Status codes: s suppressed.213.0 is directly connected.2. 1 subnets O 203. which is OSPF.2 0 100 0 200 i *i192. is unreachable.213. M − mobile.213.63.0 128.250.63. in this way.14.2 100 0 200 400 500 300 i *i195. 128.0 128.2.250.2.213.250. For example.14.200. O − OSPF. these notations appear: • An i at the beginningIndicates that the entry was learned via an iBGP peer. 02:50:45. The first problem is that the next hop for these entries. 02:50:46.41 0 100 0 i *>203. B − BGP D − EIGRP.0 255. Serial0 Apparently. RTB knows about 128. • An > symbolIndicates that BGP has chosen the best route.2 100 0 200 400 i *>i203. IA − OSPF inter area E1 − OSPF external type 1. R − RIP.250.0 via OSPF.255.13.10.1. BGP uses the decision steps that the document BGP Best Path Selection Algorithm outlines.0 is learned via path 200 with a next hop of 128. I − IGRP. • Path informationThis information is intuitive.2. Here. has a next hop 0. Look at the IP routing table: RTB# show ip route Codes: C − connected.255.63.41 0 100 0 i *>i203.0. * − candidate default Gateway of last resort is not set 203. This method is better than a redistribution of IGP into BGP.15. and advertises the path to other BGP peers.0. none of the BGP entries has reached the routing table.0. Two problems exist here.0 255.252 is subnetted.0 203.250. e − EGP.0. Note: Any locally generated entry.250.Always use the network command or redistribute static entries into BGP to advertise networks.213. which is the eBGP next hop carried into iBGP.250. as if the link between RTB and RTD does not exist.

L1 − IS−IS level−1. 1 subnets C 203.213. which means that BGP can reach the next hop.255.255 is subnetted.213.200.213.250.255. B − BGP D − EIGRP.15. * − candidate default Gateway of last resort is not set 203.15.250. i − internal Origin codes: i − IGP.63. d damped.250. E − EGP i − IS−IS.2 update−source Loopback0 Note: You can issue the bgp nexthopself command between RTA and RTB in order to change the next hop.0.2 remote−as 100 neighbor 203.1. 1 subnets O 128.250.13.255.63.10.213.0 255.255.0 0 32768 i Note: All the entries have >.0 interface Ethernet0 ip address 203. local router ID is 203.41 0 100 0 i *> 203.255.2 100 0 200 400 500 i *>i200. E2 − OSPF external type 2.15.255 area 0 router bgp 100 network 203.1 255.15.63. M − mobile.2 0 100 0 200 i *>i192.0 203.0.0 [110/138] via 203.255.213.0 [110/74] via 203.13. EX − EIGRP external.63.0 neighbor 128.1.250.0 128.0 255.250.250.14.14. IA − OSPF inter area E1 − OSPF external type 1.0.0.250.13.250.0 128.0 0.255. Serial0 128.13.0 is directly connected.10.63.15. Serial0 O 203.255.255.0 mask 255.13.1 255.208. * valid.250. The new BGP table on RTB looks like this: RTB# show ip bgp BGP table version is 10.0.255.41 0 100 0 i *>i203. S − static.13.0 128.213. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.0.250. 1 subnets O 203.15. e − EGP.0.252 is subnetted.213.63.255. RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203.15.0 255.250. I − IGRP.63. Serial0 .0 0.0 interface Serial0 ip address 128.213.250.2 remote−as 200 neighbor 203.252 router ospf 10 passive−interface Serial0 network 203. h history. Serial0 203. 00:04:46.0 203. O − OSPF.255.250.250.15. Look at the routing table: RTB# show ip route Codes: C − connected.15.250.0 0.255 area 0 network 128.213.0.2 100 0 200 400 500 300 i *>i195.41 255.255. 00:04:46.250.250.41 [110/75] via 203. R − RIP.213.1.250.0.255.10.0.2 Status codes: s suppressed. 00:04:47.14.0 128.252 is subnetted. L2 − IS−IS level−2.250.211.255.2 100 0 200 400 i *>i203. > best.

13. Ethernet0 When you turn off synchronization in this situation.252 is subnetted.14.255. 00:01:07 B 192.213.255. B − BGP D − EIGRP. Ethernet0 128. 1 subnets C 203. EX − EIGRP external.1. In this scenario.250.255.0 [200/0] via 203.1.0 [200/0] via 128.255. RTF in the middle does not know how to reach the networks: RTF# show ip route Codes: C − connected.250. 1 subnets O 203. M − mobile.255.41 [110/11] via 203.1.255.63.255.1.13.0 is now reachable via OSPF.41 255.211.The second problem is that you still do not see the BGP entries in the routing table.63.255. the entries appear in the routing table. L1 − IS−IS level−1.0 is variably subnetted.250.0 is directly connected.213.0 255.250. Ethernet0 203.0 255. Serial1 C 203.1. E2 − OSPF external type 2.41.10. * − candidate default Gateway of last resort is not set 203.13. 00:01:08 O 128. E − EGP i − IS−IS.0 [200/0] via 128.0 255.2.200. BGP does not put these entries in the routing table and does not send the entries in BGP updates because of a lack of synchronization with the IGP. If you turn off synchronization on RTB. 00:01:07 203.0 is directly connected.0 [200/0] via 128. this is what happens: RTB# show ip route Codes: C − connected.250.0 255.15.208.0. E − EGP i − IS−IS. Serial0 128.0. I − IGRP. 2 subnets.213. 00:12:37.63.250.14.0 [110/74] via 203.0 and 195.213.15.10.15. 1 subnets C 203. Serial0 O 203.63.250.250.213.0 255. L1 − IS−IS level−1.255 is subnetted.213. 2 subnets. The only difference is that 128. but there is no way to reach those networks.213. 00:12:37.213. But you need synchronization later for other issues. IA − OSPF inter area E1 − OSPF external type 1.0 is directly connected.255. 00:01:08 203. 2 masks B 128.15.255 [110/75] via 203. * − candidate default Gateway of last resort is not set B 200. with a metric of 2000: RTA# hostname RTA .250. This problem is a synchronization issue.14. R − RIP. M − mobile.13.2.255.250.250.250.63. S − static. the problem still exists. Redistribute BGP into OSPF on RTA. L2 − IS−IS level−2. L2 − IS−IS level−2. Note: RTF has no notion of networks 192. 00:12:37.0. 00:14:15. IA − OSPF inter area E1 − OSPF external type 1.63.255.252 [110/138] via 203.252 is subnetted. O − OSPF.252 is subnetted.255.14.255. 1 subnets O 128. if you turn synchronization off. S − static. 00:01:07 B 195. But connectivity is still broken.213.208.0 [200/0] via 128. B − BGP D − EIGRP. EX − EIGRP external. E2 − OSPF external type 2.13.0 is variably subnetted.213.250. 00:14:15. O − OSPF. Serial0 The routing table looks fine.10.250.0 255.0. R − RIP.63.10. 2 masks O 203.250.13.10.15.255. Serial0 B 203.0 because you have not redistributed BGP into OSPF yet.211.15.250.0 [110/74] via 203. I − IGRP.0 255.2.255.250.2.15.

15.255. Serial0 128.1.15.10. 00:00:14.255.0 mask 255.250. 2 subnets. E2 − OSPF external type 2. bring up the RTB s1 interface to see what the routes look like.0.10.0.15.0.13. If you do not take this step.250.0 255.0.1.255. 2 masks O 203.13.255 [110/75] via 203.15.0 is variably subnetted. Also.250.213.0 interface Serial0 ip address 128.250.250.252 [110/138] via 203.250.255.1.213.0 neighbor 128. 2 subnets C 203.0 [110/74] via 203. 00:00:15. Serial0 O E2 192.15.0 is variably subnetted.14.250. you need to go the other way .250.15. Turn off synchronization on RTA so that RTA can advertise 203.208.250. Serial0 O E2 203. E − EGP i − IS−IS.250.255.0 [110/2000] via 203. Serial0 The BGP entries have disappeared because OSPF has a better distance than iBGP.250.13.14.0 [110/2000] via 203. in order to reach next hop 192.10.0 255.255.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 network 203.255. Serial0 O 203. EX − EIGRP external. 2 subnets.5.1.213.0. * − candidate default Gateway of last resort is not set O E2 200.10.0 is directly connected.1 255. ip subnet−zero interface Loopback0 ip address 203.0.63.255.13. 00:00:14.0.41 255.15.0. Serial0 203.13.250.211. O − OSPF.255. Now.250.250. 2 masks O E2 128.63.8 is directly connected.0.0 0.255.255.1.250.208.255. 00:00:16. IA − OSPF inter area E1 − OSPF external type 1.255 area 0 network 128.255. 00:00:15.15. Loopback1 C 203.2 remote−as 200 neighbor 203.63.15. R − RIP.250.0 interface Ethernet0 ip address 203.255.0 [110/2000] via 203.213.15.255.1. The OSPF distance is 110. Keep synchronization off on RTB so that RTB can advertise 203. S − static. 00:00:14.0 0. Serial0 O E2 195.1 255.0 255. routing loops occur because.250. This action is necessary because RTA does not synchronize with OSPF because of the difference in masks. B − BGP D − EIGRP.250.2 update−source Loopback0 The routing table looks like this: RTB# show ip route Codes: C − connected.208.213.Serial0 O 128.15.1.0.252 is subnetted.1.41 255.250. Serial0 203. enable OSPF on serial 1 of RTB to make it passive.15. 00:00:15.15.250.255.200. while the iBGP distance is 200. I − IGRP.10.213.0 [110/2000] via 203. L1 − IS−IS level−1.5 via IGP. This action is necessary on RTB for the same reason.250.0.255 area 0 router bgp 100 network 203.255. 00:00:15.250. This step allows RTA to know about the next hop 192.0 255. L2 − IS−IS level−2.250.2 remote−as 100 neighbor 203.0 [110/2000] via 203.255. M − mobile.15.

h history.0 network 203.14.15.0 128.10.250.255.15.10.255.250.2 remote−as 200 neighbor 203.1 255.250.255 area 0 router bgp 100 no synchronization network 203.208.213.0 192.0 0.15.41 255.250.255 area 0 network 128.13.10.10.0 0.208.2 update−source Loopback0 RTB# hostname RTB ip subnet−zero interface Serial0 ip address 203.0 interface Ethernet0 ip address 203.2 0 0 200 i *>i192.213.255.13.255.255.255.255.0 0.0 interface Serial0 ip address 128.208.250.255.250.0. local router ID is 203. e − EGP.255 area 0 network 192.41 Status codes: s suppressed.213.0.63.255.255.2 0 200 400 500 i .5 remote−as 300 neighbor 203.5 0 100 0 300 i *>i195.via eBGP.213. d damped.0 neighbor 128.255.208.255.250.0.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive−interface Serial1 network 203.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 network 203.0 192.250.0 neighbor 192.208. These are the new configurations of RTA and RTB: RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203.0.15.2 remote−as 100 neighbor 203.250.250.0.6 255.13. * valid.0 0.41 remote−as 100 The BGP tables look like this: RTA# show ip bgp BGP table version is 117.10.63.255 area 0 router bgp 100 no synchronization network 203.13.14.0.211.0.10.2 255.250. > best.63.5 100 0 300 500 i * 128.208. i −internal Origin codes: i − IGP.63.213.1 255.255.213.250.0.252 interface Serial1 ip address 192. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 128.255.0.

10.0 0 32768 i There are multiple ways to design your network to talk to the two different ISPs.0 128.250.255.213.255.2 100 0 200 400 i * 192.208. You can discover that all incoming traffic to your AS arrives via one single point.255.13.208. d damped.250.13. This configuration is the final configuration for all the routers: RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203. the same major net. even though you have multiple points to the Internet.10. In this example.10. This situation can occur if you use the same pool of IP addresses. or weight.0 0. Both RTA and RTB generate default routes into OSPF. i −internal Origin codes: i − IGP.13. Because of aggregation.2 0 100 0 i RTB# show ip bgp BGP table version is 12.250.250. you receive partial routes from AS200 and only local routes from AS300. Another potential reason for asymmetry is the different advertised path length to reach your AS. > best.213.63. you have two different major nets when you talk to the two ISPs. Potential asymmetry can occur if traffic that leaves RTA comes back via RTB.200.0.200. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.0 128.2 0 200 400 i *> 203.0 192. In the example.0 0.14.13.5 0 300 500 i *>i200. One way is to have a primary ISP and a backup ISP.250.208.15. You can try to effect that decision.1 255.41 0 100 0 i *> 203. with attributes such as local preference.10 Status codes: s suppressed.0. traffic from AS400 that has your network as the destination always comes in via RTA because of the shorter path. But. In the example.10.250.0 203.213.41 255.15.14.63. e − EGP.250.0 203.5 0 0 300 i *> 195. with RTB as the preference because of the lower metric. You can use the set as−path prepend command in order to prepend path numbers to your updates and make the path length look longer. AS200 and AS300.0.63. you can balance outgoing traffic between the two ISPs.2 0 100 0 200 i * 192.10.250.0 128.208. h history. *> 200.14.0 interface Ethernet0 ip address 203.213. AS400 can have set the exit point to be AS200.5 0 300 500 400 200 i *> 192.250. Perhaps one service provider is closer to a certain destination than another.211. * valid. In this case.0 0. Entry points to your network can occur via RTA or RTB.255.15.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 .5 0 300 500 400 i *>i203. local router ID is 203.0 192.0 interface Serial0 ip address 128.15.0. when you talk to the two ISPs.250.41 0 100 0 i *>i203. In this way.0. metric.13. there is nothing that you can do.10.0 0 32768 i *>i203.213.0. your whole AS can look like one whole entity to the outside world.0.10.0 0 32768 i *> 203. You can learn partial routes from one of the ISPs and default routes to both ISPs.255.208.63.1 255.255.0 203.250.250.10.

10.255. injection of the default information into the IGP domain occurs after redistribution of BGP into IGRP and EIGRP.0 0.255.0 area 0 . without additional configuration.252 interface Serial0 ip address 203.250.0 0.6 0.10.0.63.13.255.200. with IGRP and EIGRP.2 remote−as 200 neighbor 128.14.255 area 0 default−information originate metric 2000 router bgp 100 no synchronization network 203.255.0 0.0.15.14.0 into the IGP domain.255.0.255.250. Also.208.255.0.0.252 router ospf 10 network 203.15.6 255.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive−interface Serial1 network 203.250.0 neighbor 128.255.15.0.255. you can redistribute a static route to 0. network 200.250.0 interface Serial1 ip address 203.250.2 update−source Loopback0 ip classless ip default−network 200.255 area 0 network 128.0 route−map setlocalpref permit 10 set local−preference 200 On RTA.0 is the choice for the candidate default. the local preference for routes that come from AS200 is set to 200.255.255.63.250.0. Also in this example. The ip default−network command enables you to choose the default.252 ! interface Serial1 ip address 192.250.0.0.0.213.0.15. Also.208.255 area 0 ip classless RTB# hostname RTB ip subnet−zero interface Loopback1 ip address 203.0.0 network 203.0.0 0.2 255.0. For RIP.2 remote−as 100 neighbor 203.2 255.0.2 route−map setlocalpref in neighbor 203.250. For IGRP and EIGRP.213.1 255.255.250.200. there is an automatic redistribution into RIP of 0. This example also uses this command with Intermediate System−to−Intermediate System Protocol (IS−IS Protocol) and BGP.0.255.0. use of the default−information originate command with OSPF injects the default route inside the OSPF domain.10 255.250.255. network 203. RTF# hostname RTF ip subnet−zero interface Ethernet0 ip address 203.250.255 area 0 network 192.213.15.

d damped.255.5 255.0 192.0.252 ! interface Serial2/1 ip address 128. the local preference for updates that come from AS300 is set to 300.255 access−list 1 permit any On RTC. which comes from RTA.10.5 remote−as 300 neighbor 192.208. Here is the output of the regular expression that indicates the AS300 local routes: RTB# show ip bgp regexp ^300$ BGP table version is 14.208.130 255. Any path information that does not match ^300$ drops.2 255.41 remote−as 100 ! ip classless ip default−network 192.0/16 and indicate the specific routes for injection into AS100.208. if other routes exist.0 neighbor 128.250.211. you must filter on the incoming end of AS100.213.192 interface Serial2/0 ip address 128. i − internal Origin codes: i − IGP.255. local router ID is 203.213. default−information originate metric 1000 ! router bgp 100 no synchronization network 203.255.10.255. AS100 picks RTB for the local routes of AS300. This value is higher than the local preference value of iBGP updates that come from RTA.1 distribute−list 1 out neighbor 128.1 remote−as 100 neighbor 128.10.0.250.10. transmit internally with a local preference of 100.13.213.0. * valid.0 0.208. .63.250. If the ISP refuses to do this task.63. This value is lower than the local preference of 200.63.0 neighbor 192. use ^300_[0−9]*.0 ip as−path access−list 1 permit ^300$ route−map localonly permit 10 match as−path 1 set local−preference 300 For RTB.63.10.63.208.15. > best.255.213.213.252 router bgp 200 network 128.213. Any other routes on RTB. Note: You only advertised the AS300 local routes.10 Status codes: s suppressed.255.255. In this way.213.5 route−map localonly in neighbor 203. If you want to advertise the local routes and the neighbor routes.5 0 300 0 300 RTC# hostname RTC ip subnet−zero interface Loopback0 ip address 128. e − EGP. you aggregate 128. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 192. which are the customers of the ISP. h history.15.213.0. So RTA is the preference.63.6 remote−as 400 ip classless access−list 1 deny 195.

1 255.255.255.10.2 remote−as 400 ! ip classless access−list 1 permit 195.255.174 255. RTE# hostname RTE ip subnet−zero interface Loopback0 ip address 200.255.2 send−community neighbor 192.208.255.255.252 router bgp 500 network 195. RTD# hostname RTD ip subnet−zero interface Loopback0 ip address 192.10.255.208.192 ! interface Serial0/0 ip address 192.10.5 255.0.0 255.252 router bgp 300 network 192.0 updates toward RTD.192 interface Serial0 ip address 192.0 summary−only neighbor 192.255.0 0.10.255.10.208.208.255.10. In this way.200.10.208. in this case.208.208.208.252 ! interface Serial0/1 ip address 192.211.174 255.208. You add a no−export community to 195.255.255.10.10.0 neighbor 192.211.0.211.2 route−map setcommunity out neighbor 195.0.0.2 remote−as 300 neighbor 192.6 remote−as 100 RTG# hostname RTG ip subnet−zero interface Loopback0 ip address 195.255.10.10.10.255.211.252 interface Serial1 ip address 195.10. RTD does not export that route to RTB.252 .1 255.1 255.1 remote−as 500 neighbor 192.211.255.211.255 access−list 2 permit any route−map setcommunity permit 20 match ip address 2 ! route−map setcommunity permit 10 match ip address 1 set community no−export A demonstration of the use of community filtering is on RTG.0 aggregate−address 195.211.208.0.211.10. However.0 interface Serial0 ip address 195. RTB does not accept these routes anyway.2 255.10.255.255.255.2 255.10.

0 255.0.250. 00:41:25 C 203.0.213.250.14.250.63. Ethernet0 128. Ethernet0 C 203. Serial0 O*E2 0.200. 00:02:38 RTF# show ip route Codes: C − connected. * − candidate default Gateway of last resort is 128. L2 − IS−IS level−2. 3 subnets. * valid. i − internal Origin codes: i − IGP.63.1 remote−as 500 ip classless RTE aggregates 200.213. B − BGP D − EIGRP.2.6 255. E2 − OSPF external type 2.255.14.208.2.255. Loopback0 203.213. S − static.0.5 remote−as 200 neighbor 195. IA − OSPF inter area E1 − OSPF external type 1.0.10. 00:41:25.255. 2 masks B 128.0.0/0 [110/1000] via 203.255. R − RIP.0.0 128.0 192.255.208. E2 − OSPF external type 2. e − EGP.200.0/16.250. 00:41:25. E − EGP i − IS−IS.15. L2 − IS−IS level−2.10.252 is directly connected.2 to network 200. I − IGRP.255.0.14. and RTB: RTA# show ip bgp BGP table version is 21.208.13. L1 − IS−IS level−1.255.255 [110/75] via 203.208.0. L1 − IS−IS level−1.14.10. d damped.63.0 255.0 203.63.0 255.0 0. E − EGP i − IS−IS.250. Ethernet0 O 203. 00:41:25.0.0 0 32768 i *> 203.10.5 0 300 0 300 i *> 200.0 is variably subnetted.0. R − RIP.255.15.0.2.250. > best.41 Status codes: s suppressed.63.213.10 255.250.0 [20/0] via 128.63.0 0.63. O − OSPF.213. h history.0 192.0. Here are the final BGP and routing tables for RTA.255.10.0 255.15. B − BGP D − EIGRP.14.250.250. 2 subnets. 2 masks O E2 192.250.14.213.213.0 [110/1000] via 203.213.2.0.15. S − static. O − OSPF.255. IA − OSPF inter area E1 − OSPF external type 1.255.0/16 128.200.213.0.255. 2 subnets. 00:41:26 C 128.211.0.0.2 0 100 0 i RTA# show ip route Codes: C − connected.0. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 128. interface Serial1 ip address 128.255.252 [110/138] via 203.250. EX − EIGRP external.0 is directly connected.213.2.250.2. local router ID is 203.250. * − candidate default Gateway of last resort is 203.2.14.252 [110/74] via 203. 3 masks O 203.0 [20/0] via 128.0 is variably subnetted.0 0 32768 i *>i203. 00:41:25.213. Ethernet0 O 192.13.0 255.15.15.250.0 [200/0] via 203. M − mobile. Ethernet0/0 B* 200.0 255.2.0 is variably subnetted.255. Ethernet0 B 203.10.15.0 summary−only neighbor 128.250.200. M − mobile.255.63.0 192.255.0. EX − EIGRP external.0 255. 2 subnets.0 is variably subnetted.0 aggregate−address 200.200.208.250.252 router bgp 400 network 200. 2 masks .0.2 0 200 0 200 i *>i192.2 200 0 200 400 i *> 203.250.4 255.2 to network 0.15.200.255. RTF.13.208. I − IGRP.0 is directly connected.10.10.250.

0 203.0. d damped.14.15. Serial0 128. 2 subnets. L2 − IS−IS level−2.213.1. O E2 192.208. Loopback1 C 203.250.0. Serial0 203. 2 subnets.14.0.250. 01:12:09.250. EX − EIGRP external.255. Ethernet0 203.0 [110/2000] via 203.10.255.0 is variably subnetted.0.252 [110/128] via 203.2. Serial1 Note: The RTF routing table indicates that the way to reach networks local to AS300.250. Serial1 O 192.2.208.255.250.10 255.0 is variably subnetted. Serial0 O E2 203.0 * 192. The way to reach other known networks.0.15.15. If something happens to the connection between RTB and RTD.41 0 100 0 i *>i203.0 [110/2000] via 203. 00:03:47. R − RIP.0 is variably subnetted. Ethernet0 O E2 200.255. 2 masks O 203.0 255.250.250.0 255.255.10.0.250.13.208.0 [110/74] via 203. is through RTA. 2 subnets.0 255.0.0 [110/1000] via 203.10.1.0 is variably subnetted.15.208. B − BGP D − EIGRP. local router ID is 203.255.250.208.15.15. 2 masks O 203. 01:12:11. 01:12:09. is through RTB. Ethernet0 O*E2 0.250.255. Serial0 O 203.255.213.250.250.0.250. Serial1 C 203.255.208.2 0 200 0 200 i *> 192.213. 2 subnets.0.10.10.255.0. e − EGP. 2 subnets.213.255 [110/65] via 203.250.208. 01:20:33.13.255.250. RTB# show ip bgp BGP table version is 14. 2 subnets.4 255.0 [110/2000] via 203.41 255.2. 00:50:46 C 192. S − static.250.208.255.250.0 255.0 203.0 is directly connected. Serial1 203. 01:12:09. IA − OSPF inter area E1 − OSPF external type 1.14.0 0 32768 i RTB# show ip route Codes: C − connected.0 255. 01:20:33.250. O − OSPF. M − mobile.15.0 255.10.1.15.250.0 0.200.1.10.250.0 0. i − internal Origin codes: i − IGP.14.255 [110/75] via 203.250.0 is variably subnetted.255. 00:48:50.213.255.10.13.0.0 [110/2000] via 203.0 192.200.0 [20/0] via 192.0 [110/2000] via 203.0. 2 masks B* 192. I − IGRP.255.14.15.250.255. Serial1 203.250. 00:45:01.0 is variably subnetted.250.255.250.5. L1 − IS−IS level−1.1.250.255.14.13.13. Serial0 .2 200 0 200 400 i *>i203.0.63.255. the default that RTA advertises kicks in with a metric of 2000.10.250.255.14. 01:15:40.0. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.252 [110/74] via 203. Serial1 C 203.5 to network 192. such as 192.15.0 is directly connected.13.250. such as 200.250.250.1.10 Status codes: s suppressed.255.0. Ethernet0 O 128.250.63.208.41 0 100 0 i *> 203.15.13.1.208.255.14. Ethernet0 O E2 203.250.15.213. Ethernet0 128. 2 masks O 203. * − candidate default Gateway of last resort is 192. 2 masks O E2 128.15.10.255.13.255.15.0 255.255. > best.252 is subnetted. 2 subnets C 203.213.13. E − EGP i − IS−IS. 2 masks O E2 128.0/16 128.1.255.0 [110/1000] via 203. 01:12:09.2. h history.41 255.15.255 [110/11] via 203. The gateway of last resort is set to RTB.252 is directly connected.0 255.0.255.213.250.15.0. E2 − OSPF external type 2.5 0 300 0 300 i *>i200.0 255. 00:03:33.208. * valid.0.4 255.1.8 is directly connected.0 255.10. 00:46:55.0.0 128.252 is directly connected.200.63.

1.0/0 [110/2000] via 203.255.0. NetPro Discussion Forums − Featured Conversations for RP Service Providers: MPLS Virtual Private Networks: Services Virtual Private Networks: Security Related Information • BGP: Frequently Asked Questions • Sample Configurations of BGP Across a PIX Firewall • How to Use HSRP to Provide Redundancy in a Multihomed BGP Network • Configuring Single Router Mode Redundancy and BGP on a Cat6000 MSFC • Achieve Optimal Routing and Reduce BGP Memory Consumption • Troubleshooting BGP • Troubleshooting High CPU Caused by the BGP Scanner or BGP Router Process • Load Sharing with BGP in Single and Multihomed Environments: Sample Configurations • BGP Support Page • Technical Support & Documentation − Cisco Systems All contents are Copyright © 2006−2007 Cisco Systems.15. Updated: Feb 13. suggestions. Inc. 00:08:33.255. Serial0 NetPro Discussion Forums − Featured Conversations Networking Professionals Connection is a forum for networking professionals to share questions.15. products. All rights reserved. Serial0 O E2 200.250. Important Notices and Privacy Statement. 01:20:34.250. 2008 Document ID: 26634 . Serial0 O*E2 0.0 255. and information about networking solutions.0 [110/2000] via 203.252 [110/138] via 203.0.63.255. O 128.1.15.213.200.0 255.0. 00:05:42. and technologies.1. The featured links are some of the most recent conversations available in this technology.0.250.