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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

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

0 in queue Sent 2826 messages. The remote AS number points to either an external or an internal AS. RTA# router bgp 100 neighbor 129. Note: Pay special attention to the BGP state. Also. 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. Normally. and loopback does not apply. 0 notifications. RTA and RTB run eBGP. external link BGP version 4. issue this command: neighbor ip−address update−source interface . dropped 10 BGP and Loopback Interfaces The use of a loopback interface to define neighbors is common with iBGP. which indicates either eBGP or iBGP. notice these items: • The BGP version. which is 4 • The remote router ID This number is the highest IP address on the router or the highest loopback interface. If you use the IP address of a loopback interface in the neighbor command. RTB and RTC run iBGP.1.220. iBGP routers do not need to have direct connection.220.1 BGP state = Established. remote AS 200. But. Note: Also. remote router ID 175.220.1 remote−as 200 In this example. A version that continues to increment indicates that there is some route flap that causes the continuous update of routes. keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 2828 messages.1. Any time that new information comes in.2 remote−as 200 RTC# router bgp 200 neighbor 175.212.1. In the case of eBGP. This section provides an example of the information that the show ip bgp neighbors command displays. 0 in queue Connections established 11. table version = 3. the table increases the version.1.213. 0 notifications. • The table version The table version provides the state of the table. there must be some IGP that runs and allows the two neighbors to reach one another. you need some extra configuration on the neighbor router. hold time is 180.1.213.12. but is not common with eBGP. the eBGP peers have direct connection. up for 0:10:59 Last read 0:00:29. peer routers frequently have direct connection. # show ip bgp neighbors BGP neighbor is 129.2 remote−as 100 neighbor 175. In order to indicate a loopback interface. if existent. but the iBGP peers do not have direct connection.1 remote−as 200 RTB# router bgp 200 neighbor 129. Anything other than the state Established indicates that the peers are not up.213. 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.

1 remote−as 100 neighbor 190.1.This example illustrates the use of this command: RTA# router bgp 100 neighbor 190.212.1 update−source loopback 1 RTB# router bgp 100 neighbor 150.11. In order to force this action. RTA must force BGP to use the loopback IP address as the source in the TCP neighbor connection.1. RTA and RTB run iBGP inside AS100.225.225. Note: RTA has used the physical interface IP address of RTB. eBGP Multihop In some cases. This statement forces BGP to use the IP address of the loopback interface when BGP talks to neighbor 190. 150. you can use eBGP multihop.11.1 remote−as 100 In this example. The eBGP multihop allows a neighbor connection between two external peers that do not have direct connection. The multihop is only for eBGP and not for iBGP.1. RTB uses the loopback interface of RTA. RTA adds update−source interface−type interface−number so that the command is neighbor 190. Refer to Sample Configuration for iBGP and eBGP With or Without a Loopback Address for a complete network scenario sample configuration.225.225.212.1 update−source loopback 1. This example illustrates eBGP multihop: RTA# .1. To achieve the connection.11.11. 190. Use of this IP address is why RTB does not need any special configuration. a Cisco router can run eBGP with a third−party router that does not allow direct connection of the two external peers.11. In the neighbor command. In this case.225. as a neighbor.1.

1 255.0.2 and the other path via 2. On the other hand.10.10. which is 129.255. .1 ebgp−multihop neighbor 160.1 ebgp−multihop network 160.1.1. You use static routes. eBGP Multihop (Load Balancing) RTA# int loopback 0 ip address 150.213.0.10.1 ip route 150.10. and ebgp−multihop.1.0 255.0 1.10. to introduce two equal−cost paths to reach the destination.2.1: one path via 1.2.2.10. In normal situations. RTB does not need the ebgp−multihop command.2.10.2.0 2.1 255.10.1 This example illustrates the use of loopback interfaces. or an IGP.10.255.255.0 ip route 150.0.255.1 update−source loopback 0 network 150.1. RTB has the same choices. RTA needs to indicate its use of the ebgp−multihop command.1 update−source loopback 0 neighbor 150.1.1.1.2.0 router bgp 200 neighbor 150.0.2 remote−as 100 RTA indicates an external neighbor that does not have direct connection.1.1.255.1. Because of this direct connection.255.0 router bgp 100 neighbor 160.225.10.10.10.2 ip route 160. 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.0 1.0 255.1 remote−as 100 neighbor 150. BGP picks one of the lines on which to send packets. router bgp 100 neighbor 180.1.255.2.1.1. RTA has two choices to reach next hop 160.0 ip route 160.0 255.1. With the introduction of loopback interfaces.0.0.213.11.0.10.1 remote−as 200 neighbor 160.1 ebgp−multihop RTB# router bgp 300 neighbor 129. the next hop for eBGP is the loopback interface.1. update−source.0 2. and load balancing does not happen.225. The example is a workaround in order to achieve load balancing between two eBGP speakers over parallel serial lines.0. You should also configure an IGP or static routing to allow the neighbors without connection to reach each other.1 remote−as 300 neighbor 180.1.1.2 RTB# int loopback 0 ip address 160.255.0.0.10. RTB indicates a neighbor that has direct connection.0 255.2.10.11.

with the name MYMAP. This next−instance check continues until you either break out or finish all the instances of the route map. there is no redistribution or control of the route. A filter on the outbound is acceptable. there is a redistribution or control of the routes.1.1. The control and modification of routing information occurs through the definition of conditions for route redistribution from one routing protocol to another. These commands illustrate the example: match ip address 1. if the match criteria are met and you have a permit.2 and later releases do not have this restriction. If you finish the list without a match. The related commands for match are: • match as−path • match community . If the match criteria are met and you have a deny. you can define a route map that checks outgoing updates. and set specifies a set action if the criteria that the match command enforces are met. If the match criteria are not met and you have a permit or deny. 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. Or the control of routing information can occur at injection in and out of BGP. If there is a match for IP address 1. the route is not accepted nor forwarded.1. In Cisco IOS® Software releases earlier than Cisco IOS Software Release 11.1 set metric 5 Now. when you use route maps to filter BGP updates rather than redistribute between protocols. • route−map MYMAP permit 10 (The first set of conditions goes here. In the BGP context. For example. and the second has a sequence number of 20. the next instance of the route map is checked. Cisco IOS Software Release 11.1. The first instance has a sequence number of 10. For example.Route Maps There is heavy use of route maps with BGP. the first set of conditions are applied via instance 10. as the set action specifies. You break out of the list.1. If the first set of conditions is not met. the route map is a method to control and modify routing information. You can define multiple instances of the same route map.2. In this example. there are two instances of the route map defined. you cannot filter on the inbound when you use a match command on the IP address. or the same name tag. The match specifies a match criteria.) • route−map MYMAP permit 20 (The second set of conditions goes here. you proceed to a higher instance of the route map.) When you apply route map MYMAP to incoming or outgoing routes. 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. match and set Configuration Commands Each route map consists of a list of match and set configuration commands. You break out of the list. instance 20 is checked. the metric for that update is set to 5.

• 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: .

0.10.0 route−map SETMETRIC permit 10 match ip−address 1 set metric 2 route−map SETMETRIC permit 20 set metric 5 access−list 1 permit 170.0.0.2.0.10. There are multiple ways to send network information with use of BGP. you break out of the route map list.0.0 network 2.255 Now that you feel more comfortable with how to start BGP and how to define a neighbor.0.2. by default.10.10. These sections go through the methods one by one: . RTA gets updates via BGP and redistributes the updates to RIP. Suppose that RTA wants to redistribute to RTB routes about 170. you can use this configuration: RTA# router rip network 3. Note: Always ask the question "What happens to routes that do not match any of the match statements?" These routes drop.0. the route has a metric of 2.0. If there is no match. in Example 1.0 neighbor 2.0 0. Therefore.10.10.255. look at how to start the exchange of network information.255. You cannot apply route maps on the inbound when you match with an IP address as the basis.0. if a route matches the IP address 170.0. In this case.Example 1 Assume that RTA and RTB run Routing Information Protocol (RIP).0.0. you proceed down the route map list.2.255 In this example.0 passive−interface Serial0 redistribute bgp 100 route−map SETMETRIC router bgp 100 neighbor 2.0.0.2. which indicates setting everything else to metric 5.2. Example 2 Suppose that.0.255 access−list 1 permit 0.2.2 route−map STOPUPDATES out route−map STOPUPDATES permit 10 match ip address 1 access−list 1 deny 170.255.0 0.0.0 with a metric of 2 and all other routes with a metric of 5. you must use an outbound route map on RTC: RTC# router bgp 300 network 170.0 255.10.0 network 150. and RTA and RTC run BGP.2 remote−as 100 neighbor 2. you do not want AS100 to accept updates about 170.0.3 remote−as 300 network 150. Then.0.10.255.

Redistribution The network command is one way to advertise your networks via BGP. The command uses a mask portion because BGP version 4 (BGP4) can handle subnetting and supernetting. The network command works if the router knows the network that you attempt to advertise.255.255. A maximum of 200 entries of the network command are acceptable.213.213. you do not try to run BGP on a certain interface.0.0 and RTC announces 175. Here is an example: RTA announces 129. Another way is to redistribute your IGP into BGP. The /16 indicates that you use a supernet of the class C address and you advertise the first two octets. static.0/16. Note: You need the static route to get the router to generate 192. With this command.0.0. or another protocol.0 null 0 This example indicates that router A generates a network entry for 192.213.0 ip route 192. • 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.0 mask 255.0 255. This concept is different than the familiar configuration with Interior Gateway Routing Protocol (IGRP) and RIP. or learned dynamically. some of these routes can have been learned via BGP and you do not need to send them out again. 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.0. you try to indicate to BGP what networks BGP should originate from this box.0.213.1.0. RIP.0. An example of the network command is: RTA# router bgp 1 network 192. Enhanced Interior Gateway Routing Protocol (EIGRP). Look at the RTC configuration: .213.0 because the static route puts a matching entry in the routing table.0. Your IGP can be IGRP.220. or first 16 bits. Instead. Open Shortest Path First (OSPF) protocol. This redistribution can seem scary because now you dump all your internal routes into BGP. whether connected.

220.1.1 remote−as 300 network 175.220.220.0.0. you have: RTC# router eigrp 10 network 175. The correct configuration is: RTC# router eigrp 10 network 175.220.0 mask 255.255.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 .1.213.0. you have: RTC# router eigrp 10 network 175.0.1.1. AS100 is the source. You are not the source of 129.213.1.213.0.If you issue the network command.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1.0 !−−− This limits the networks that your AS originates to 175.0.1.220. This redistribution causes the origination of 129.0.1.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1.1 remote−as 300 redistribute eigrp 10 !−−− EIGRP injects 129. If you use redistribution instead.0 again into BGP.0 by your AS. So you have to use filters to prevent the source out of that network by your AS.0.

either internal or external.0 255. Here is an example: .220...1.1 remote−as 300 redistribute static .0.0. router bgp 200 neighbor 1.1.0.0. Otherwise. The difference is that routes that generate from the network command.1.255.1. the router disregards the packet..0 0.255 You use the access−list command to control the networks that originate from AS200.0 null0 .1 remote−as 300 neighbor 1. or static indicate your AS as the origin of these networks. The only difference is that BGP considers these routes to have an origin that is incomplete.220. Refer to Understanding Redistribution of OSPF Routes into BGP for more details. external. So if you get the packet and there is a more specific match than 175. the router sends the packet to the specific match.0.. or unknown. Redistribution of OSPF into BGP is slightly different than redistribution for other IGPs. Static Routes and Redistribution You can always use static routes to originate a network or a subnet. redistribution. BGP passes on information that BGP learns from one peer to other peers. ip route 175.0 redistribute bgp 200 default−metric 1000 100 250 100 1500 router bgp 200 neighbor 1.0. and nssa−external are necessary to redistribute respective routes. The null0 interface means disregard the packet. which exists. Specific keywords such as internal. This method is a nice way to advertise a supernet.1 distribute−list 1 out redistribute eigrp 10 access−list 1 permit 175. This document has discussed how you can use different methods to originate routes out of your AS. Redistribution is always the method for injection of BGP into IGP. You can accomplish the same result that the example in the Redistribution section accomplished with this: RTC# router eigrp 10 network 175. The simple issue of redistribute ospf 1 under router bgp does not work.1.1.220.255.255. Remember that these routes are generated in addition to other BGP routes that BGP has learned via neighbors.220..

0. Is it true that you can do the same thing by learning via eBGP.0.10.10.0.10.1 remote−as 200 network 170.2 remote−as 300 network 150. the difference is that the network command adds an extra advertisement for these same networks.0.10. but iBGP offers more flexibility and more efficient ways to exchange information within an AS.10.10. For example. For example. which indicates that AS300 is also an origin for these routes.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. Again.20. iBGP provides ways to control the best exit point out of the AS with use of local preference. assume that AS200. Note: Remember that BGP does not accept updates that have originated from its own AS.10. RTA generates a route 150.10.20. .00 Note: You do not need network 150. iBGP You use iBGP if an AS wants to act as a transit system to other ASs. RTA# router bgp 100 neighbor 150.10. RTC passes this route to AS200 and keeps the origin as AS100.10.0 and sends the route to AS300. The section Local Preference Attribute provides more information about local preference. from the example in this section.0 or network 160.0.10.0 RTB# router bgp 200 neighbor 160. has a direct BGP connection into AS100. RTB passes 150. redistributing into IGP.0 RTC# router bgp 300 neighbor 150. Then.0 to AS100 with the origin still AS100. This refusal ensures a loop−free interdomain topology. and then redistributing again into another AS? Yes. RTA notices that the update has originated from its own AS and ignores the update.1 remote−as 100 neighbor 160.20.0.20.2 remote−as 300 network 160.

which is inside the AS. make an iBGP peering between RTB and RTD in order to not break the flow of the updates. BGP chooses only a single path to reach a specific destination. such as next hop. administrative weights.30.0 RTB# router bgp 100 neighbor 150.10. In the diagram in this section. .20. The BGP updates that come from RTB to RTA transmit to RTE.10. origin code.2 remote−as 300 network 150.50. the protocol must choose paths to reach a specific destination. The BGP Decision Algorithm After BGP receives updates about different destinations from different autonomous systems. RTA and RTD also run iBGP.10. BGP bases the decision on different attributes. and other attributes. Therefore. which is outside the AS. The BGP speaker that receives the update redistributes the information to other BGP speakers outside of its AS.1 remote−as 400 network 190. RTA and RTB run iBGP.1 remote−as 100 neighbor 170. RTA# router bgp 100 neighbor 190.0.40.50.0. sustain a full mesh between the iBGP speakers within an AS. Therefore.40.10. metric.1 remote−as 100 neighbor 175.10. path length.0 RTC# router bgp 400 neighbor 175.10. local preference. The updates do not transmit to RTD.0 Note: Remember that when a BGP speaker receives an update from other BGP speakers in its own AS (iBGP).10. route origin.2 remote−as 100 network 175.10. the BGP speaker that receives the update does not redistribute that information to other BGP speakers in its own AS.

0 reaches RTA. the path to reach 190.0. the network has two AS numbers attached: first 200. BGP Case Studies 2 AS_PATH Attribute Whenever a route update passes through an AS. RTB advertises network 190.10. INCOMPLETE usually occurs when you redistribute routes from other routing protocols into BGP and the origin of the route is incomplete. RTC appends its own AS number to the network.0. The same process applies to 170.0 and path (100) in order to reach 170.0. An e in the BGP table indicates EGP. • INCOMPLETENLRI is unknown or learned via some other means. In the example in this section. An i in the BGP table indicates IGP. Origin Attribute The origin is a mandatory attribute that defines the origin of the path information. the AS number is prepended to that update. RTB has to take path (300. An AS_SET is an ordered mathematical set {} of all the ASs that have been traversed.0. For RTA.0. The AS_PATH attribute is actually the list of AS numbers that a route has traversed in order to reach a destination.0 in AS200. Refer to BGP Best Path Selection Algorithm for more information.0 is (300.0.10.10.BGP always propagates the best path to the neighbors. The origin attribute can assume three values: • IGPNetwork Layer Reachability Information (NLRI) is interior to the AS of origination. When that route traverses AS300. The section BGP Case Studies 2 explains these attributes and their use.0. The CIDR Example 2 (as−set) section of this document provides an example of AS_SET.0 and 180.10. then 300. This normally happens when you issue the bgp network command . RTB traverses AS300 and then AS100 in order to reach 170.0. 200).10. RTC has to traverse path (200) in order to reach 190. • EGPNLRI is learned via exterior gateway protocol (EGP).0. So when 190. 100).0.10.0. .10.10. An ? in the BGP table indicates INCOMPLETE.

0 via 300 i. RTE also reaches 190.0 via 100 ?.50.50. The "100 ?" means that the next AS is 100 and that the origin is incomplete and comes from a static route.0 255.10.0 via 100 i. This "i" means that the entry is in the same AS and the origin is IGP. The "300 i" means that the next AS path is 300 and the origin of the route is IGP.10.2 remote−as 300 network 150.0.1 remote−as 100 network 190. RTA# router bgp 100 neighbor 190. The "100 i" means that the next AS is 100 and the origin is IGP.10.0 redistribute static ip route 190.0 RTE# router bgp 300 neighbor 170.10.20.10.0.0.0.10.1 remote−as 100 network 170.10.20.255. RTE reaches 150.10.10. RTA also reaches 190.10.0.1 remote−as 100 neighbor 170.30.0 via i.50.0.0 RTA reaches 170. BGP Next Hop Attribute .0 null0 RTB# router bgp 100 neighbor 150.10.0.10.

0. RTC advertises 170.10. Note: RTA advertises 170.0.20.10.20.20. the next hop to reach 170. the next hop is always the IP address of the neighbor that the neighbor command specifies. In the example in this section. Because of this rule. RTA# router bgp 100 neighbor 170.2. Otherwise.10. The eBGP next hop is carried in iBGP. Make sure that RTB can reach 170.30. you can also run iGRP on RTA network 170.10.0 because the next hop address is inaccessible.20.10.1.10.0 Note: RTC advertises 170.10.10. For eBGP.2 and not 150.20.0 to RTA with a next hop of 170.10. RTB drops packets with the destination of 170.20.10.10.0.20.0.20.10.10.0.0 is 170. RTA advertises 170.0.10.2 remote−as 300 neighbor 150.2 via IGP.0.0.0 to RTA with a next hop equal to 170.10. . For iBGP.10. RTA advertises 150. if RTB runs iGRP.2.0.0 to RTB with a next hop equal to 170.10.50.0. So.10.10.0 to RTC with a next hop of 170.30. For example.1 remote−as 100 network 150.0 to its iBGP peer RTB with a next hop of 170.1 remote−as 100 network 170.10.1 remote−as 100 RTC# router bgp 300 neighbor 170.10. according to RTB.1.The BGP next hop attribute is the next hop IP address to use in order to reach a certain destination.10.0. the protocol states that the next hop that eBGP advertises should be carried into iBGP.2.20. You want to make iGRP passive on the link to RTC so that BGP is only exchanged.0 RTB# router bgp 100 neighbor 150.2.

RTC uses as next hop 170.10.0.3. Assume that RTC and RTD in AS300 run OSPF.0. and RTD is a multiaccess network. but NBMA.0 to RTA with a next hop 170. The sections BGP Next Hop (Multiaccess Networks) and BGP Next Hop (NBMA) provide more details. Note: RTC advertises 180.0. RTC can reach network 180.20.2.10. and RTD is not multiaccess. RTC does not use its own IP address.3.3. further complications occur.0 via 170. BGP Next Hop (Multiaccess Networks) This example shows how the next hop behaves on a multiaccess network such as Ethernet.0 is more sensible than the extra hop via RTC. RTC.10. RTC runs BGP with RTA.20. RTC uses this address because the network between RTA. The RTA use of RTD as a next hop to reach 180.20.20.10.0.20.20.20. RTC. BGP Next Hop (NBMA) .0. 170. When RTC sends a BGP update to RTA with regard to 180.Take special care when you deal with multiaccess and nonbroadcast multiaccess (NBMA) networks.20. If the common medium to RTA.

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. routing fails. The next−hop−self command remedies this situation.10. For the BGP Next Hop (NBMA) example.20. RTC advertises 180. The problem is that RTA does not have a direct permanent virtual circuit (PVC) to RTD and cannot reach the next hop.3. In this case.1 remote−as 100 neighbor 170.20.The common medium appears as a cloud in the diagram.0.20.20.0 with a next hop equal to 170. you can use the next−hop−self command.10.0 to RTA with a next hop of 170. this configuration solves the problem: RTC# router bgp 300 neighbor 170. If the common medium is a frame relay or any NBMA cloud. next−hop−self Command For situations with the next hop.0.20.10. the exact behavior is as if you have connection via Ethernet.2.10. .20. as in the BGP Next Hop (NBMA) example.1 next−hop−self RTC advertises 180.

. or another protocol.0. If you want RTA to learn about 160. eBGP updates have a distance of 20. Note: This change is not recommended.BGP Backdoor In this diagram.0. The default distances are: • 120 for RIP • 100 for IGRP • 90 for EIGRP • 110 for OSPF RTA receives updates about 160.10. IGRP. RTA and RTB run some kind of IGP.0 via RTB (IGP).0 via two routing protocols: • eBGP with a distance of 20 • IGP with a distance that is greater than 20 By default. then you have two options: • Change the external distance of eBGP or the IGP distance. RTB and RTC run eBGP. 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. • Use BGP backdoor. By definition. either RIP.10. RTA and RTC run eBGP. which is less than the IGP distances.

look at this scenario. EIGRP is the preference. so RTB gets the update and is able to reach 170.10. but because of the backdoor command. Remember that the next hop is carried via iBGP.0.0 via next hop 2.2.2. RTA learns 160.0. this network gets the same treatment as a locally assigned network.0 from RTB via EIGRP with distance 90.2.0.10. For BGP.0. RTB must send the traffic to RTE.0 backdoor Network 160. RTC in AS300 sends updates about 170.1 remote−as 300 network 160. Normally eBGP is the preference. . Issue the network address backdoor command.1.BGP backdoor makes the IGP route the preferred route.10.0 is treated as a local entry. In order to reach the next hop. The configured network is the network that you want to reach via IGP. RTA also learns the address from RTC via eBGP with distance 20.10.2.0. except BGP updates do not advertise this network.0. RTA and RTB run iBGP. Synchronization Before the discussion of synchronization.0.0 router bgp 100 neighbor 2.10. RTA# router eigrp 10 network 150.10. but is not advertised as a normal network entry.

10. Make sure that other routers can reach 170. BGP advertises the route to external peers. If you do not pass traffic from a different AS through your AS. . you can disable synchronization. RTB waits to hear about 170. You can make RTB think that IGP has propagated the information if you add a static route in RTB that points to 170.0 via IGP.0. Then.0. 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. BGP waits until IGP has propagated the route within the AS. In the example in this section.0 flows in and drops at RTE. you do not need synchronization.Assume that RTA has not redistributed network 170.0.0 into IGP.0 even exists.10.10. if your AS passes traffic from another AS to a third AS. RTE has no idea that 170.0. BGP should not advertise a route before all the routers in your AS have learned about the route via IGP.0. Your router waits indefinitely for an IGP update about a certain route before the router sends the route to external peers. Then. Synchronization states that. The disablement of synchronization is not automatic. the router has no way to know.10. You can also disable synchronization if all routers in your AS run BGP.0.0.0.0. The disablement of this feature can allow you to carry fewer routes in your IGP and allow BGP to converge more quickly. RTB starts to send the update to RTD. Disable Synchronization In some cases. traffic that comes from RTD to RTB with destination 170.10.10. If RTB starts to advertise to AS400 that RTB can reach 170. At this point.10. If all your routers in the AS run BGP and you do not run IGP at all.

0.0.2 remote−as 400 neighbor 3. RTD# router bgp 400 neighbor 1.1.3.0.3.1. even if RTB does not have an IGP path to 170.1.10.0 neighbor 3.10.10.3. RTB# router bgp 100 network 150.0 in its IP routing table and advertises the network !−−− to RTD.0.0 neighbor 1.0.4 remote−as 100 Weight Attribute .0 RTA# router bgp 100 network 150.1 remote−as 100 network 175.1.3.0.3 remote−as 100 no synchronization !−−− RTB puts 170.10.10.

0.2.1 remote−as 100 neighbor 1. The value is not propagated or carried through any of the route updates. .0 from RTB has a 100 weight.The weight attribute is a Cisco−defined attribute. Paths that the router originates have a weight of 32.1.2.0 from RTA has a 200 weight.10. The value only makes sense to the specific router.10. and other paths have a weight of 0.0 from AS4. This attribute uses weight to select a best path.0.0 from AS4.10. 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. ♦ neighbor {ip−address | peer−group} weight weight • Use AS_PATH access lists. Look at the example in this section.0. Multiple methods achieve this weight set: • Use the neighbor command. Routes with a higher weight value have preference when multiple routes to the same destination exist. ♦ 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. neighbor 2. which has a higher weight value. The weight is assigned locally to the router.2.0.1.2 remote−as 200 neighbor 2. has preference as the next hop. you force RTC to use RTA as a next hop to reach 175.1 weight 200 !−−− The route to 175. RTC# router bgp 300 neighbor 1.10.2. A weight can be a number from 0 to 65. RTB propagates the update to RTC.1.10.0. You can achieve the same outcome with IP AS_PATH and filter lists. RTA propagates the update to RTC.1. RTA. RTC now has two ways to reach 175.0.10.0.768 by default.2 weight 100 !−−− The route to 175.0 and has to decide which way to go. RTA has learned about network 175. RTB has also learned about network 175.535.

2.1..2 remote−as 200 neighbor 2.2 remote−as 200 neighbor 2.1.1.2.1.1. Local Preference Attribute .2 filter−list 6 weight 100 ... route−map setweightin permit 20 set weight 100 !−−− Anything else has weight 100.1 filter−list 5 weight 200 neighbor 2..1 remote−as 100 neighbor 1.1.2. ip as−path access−list 5 permit ^100$ !−−− This only permits path 100. ip as−path access−list 6 permit ^200$ .. RTC# router bgp 300 neighbor 1.1 route−map setweightin in neighbor 2. ip as−path access−list 5 permit ^100$ .2 route−map setweightin in .1. route−map setweightin permit 10 match as−path 5 set weight 200 !−−− Anything that applies to access list 5.2. RTC# router bgp 300 neighbor 1...1 remote−as 100 neighbor 1.2.1. has weight 200. such as packets from AS100.2.2..2. You also can achieve the same outcome with the use of route maps.

RTC sets the local preference of all updates to 150. 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. The same RTD sets the local preference of all updates to 200. local preference is an attribute that routers exchange in the same AS. Local preference helps you determine which way to exit AS256 in order to reach that network.1 remote−as 256 bgp default local−preference 200 In this configuration. A path with a higher local preference is preferred more.0.3.213. You can also set local preference with route maps.Local preference is an indication to the AS about which path has preference to exit the AS in order to reach a certain network. 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 1.3.0.4 remote−as 300 neighbor 128. In the diagram in this section.0 from two different sides of the organization.2 remote−as 256 bgp default local−preference 150 RTD# router bgp 256 neighbor 3.0 has a higher local preference when updates come from AS300 rather than from AS100. You set local preference with the issue of the bgp default local−preference value command.1.213. Therefore. The default value for local preference is 100. both RTC and RTD realize that network 170. which is only relevant to the local router.1 remote−as 100 neighbor 128.11.10.1. Assume that RTD is the exit point preference.10.11. AS256 receives updates about 170. All traffic in AS256 that has that network as a destination transmits with RTD as an exit . Unlike the weight attribute. There is an exchange of local preference within AS256.

This tag can be unnecessary. Updates that come from AS34 also are tagged with the local preference of 200. such as updates that come from AS34. all updates that RTD receives are tagged with local preference 200 when the updates reach RTD. Metric Attribute .. For this reason.3. have a value of 150.point.1 remote−as 256 .3. In the example in this section. The use of route maps provides more flexibility.213.11.4 route−map setlocalin in neighbor 128. ip as−path access−list 7 permit ^300$ . you can use route maps to specify the specific updates that need to be tagged with a specific local preference. any update that comes from AS300 has a local preference of 200...3.3.4 remote−as 300 neighbor 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. Here is an example: RTD# router bgp 256 neighbor 3.. Any other updates..

2. RTD.. The bgp always−compare−med command is useful when multiple service providers or enterprises agree on a uniform policy for how to set MED. RTA chooses RTC as the best next hop because 120 is less than 200. Assume that you have set the metric that comes from RTC to 120.4. In order to force RTA to compare the metrics. A metric is carried into an AS but does not leave the AS.10. Note: There are two BGP configuration commands that can influence the multi−exit discriminator (MED)−based path selection. When an update enters the AS with a certain metric. 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. RTA must choose based on some other attributes.1 remote−as 300 .2. Therefore. the metric that comes from RTD to 200. RTC# router bgp 300 neighbor 2.2. The diagram in this section shows the set of metric.3.. a router compares metrics that come from neighbors in the same AS. When RTA gets an update from RTB with metric 50.The metric attribute also has the name MULTI_EXIT_DISCRIMINATOR. and RTB is in AS400.0.2.3. When the same update passes on to a third AS. These configurations illustrate this process: RTA# router bgp 100 neighbor 2. In the diagram in this section. 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.4.1.2 route−map setmetricout out neighbor 1.1.2 remote−as 100 neighbor 3. the router compares metrics for paths from neighbors in the same AS. and the metric that comes from RTB to 50. An issue of the bgp always−compare−med command ensures the comparison of the MED for paths from neighbors in different ASs. A lower metric value is preferred more. RTC and RTD are in AS300. Unless a router receives other directions. you need to issue the special configuration command bgp always−compare−med on the router.3. metric is exchanged between ASs.1 remote−as 300 neighbor 3.. that metric is used to make decisions inside the AS. 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. RTA can only compare the metric that comes from RTC to the metric that comes from RTD.3. AS100 gets information about network 180. By default.0 via three different routers: RTC. The metric default value is 0. In order for the router to compare metrics from neighbors that come from different ASs. MED (BGP4).3 remote−as 300 neighbor 4. you must issue the bgp always−compare−med command on RTA. RTA cannot compare the metric to 120 because RTC and RTB are in different ASs.3.2 route−map setmetricout out neighbor 1.2 remote−as 300 route−map setmetricout permit 10 set metric 120 RTD# router bgp 300 neighbor 3. or INTER_AS (BGP3).3 remote−as 400 . Unlike local preference.2 remote−as 100 neighbor 2. and RTB. The commands are the bgp deterministic−med command and the bgp always−compare−med command. The attribute is a hint to external neighbors about the path preference into an AS.2.3.2.1.1. that metric returns to 0.

Here is the configuration: RTB# router bgp 400 redistribute static default−metric 50 ip route 180.4.967. Assume that.0.0.4. prefer.4.3.200. Community Attribute The community attribute is a transitive.21 remote−as 300 neighbor 3. • local−asUse in confederation scenarios to prevent the transmit of packets outside the local AS.3 remote−as 400 bgp always−compare−med In this case. internal or external.4.4 remote−as 100 neighbor 4. route−map setmetricout permit 10 set metric 200 RTB# router bgp 400 neighbor 4. Keep this route within an AS.10.3.255.294.4. Any router belongs to this community.0. optional attribute in the range of 0 to 4.0 with a metric of 50.4 route−map setmetricout out route−map setmetricout permit 10 set metric 50 With these configurations. The routing decisions are accept.10.2. well known communities for use in this command are: • no−exportDo not advertise to eBGP peers.10.0. Here are two examples of route maps that set the community: . The route map set command has this syntax: set community community−number [additive] [well−known−community] A few predefined.0. You can use route maps to set the community attributes. RTA picks RTB as the best next hop in order to reach network 180. • internetAdvertise this route to the Internet community.4. you must configure RTA in this way: RTA# router bgp 100 neighbor 2. in the example in this section. among others. RTA picks RTC as next hop.3 remote−as 300 neighbor 4. and redistribute. In order to include RTB in the metric comparison.0 null 0 !−−− This causes RTB to send out 180. The community attribute is a way to group destinations in a certain community and apply routing decisions according to those communities.0 255. You can also set metric during the redistribution of routes into BGP if you issue the default−metric number command. • no−advertiseDo not advertise this route to any peer. RTB injects a network via static into AS100. with consideration of the fact that all other attributes are the same.

valid. and the second part represents a 2−byte number. table Default−IP−Routing−Table) Not advertised to any peer . If you use the keyword additive.0.0 BGP routing table entry for 6. • 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. 200 replaces any old community that already exits.0. version 7 Paths: (1 available. issue the ip bgp−community new−format command globally on this router. The value appears as 100:20 in the output of the show ip bgp 6.0.0.3 route−map setcommunity out In Cisco IOS Software Release 12.0 command in this example: Router# show ip bgp 6. version 9 Paths: (1 available. issue the ip bgp−community new−format global configuration command.0.0.10.200. metric 0.0 and later. the community value displays in AA:NN format. In order to send the attribute to a neighbor.0. By default. best #1.10.0. external.0. the community attribute value appears as 6553620. one per line.0 command displays the community attribute value in decimal format.3. you must use this command: neighbor {ip−address | peer−group−name} send−community Here is an example: RTA# router bgp 100 neighbor 3. Here is an example: Without the ip bgp−community new−format command in global configuration.1) Origin IGP.3. an addition of 200 to the community occurs. and AA:NN. Even if you set the community attribute. Cisco IOS Software uses the older decimal format.0.3. Router# show ip bgp 6. localpref 100.200. hexadecimal. End with CNTL/Z.3 remote−as 300 neighbor 3.3.3 send−community neighbor 3.0. In this example.10. Router(config)# ip bgp−community new−format Router(config)# exit With the ip bgp−community new−format global configuration command. The first part of AA:NN represents the AS number. this attribute does not transmit to neighbors by default.3. an issue of the show ip bgp 6. best Community: 6553620 Now.0/8.0 BGP routing table entry for 6.1 from 10.0/8.0.1 (200. In order to configure and display in AA:NN. you can configure communities in three different formats: decimal.10.3. best #1. table Default−IP−Routing−Table) Not advertised to any peer 1 10. Router# configure terminal Enter configuration commands.

RTB originates network 160.0 0.2 distribute−list 1 out access−list 1 deny 160.10. metric 0. external. you must define an access list to filter those updates and apply the access list during communication with RTA: RTC# router bgp 300 network 170.200. The choice of one method over another method depends on the specific network configuration.3. Issue this command in the router configuration mode: neighbor {ip−address | peer−group−name} distribute−list access−list−number {in | out} In this example.1 (200. or with path information or communities as a basis.255. All methods achieve the same results.0 neighbor 3. 1 10. 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.0.10.10.1) Origin IGP.0.0.0 and sends the update to RTC.0.255 access−list 1 permit 0. localpref 100.0 255.10.0.2.10. valid.2. If RTC wants to stop the propagation of the updates to AS100. you can filter BGP with the use of routing updates to or from a particular neighbor.255 .255.1 from 10. You define an access list and apply the access list to the updates to or from a neighbor.3.10.255. You can filter BGP updates with route information as a basis.10. Route Filtering In order to restrict the routing information that the router learns or advertises.0.2.2 remote−as 100 neighbor 2.3 remote−as 200 neighbor 2.200.2.

This address is equivalent to 160.0. The method uses the distribute−list command with standard and extended access control lists (ACLs).0. Path Filtering Another type of filtering is path filtering. define an access list on RTC that prevents the transmit to AS100 of any updates that have originated from AS200.0.0. you must use an extended access list of this format: access−list 101 permit ip 160.0. In the diagram in this section.0.0.0/8.0 so that they do not go to AS100. The command access−list 1 permit 160.0. Assume that.0/8 only. you can block updates about 160. Your goal is to filter updates and advertise only 160. in the example in this section.x.0.0. !−−− Filter out all routing updates about 160.0.0.x.0/8. You can specify an access list on both incoming and outgoing updates with use of the BGP AS paths information.0.0/9.10. This list permits 160.0.0/8.0.x. The use of access lists is a bit tricky when you deal with supernets that can cause some conflicts. and so on.0. Note: The /8 notation means that you use 8 bits of subnet mask. which start from the far left of the IP address. 160.255.x.0.0 0.255.0.255 255.0.0.0.255 permits 160.10.0 255. Refer to How to Block One or More Networks From a BGP Peer for sample configurations on how to filter networks from BGP peers.255.10. RTB has different subnets of 160.0 0.255. as well as prefix list filtering.0.0.0 0. To block the updates. Issue these commands: .0. In order to restrict the update to only 160.0.

instead of the use of ^200$. The access list denies these updates.* is another regular expression in which the . you specify a string that input must match.2. you specify a string that consists of path information that an input must match. A regular expression comprises: • Range A range is a sequence of characters within left and right square brackets.3.3. issue the show ip bgp regexp regular−expression command. ip as−path access−list access−list−number {permit | deny} as−regular−expression neighbor {ip−address | peer−group−name} filter−list access−list−number {in | out} This example stops the RTC send of updates about 160.0 with path information that starts with 200 and ends with 200. as in the diagram in this section.* represents any path information. The . These updates match the access list ^200 because the path information starts with 200. What happens if.3 remote−as 200 neighbor 2. ip as−path access−list 1 deny ^200$ ip as−path access−list 1 permit . In order to check if you have implemented the correct regular expression. In the example in the section Path Filtering. In the case of BGP. In this path information.0 to RTA: RTC# router bgp 300 neighbor 3.10. means "any character" and the * means "the repetition of that character". When you build a regular expression. An example is [abcd]. The ^200$ in the command is a "regular expression". 200 is first and 400 is last.0. you use ^200? With an AS400. the updates match the access list. which is not the requirement. which is necessary to permit the transmission of all other updates.10.2 filter−list 1 out !−−− The 1 is the access list number below. • Atom An atom is a single character. you specified the string ^200$.2. The access list prevents the transmission of these updates to RTA. AS Regular Expression This section explains the creation of a regular expression. Here are some examples: . So . in which ^ means "starts with" and $ means "ends with".0. This command shows all the paths that have matched the regular expression configuration.* 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. Since RTB sends updates about 160.2 remote−as 100 neighbor 2. updates that AS400 originates have path information of the form (200.2. A regular expression is a pattern to match against an input string. You wanted path information that comes inside updates to match the string in order to make a decision.2. 400).

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

1 route−map setcommunity out route−map setcommunity match ip address 1 set community no−export access−list 1 permit 0. In this example.3. ^$ • This expression indicates origination from this AS. RTC does not propagate the updates to external peer RTA.255. . Refer to Using Regular Expressions in BGP for sample configurations of regular expression filtering. BGP Community Filtering This document has covered route filtering and AS−path filtering.255. Another method is community filtering.0.3. and this section provides a few examples of how to use community. Use the no−export community attribute.3.1 remote−as 300 neighbor 3.10.3.0 255.0. • This expression indicates transmission from AS100.255 Note: This example uses the route−map setcommunity command in order to set the community to no−export.3. RTB# router bgp 200 network 160. 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.3.1 send−community neighbor 3.0 neighbor 3. The section Community Attribute discusses community. Note: The neighbor send−community command is necessary in order to send this attribute to RTC.0. When RTC gets the updates with the attribute NO_EXPORT.

255.In this example. like weight and metric. match−on−community: route−map match−on−community match community 10 !−−− The community list number is 10. RTB sent updates to RTC with a community of 100 200. set weight 20 ip community−list 10 permit 200 300 !−−− The community number is 200 300.3. you can define this route map.3.0 255.255.1 route−map setcommunity out route−map setcommunity match ip address 2 set community 100 200 additive access−list 2 permit 0.0 neighbor 3.0. any route that has 100 in the community attribute matches list 1.3.3.3.0. The community list allows you to filter or set attributes with different lists of community numbers as a basis.0.3.3. If RTC wants to set the weight with those values as a basis. ip community−list community−list−number {permit | deny} community−number For example. This action adds the value 100 200 to any existing community value before transmission to RTC. RTB has set the community attribute to 100 200 additive.3.10. The keyword exact states that the community consists of 200 only and nothing else.3 remote−as 200 neighbor 3.255 A community list is a group of communities that you use in a match clause of a route map.1 remote−as 300 neighbor 3. You can use the community list in order to filter or set certain parameters.3. in certain updates with the community value as a basis. In the second example in this section. The last community list is here to make sure .1 send−community neighbor 3. The weight of this route is set to 20. Any route that has only 200 as community matches list 2 and has a weight of 20.3. you can do this: RTC# router bgp 300 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. RTB# router bgp 200 network 160.

0. 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 3.0 neighbor 3. you want RTC to learn from AS200 about networks that are local to AS200 and nothing else. you want to set the weight on the accepted routes to 20. Use a combination of neighbor and as−path access lists: RTC# router bgp 300 network 170. Refer to Using BGP Community Values to Control Routing Policy in an Upstream Provider Network for more information.3.3 route−map stamp in route−map stamp match as−path 1 set weight 20 . Remember that anything that does not match drops. in the diagram in this section. by default. The keyword internet indicates all routes because all routes are members of the Internet community.10. Also.3.that other updates do not drop.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.

3.3. When the information is propagated to AS600. 300).0 neighbor 2. If all other attributes are the same.10.2.3 route−map stamp in route−map stamp permit 10 match as−path 1 set weight 20 route−map stamp permit 20 match as−path 2 set weight 10 ip as−path access−list 1 permit ^200$ ip as−path access−list 2 permit ^200 600 .0 to two different ASs.2. you must manipulate the path information in order to manipulate the BGP decision process.0 neighbor 3.0. AS100 and AS200.2 route−map SETPATH out route−map SETPATH set as−path prepend 300 300 .0.0 via two different routes.10. AS300 gets all traffic via AS100. 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. A common practice is to repeat your own AS number in this way: RTC# router bgp 300 network 170.3. The statement also sets a weight of 10 for updates that are behind AS400. You can do this if you prepend AS numbers to the existing path information that is advertised to AS100. Any other updates drop. 300). 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 170.10. 200.* This statement sets a weight of 20 for updates that are local to AS200.0. in the diagram in the section BGP Neighbors and Route Maps.3.2 remote−as 100 neighbor 2. the routers in AS600 have network reachability information about 150. If you want to influence this decision from the AS300 end.2. The first route is via AS100 with path (100. and the second one is via AS400 with path (400. Use of set as−path prepend Command In some situations.3 remote−as 200 neighbor 3.10. These updates are permitted.2. 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. AS600 picks the shortest path and chooses the route via AS100.

You do not define the same policies for each separate neighbor.Because of this configuration. 300) that AS600 received from AS400.0. 200. Also. You can only override options that are set on the inbound.5.5. The configuration applies the peer group to all internal neighbors. RTF. instead. and RTG. Route maps. the . Members of the peer group inherit all the configuration options of the peer group.3.3. distribute lists. BGP Peer Groups A BGP peer group is a group of BGP neighbors with the same update policies. 300.2 filter−list 3 in This configuration defines a peer group with the name internalmap. 300). RTE.10. AS600 receives updates about 170.2 peer−group internalmap neighbor 3. 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 5.6. you define a peer group name and assign these policies to the peer group. such as a route map SETMETRIC to set the metric to 5 and two different filter lists. In order to define a peer group.3.6. This path information is longer than the (400. The configuration defines some policies for the group. You can also configure members to override these options if the options do not affect outbound updates.2 peer−group internalmap neighbor 5. and filter lists typically set update policies.3. 1 and 2. 300.0 via AS100 with path information of: (100.2 peer−group internalmap neighbor 3.

Cisco introduced the BGP Dynamic Update Peer Groups feature.2 with the assignment of filter list 3. This filter list overrides filter list 2 inside the peer group. you define the remote−as statements outside of the peer group because you must define different external ASs. This separation improves the convergence time and the flexibility of neighbor configuration. The feature introduces a new algorithm that dynamically calculates and optimizes update groups of neighbors that share the same outbound policies.2 peer−group externalmap neighbor 1.2.1.1. 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. you configure RTC with a peer group externalmap and apply the peer group to external neighbors.1. For more information on peer groups. BGP Case Studies 4 CIDR and Aggregate Addresses .4.2.1. This method to group updates limited outbound policies and specific session configurations.1.1.1. These neighbors can share the same update messages. the group of BGP update messages was on the basis of peer group configurations. Also. refer to BGP Peer Groups.2 remote−as 600 neighbor 4. The feature is available in later Cisco IOS Software releases as well.configuration defines a separate filter list 3 for neighbor RTE.2 peer−group externalmap neighbor 4.4.4.1.2 remote−as 200 neighbor 1. Now.2. Note: In Cisco IOS Software Release 12.0(24)S. Note: You can only override options that affect inbound updates.2 remote−as 100 neighbor 2. look at how you can use peer groups with external neighbors.2 peer−group externalmap neighbor 1. you override the inbound updates of neighbor 1. The BGP Dynamic Update Peer Group feature separates update group replication from peer group configuration. In earlier releases of Cisco IOS Software. With the same diagram in this section. Refer to BGP Dynamic Update Peer−Groups for more details.4.2 filter−list 3 in Note: In these configurations.2.

0 to . RTB generates network 160. This representation is similar to 192.10. Aggregate Commands There is a wide range of aggregate commands.0.0. Now.0 but does not prevent the propagation of 160. 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. With CIDR.10. Aggregation is the process that combines the characteristics of several different routes in such a way that advertisement of a single route is possible.3.0.10. B. You use aggregates in order to minimize the size of routing tables.0.0.2.3 remote−as 200 neighbor 2. The "16" represents the number of bits in the subnet mask. such as class A. 192.0.2 remote−as 100 network 170.0. CIDR or supernetting is a new way to look at IP addresses.0. You configure RTC to propagate a supernet of that route 160.0 aggregate−address 160. The command aggregate−address 160.0 255.0 #RTC router bgp 300 neighbor 3. or C.0.213.0 propagates an additional network 160.0/16.0. You must understand how each one works in order to have the aggregation behavior that you desire.One of the main enhancements of BGP4 over BGP3 is classless interdomain routing (CIDR).10.3.0.0.0.255. when you count from the far left of the IP address.0. the network is a legal supernet.3.0 to RTA: RTB# router bgp 200 neighbor 3.0. network 192.0.0.2.0.3.1 remote−as 300 network 160.213. there is no notion of classes.0 255.0.0 RTC propagates the aggregate address 160.0 to RTA.0 was once an illegal class C network.213.0. For example. In this example.0.0.

0. The command aggregate 160. suppress the more−specific route 160. .0.RTA.20.0. the network entry always injects into BGP updates.0 255. you want to aggregate 160.0.0 0. Note: You cannot aggregate an address if you do not have a more−specific route of that address in the BGP routing table.0.0. with the diagram in the section CIDR and Aggregate Addresses.0 summary−only propagates network 160. define a route map and apply the route map to the aggregates.0.0.0. The outcome is the propagation of both networks 160.0.0.0.0 If you want RTC to propagate network 160. If you want to suppress more−specific routes when you do the aggregation.0.0.0.0. which is the advertisement of both the prefix and the more−specific route.255 access−list 1 deny 0.0.0 to RTA.10.0. Suppose that.0.0.0. aggregate−address address−mask suppress−map map−name This command advertises the prefix and the more−specific routes.20.255. But the command includes as−set information in the path information of the routing updates. Use this route map: route−map CHECK permit 10 match ip address 1 access−list 1 permit 160. there is a suppression from the updates of any packets that the access list permits.0 and suppresses the more−specific route 160. An injection of the more−specific route into the BGP table is possible.10. Note: If you aggregate a network that injected into your BGP via the network statement.0.255 By definition of the suppress−map.10.0.0 only and not the more−specific route. aggregate−address address−mask as−set This command advertises the prefix and the more−specific routes.0. RTB cannot generate an aggregate for 160.255.0.0 255. The action allows you to be selective about which more−specific routes to suppress. issue this command: aggregate−address address mask summary−only This command advertises the prefix only. The command suppresses all the more−specific routes.0.0. For example.0.10. The route injection can occur via: • Incoming updates from other ASs • Redistribution of an IGP or static into BGP • The network command. for example.0.0. and allow the propagation of 160. network 160.0 255. But the command suppresses advertisement with a route map basis.0.255. This injection occurs even though you use the aggregate summary−only command.0 and 160.0 if RTB does not have a more−specific entry of 160.0.0 as−set The section CIDR Example 2 (as−set) discusses this command.0 in the BGP table. aggregate 129. The example in the section CIDR Example 1 discusses this situation.0.

0. You cannot have RTB generate a prefix for 160.0 255. The outcome is that RTB advertises the aggregate with an origin of incomplete (?). such as metric.3.0.0 255.10.0. RTC# router bgp 300 neighbor 3.3 remote−as 200 neighbor 2. There are two solutions to this problem. at the time of the send of aggregates.0.0.3. apply this route map to the aggregate attribute−map command: route−map SETMETRIC set origin igp aggregate−address 160.10.0.2.2.0. CIDR Example 1 Request: Allow RTB to advertise the prefix 160. which means that AS200 is the originator of 160.2 remote−as 100 network 170.10.0.0.10.0 is local to AS200.0.0.0.Then.2.2 remote−as 100 neighbor 2.0.0 suppress−map CHECK Here is another variation: aggregate−address address−mask attribute−map map−name This command allows you to set the attributes.0 attribute−map SETORIGIN For more information. refer to Understanding Route Aggregation in BGP. even if you use the aggregate summary−only command. The first solution is to use a static route and redistribute into BGP.0. In order to set the origin of the aggregates to IGP.0.0 and suppress all the more−specific routes.10.0 without the generation of an entry for 160. apply the route map to the aggregate statement.0.0.0. RTB# router bgp 200 .2. The problem with this request is that network 160. RTB generates both networks because RTB is the originator of 160.0.0.0 aggregate−address 160.

0.0.3. ip route 160. With as−set. except that the entry sets the origin of the update to IGP.0.0.0.3.3.0 null0 In the second solution.3.0. in addition to the static route. you force RTC to generate path information in the form of a set {}.0.0 255.0 255.1 remote−as 300 redistribute static !−−− This generates an update for 160.0.0. RTB# router bgp 200 network 160.0 null0 CIDR Example 2 (as−set) You use the statement as−set in aggregation to reduce the size of the path information.20. RTB# router bgp 200 network 160.0. regardless of how many times the AS number appeared in multiple paths that were aggregated. If you add the aggregate as−set statement. irrespective of which path came first.0.0 from RTB. This entry has the same effect.0.0.3.0 neighbor 3.0.10.0. RTC gets updates about 160. That set includes all the path information.0 !−−− with the origin path as "incomplete".0 .0 !−−− This entry marks the update with origin IGP.0 from RTA and updates about 160. 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. you add an entry for the network command.20.1 remote−as 300 RTA# router bgp 100 network 160.10. Suppose that RTC wants to aggregate network 160.0. neighbor 3. the AS number is listed only once. In this example.0/8 and send the network to RTD. neighbor 3.0 mask 255.0.0.1 remote−as 300 redistribute static ip route 160.0. RTD does not know the origin of that route.0.3.

0 255.3 remote−as 200 neighbor 2. the ASs exchange routing as if they used iBGP.0 summary−only aggregate 160. as if the route originated from AS300. and local preference information.0.0. Even though these ASs have eBGP peers to ASs within the confederation.3.4. RTC sends an update 160.0.3. the confederation appears to be a single AS. 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. BGP Confederation The implementation of BGP confederation reduces the iBGP mesh inside an AS.3 remote−as 200 neighbor 2.0.0/8 !−−− with an indication that 160.4 remote−as 400 aggregate 160.0.0.0.4.0. The trick is to divide an AS into multiple ASs and assign the whole group to a single confederation.0 255.3.0/8 to RTD with path information (300).0.2. BGP Confederation and Route Reflectors.4.0 summary−only !−−− This command causes RTC to send RTD updates about 160.2 remote−as 100 neighbor 4.0 actually comes from two different ASs.0.0. metric.2.0. issue this command: bgp confederation identifier autonomous−system The confederation identifier is the AS number of the confederation group. Case 2: RTC# router bgp 300 neighbor 3. Each AS alone has iBGP fully meshed and has connections to other ASs inside the confederation.3. In this way.0. are for Internet service providers (ISPs) that want further control of the explosion of iBGP peering inside their ASs.1 remote−as 300 Case 1: RTC does not have an as−set statement.2. the confederation preserves next hop.0.4.0 belongs to a set {100 200}.2 remote−as 100 neighbor 4.0. !−−− This may create loops if RTD has an entry back into AS100 or AS200.0 as−set !−−− This command causes RTC to send RTD updates about 160.0. neighbor 2.0.0/8 !−−− with no indication that 160.0.2.0 255.2.4 remote−as 400 aggregate 160. RTC# router bgp 300 neighbor 3. The next two subjects. In order to configure a BGP confederation. To the outside world.0.0.0.

AS500. you need nine peer connections for each router. AS60. For each of AS50.5.10. but you only have interest in the BGP speakers that have eBGP connections to other ASs.5 remote−as 100 (EBGP connection to external AS100) RTD# .14.212. The outside world sees only one AS.11.Assume that you have an AS500 that consists of nine BGP speakers.1 remote−as 50 (IBGP connection within AS50) neighbor 128. RTC# router bgp 50 bgp confederation identifier 500 bgp confederation peers 60 70 neighbor 128. or AS70. and you define the list of confederation peers with the bgp confederation peers command. You need eight iBGP peers and one eBGP peer to external ASs. If you want to make a full iBGP mesh inside AS500. Here is a sample configuration of routers RTC.1 remote−as 70 (BGP connection with confederation peer 70) neighbor 5. and AS70. You give the AS a confederation identifier of 500. AS60. and AS70.1 remote−as 50 (IBGP connection within AS50) neighbor 129.20. and RTA: Note: RTA has no knowledge of AS50.210.1 remote−as 60 (BGP connection with confederation peer 60) neighbor 135. RTD.213. If you use confederation. you define a full mesh of iBGP peers. Other non−BGP speakers exist also. RTA has only knowledge of AS500.213. AS60. you can divide AS500 into multiple ASs: AS50.5.

You can relax this restriction a bit and provide additional control.30. Peering between RTA and RTB is not necessary because RTC is an RR for the updates that come from RTA and RTB. RTB. and RTC form a cluster with a single RR within AS100. router bgp 60 bgp confederation identifier 500 bgp confederation peers 50 70 neighbor 129. .212.1 remote−as 50(BGP connection with confederation peer 50) neighbor 135. As the iBGP section demonstrates. which allows a router to advertise.14. RTC can be elected as an RR. and the neighbors at which the command points are the clients of that RR. The combination of the RR and the clients is a "cluster". RTB.5. a BGP speaker does not advertise a route that the BGP speaker learned via another iBGP speaker to a third iBGP speaker. This route reflection reduces the number of iBGP peers within an AS. iBGP learned routes to other iBGP speakers.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).1 remote−as 70 (BGP connection with confederation peer 70) neighbor 6.210. maintain a full iBGP mesh between RTA.6.2 remote−as 60 (IBGP connection within AS60) neighbor 128. If you utilize the RR concept. In this way. In the example.6. and RTC within AS100. Other iBGP peers of the RR that are not clients are "nonclients".5.213. the RTC configuration has the neighbor route−reflector−client command that points at the RTA and RTB IP addresses. or reflect. RTC has a partial iBGP peering with RTA and RTB.30.6 remote−as 600 (EBGP connection to external AS600) RTA# router bgp 100 neighbor 5. In this example. neighbor route−reflector−client The router with this command is the RR. RTA. In normal cases.

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

10.10 remote−as 100 neighbor 8.13.3.9 remote−as 300 bgp cluster−id 10 RTD# router bgp 100 neighbor 10.7.5 remote−as 100 neighbor 5.5.13 remote−as 500 RTC# router bgp 100 neighbor 1.10 remote−as 100 neighbor 5. Then. If you configure peer groups.2.7 remote−as 100 neighbor 3.6.1.6 remote−as 100 neighbor 6.3.4.1.3.9.4. This transmission can cause problems.7. If you turn off BGP client−to−client reflection on the RR and you make redundant BGP peering between the clients.3 remote−as 100 neighbor 11.2 remote−as 100 neighbor 2. The RR scheme allows such conventional BGP speakers to coexist. Important Note: This configuration does not use peer groups.3 remote−as 100 neighbor 9.10.11.6 route−reflector−client neighbor 7.4 remote−as 100 neighbor 7.1 remote−as 100 neighbor 1. 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.10.11.6 route−reflector−client neighbor 7.7.7. RR and Conventional BGP Speakers An AS can have BGP speakers that do not understand the concept of RRs.5. These routers can be either members of a client group or a nonclient group.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.13. neighbor 6.5.5 route−reflector−client neighbor 6. The router subcommand bgp client−to−client reflection is enabled by default on the RR.8.1 route−reflector−client neighbor 2.2.1.7 remote−as 100 neighbor 10. you can create more clusters gradually.6.8 remote−as 200 Note: You do not need the bgp cluster−id command for RTC because only one RR exists in that cluster.11 remote−as 400 bgp cluster−id 10 RTF# router bgp 100 neighbor 10.6.1.3.10.7 remote−as 100 neighbor 3.5.4. .9.10.6.2. This document calls these routers conventional BGP speakers.2 route−reflector−client neighbor 4. you can safely use peer groups.6. The existence of these routers allows easy and gradual migration from the current iBGP model to the RR model.7.6.10 remote−as 100 neighbor 4.8.2.10. a potential withdrawal to the source of a route on the RR transmits to all clients inside the cluster.4 remote−as 100 neighbor 13.

3 remote−as 100 neighbor 2.6.In this diagram.14.1. you can make RTC an RR with clients RTA and RTB. Later on. .5.4.1 remote−as 100 neighbor 14.13 remote−as 300 RTC# router bgp 100 neighbor 4.1. You can do normal iBGP mesh between these routers and RTD.13.2.5 remote−as 100 neighbor 5. only the RRs require the upgrade.14. You cannot configure these routers as RRs. RTA.6. when you are ready to upgrade.6 remote−as 100 neighbor 6.1.4 remote−as 100 neighbor 2.2 remote−as 100 neighbor 1.2.6.13.1. and RTB are "conventional" routers. Clients do not have to understand the route reflection scheme.5. and RTF have the concept of route reflection.4.5. Here is the configuration of RTD and RTC: RTD# router bgp 100 neighbor 6.1 remote−as 100 neighbor 13.6 route−reflector−client neighbor 5. RTC.2. RTD. remove the iBGP full mesh and have RTA and RTB become clients of RTC.2.2 remote−as 100 neighbor 1.5.14 remote−as 400 When you are ready to upgrade RTC and make RTC an RR. RTE.6.3.3.5 route−reflector−client neighbor 3.

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

local router ID is 203. RTB# hostname RTB interface Serial0 ip address 203.10.252 interface Serial1 ip address 192.6 command on RTD.255.6 255.10.0 0. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 192.255.5 0 0 300 i *> 203.252 router bgp 300 network 192. * valid.208.10.255. issue the clear ip bgp 192. If you assume that the eBGP link to RTD is stable.2 Status codes: s suppressed. > best.250.250.0 neighbor 192.10. local router ID is 203.250.5 remote−as 300 RTD# hostname RTD interface Loopback0 ip address 192.2 255.10.208.0 192.5 255.255.0.10.250.255. * valid.208.6 remote−as 100 The configuration of RTB is for route dampening with default parameters. e − EGP.10.15.174 255.208.208.252 router bgp 100 bgp dampening network 203.10.208. i − internal Origin codes: i − IGP. d damped.208.2 Status codes: s suppressed.192 interface Serial0/0 ip address 192.10.250. d damped. i − internal Origin codes: i − IGP.0.15. The RTB BGP table looks like this: RTB# show ip bgp BGP table version is 24. e − EGP.0 neighbor 192.15. h history.208.255.0 0 32768 i In order to simulate a route flap. h history. > best. the RTB BGP table looks like this: RTB# show ip bgp BGP table version is 24.15.255. ? − incomplete Network Next Hop Metric LocPrf Weight Path .208.255.15.

10.C. metric 0. valid. external Dampinfo: penalty 910.208.250.0.10.D m.0 255.5 from 192. (suppressed due to dampening) 192.10. * valid.208. flapped 3 times in 0:05:18 . h 192. no best path) 300 (history entry) 192.208.0 255. > best. i − internal Origin codes: i − IGP.5 0 0 300 i *> 203.208.255. metric 0. RTB# show ip bgp 192.208. • show ip bgp flap−statistics filter−list list Displays flap statistics for all paths that pass the filter. you see: RTB# show ip bgp BGP table version is 32.0 is in a history state.5 from 192.B.174) Origin IGP.m. h history.208.m.B. but the penalty is still below the "suppress limit".208.15.C. • show ip bgp flap−statistics regexp regular−expression Displays flap statistics for all paths that match the regular expression.10. • show ip bgp flap−statistics A.0 192. refer to BGP Best Path Selection . • show ip bgp flap−statistics A.15. • show ip bgp neighbor [dampened−routes] | [flap−statistics] Displays flap statistics for all paths from a neighbor. the reuse value is the default. 750. • clear ip bgp flap−statistics Clears flap statistics for all routes.m Clears flap statistics for a single entry. • clear ip bgp A. or suppressed.208.m.10. no best path) 300.250.10.0 BGP routing table entry for 192.C.255. ? − incomplete Network Next Hop Metric LocPrf Weight Path *d 192.D m.10.0.15.10.208.0 0.250.0. This placement means that you do not have a best path to the route. How BGP Selects a Path Now that you are familiar with the BGP attributes and terminology. The dampening information is purged when the penalty becomes less than half of the reuse limit.10. d damped. external Dampinfo: penalty 2615. Route suppression has not yet occurred.5 (192.B. e − EGP. reuse in 0:27:00 The route has been dampened.m.C. local router ID is 203.10.m. the purge occurs when the penalty becomes 375 (750/2=375).2 Status codes: s suppressed.0 0 32768 i RTB# show ip bgp 192. The route is reused when the penalty reaches the "reuse value".0 192. but information about the route flapping still exists.0 0 32768 i The BGP entry for 192. • clear ip bgp flap−statistics A.m. These commands show and clear flap statistics information: • show ip bgp flap−statisticsDisplays flap statistics for all the paths.10.174) Origin IGP.208.0.m Displays flap statistics for a single entry.255. version 25 Paths: (1 available.0.255.0.B.208.m longer−prefixDisplays flap statistics for more specific entries.208.10. • clear ip bgp flap−statistics regexp regular−expression Clears flap statistics for all the paths that match the regular expression.D m.5 0 0 300 i *> 203.208.0 0. flapped 1 times in 0:02:03 The route has received a penalty for flapping.5 (192.0 BGP routing table entry for 192.D flap−statisticsClears flap statistics for all paths from a neighbor.208.10. The default is 2000. In this case.10.10. version 32 Paths: (1 available. In this case. • clear ip bgp flap−statistics filter−list list Clears flap statistics for all the paths that pass the filter. If the route flaps a few more times.208.

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

255.255 area 0 RTB# hostname RTB ip subnet−zero interface Serial0 ip address 203.0 0.14.250.250.41 remote−as 100 RTC# hostname RTC ip subnet−zero interface Loopback0 ip address 128.208.255.0.255.255.1 255.0.1 255.15.255.255.255.15.2 update−source Loopback0 RTF# hostname RTF ip subnet−zero interface Ethernet0 ip address 203.63.250.255 area 0 router bgp 100 network 203.0 interface Serial1 ip address 203.255.213.130 255.255.0 interface Ethernet0 ip address 203.255.252 router ospf 10 network 203.255.6 255.255.1 255.250.0.14.15.250.15.255.252 interface Serial1 ip address 192.2 255.0.10.255.0.13.0 0.0 network 203.208.213.255.250.63.252 router ospf 10 network 203.41 255.2 255.255 area 0 router bgp 100 network 203.2 remote−as 200 neighbor 203.250.255.255.0 neighbor 192.250.63.250.ip subnet−zero interface Loopback0 ip address 203.252 router ospf 10 network 203.255.192 interface Serial2/0 ip address 128.15.5 255.2 remote−as 100 neighbor 203.5 remote−as 300 neighbor 203.0.0 interface Serial0 ip address 128.14.213.0 neighbor 128.250.255.255.0 0.213.250.252 ! .250.10.255.13.250.63.250.13.

255.10.208.0 neighbor 192.208.252 ! interface Serial0/1 ip address 192.2 remote−as 400 .255.213.255.252 interface Serial1 ip address 195.10.63.255.1 remote−as 100 neighbor 128.208.255.1 255.10.1 255.0 interface Serial0 ip address 195.255.208.255.0 neighbor 128.10.10.63.211.63.1 remote−as 500 neighbor 192.2 255.174 255.211.0.1 255.211.10.2 255.2 remote−as 300 neighbor 195.213.213.211.213.10.10.5 255.255.252 interface Serial1 ip address 128.10.252 router bgp 200 network 128.192 interface Serial0/0 ip address 192.252 router bgp 500 network 195.10.10.200.192 interface Serial0 ip address 192.10.255.255.200.255.255.213.208.255.6 255.63.1 remote−as 500 RTG# hostname RTG ip subnet−zero interface Loopback0 ip address 195.255.10.0 neighbor 192.174 255.255.5 remote−as 200 neighbor 195.255.6 remote−as 400 RTD# hostname RTD ip subnet−zero interface Loopback0 ip address 192.10.211.6 remote−as 100 RTE# hostname RTE ip subnet−zero interface Loopback0 ip address 200.10.252 clockrate 1000000 router bgp 400 network 200.208.211.255.252 router bgp 300 network 192.63.10.interface Serial2/1 ip address 128.255.208.213.208.2 255.255.255.0 neighbor 128.

S − static. Note: Any locally generated entry. Here. in this way.14. B − BGP D − EIGRP.0 0.63.0 203. * − candidate default Gateway of last resort is not set 203.2. none of the BGP entries has reached the routing table.250.213.0 via OSPF. as if the link between RTB and RTD does not exist. • An > symbolIndicates that BGP has chosen the best route.0 via a next hop of 128.255. local router ID is 203. This is the RTB BGP table: RTB# show ip bgp BGP table version is 4.255 is subnetted. h history.213. There is no way to reach that next hop via this IGP. You can run OSPF on the RTA s0 interface and make it passive.0 128. L1 − IS−IS level−1.15.0 255.250.0. BGP picks one best path to reach a destination. O − OSPF. EX − EIGRP external. L2 − IS−IS level−2.0 0 32768 i In this table.0 128.13.Always use the network command or redistribute static entries into BGP to advertise networks. which is OSPF. Serial0 Apparently.250.13.63. RTB knows how to reach the next hop 128.0. Serial0 O 203.41 0 100 0 i *>203.15. This RTA configuration appears here: .2.208.15. R − RIP.1. 02:50:45.15.250. For example. 02:50:46.200.0 [110/74] via 203.2. these notations appear: • An i at the beginningIndicates that the entry was learned via an iBGP peer.41 [110/75] via 203.213.213. 128.213.213. 1 subnets O 203.0.0.63. i − internal Origin codes: i − IGP. > best.250.213. • Path informationThis information is intuitive.211. 1 subnets C 203.2 0 100 0 200 i *i192.252 is subnetted. IA − OSPF inter area E1 − OSPF external type 1. M − mobile.1.0.63.0 128.213.10.63. This method is better than a redistribution of IGP into BGP.14.0 is directly connected. RTB has not learned about 128.255.15.2. and advertises the path to other BGP peers.250.250.0.15.41 0 100 0 i *>i203.213.0 255.250.13.0 203. Serial0 203.213.13.2 100 0 200 400 500 i *i200. installs the path in the IP routing table.0 is learned via path 200 with a next hop of 128. which is the eBGP next hop carried into iBGP. ? − incomplete Network Next Hop Metric LocPrf Weight Path *i128.250.2 100 0 200 400 500 300 i *i195. you start with the s1 interface on RTB shutdown.10. d damped. BGP uses the decision steps that the document BGP Best Path Selection Algorithm outlines.213. E − EGP i − IS−IS. This example uses the network command to inject networks into BGP.250.213.15.10. E2 − OSPF external type 2.0.63. Note: Notice the Next Hop attribute. network 128.2 Status codes: s suppressed. is unreachable.63.250.255.250. Look at the IP routing table: RTB# show ip route Codes: C − connected. * valid. The first problem is that the next hop for these entries.63. • An i at the endIndicates that the origin of the path information is IGP.2 100 0 200 400 i *>i203.250. Two problems exist here.0 128. I − IGRP.0. has a next hop 0. such as 203.0. RTB knows about 128. e − EGP.255.250.13.63.

2 0 100 0 200 i *>i192.250. R − RIP. e − EGP.250.200. EX − EIGRP external.15.0 interface Ethernet0 ip address 203.213.0 0.211.250.213.213.0.252 router ospf 10 passive−interface Serial0 network 203.0.14.13. E2 − OSPF external type 2.0 0.2 Status codes: s suppressed.15.63.15.41 0 100 0 i *> 203.13.250.213.255.0. 1 subnets O 128.0 255.2 remote−as 100 neighbor 203.0.0 [110/138] via 203. O − OSPF.213. Serial0 128.63. h history.250.213.252 is subnetted. RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203.15.13.13.252 is subnetted.213.0.2 100 0 200 400 500 i *>i200.0 128. S − static. which means that BGP can reach the next hop.255.250.63.1 255.255.250.0 128.250.255 area 0 network 128.250.255 is subnetted.0 0 32768 i Note: All the entries have >.15.0 neighbor 128.250.250. L1 − IS−IS level−1.250.2 100 0 200 400 i *>i203.15. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.41 255.2 100 0 200 400 500 300 i *>i195.255. Serial0 203.250.255.0 is directly connected. 1 subnets O 203.1 255.0.14.255 area 0 router bgp 100 network 203.14.0 128. > best. local router ID is 203.0 [110/74] via 203.1. IA − OSPF inter area E1 − OSPF external type 1.0 0. Serial0 O 203.15.255.255. I − IGRP.255.0.0 interface Serial0 ip address 128.0 128.63.255. Serial0 . 00:04:46.63.213.213.255. M − mobile.250.0 255.41 0 100 0 i *>i203.1.208.10. * − candidate default Gateway of last resort is not set 203.0 255.63.0. The new BGP table on RTB looks like this: RTB# show ip bgp BGP table version is 10. d damped.250.0. 00:04:46.13. * valid.250.15. L2 − IS−IS level−2.2 remote−as 200 neighbor 203.250. Look at the routing table: RTB# show ip route Codes: C − connected.250.0 203.250.255.0 203.255.10. i − internal Origin codes: i − IGP.1.255.213.2 update−source Loopback0 Note: You can issue the bgp nexthopself command between RTA and RTB in order to change the next hop.15.10.0 mask 255. 1 subnets C 203.255. B − BGP D − EIGRP.41 [110/75] via 203. 00:04:47.13.255. E − EGP i − IS−IS.0.250.63.

1.255.255.255. E2 − OSPF external type 2.250.63.250.255.0 [110/74] via 203. IA − OSPF inter area E1 − OSPF external type 1.0 is variably subnetted.10.250.41 255. B − BGP D − EIGRP.14.10.208. R − RIP.255.0 [200/0] via 128. * − candidate default Gateway of last resort is not set 203.41. 2 subnets.0 is directly connected. RTF in the middle does not know how to reach the networks: RTF# show ip route Codes: C − connected.208. the entries appear in the routing table.13. E2 − OSPF external type 2. Ethernet0 203.0 is directly connected. 1 subnets C 203.255.211.2.0 255. 1 subnets C 203. If you turn off synchronization on RTB.255 [110/75] via 203.250. B − BGP D − EIGRP.10.250. S − static. 2 masks O 203. 00:01:08 203. 00:01:08 O 128.15.213.2. this is what happens: RTB# show ip route Codes: C − connected.252 is subnetted.41 [110/11] via 203. if you turn synchronization off.0 255.0 255. The only difference is that 128.63.14. Note: RTF has no notion of networks 192. But connectivity is still broken. Serial0 The routing table looks fine. E − EGP i − IS−IS. O − OSPF.14.14.213.0.252 [110/138] via 203.15.250. I − IGRP.250.15.255.255.0 is variably subnetted. Serial0 B 203. Ethernet0 When you turn off synchronization in this situation.250.0 [200/0] via 128.0. Redistribute BGP into OSPF on RTA. the problem still exists. M − mobile. Serial1 C 203.0 [200/0] via 203.15.213.0 255.0 [110/74] via 203.255. O − OSPF.250. In this scenario.13. 00:14:15.The second problem is that you still do not see the BGP entries in the routing table.0.15.13.250. M − mobile.63.255.1.250. but there is no way to reach those networks.213.0 255.213.0 because you have not redistributed BGP into OSPF yet.1.250. Serial0 128. IA − OSPF inter area E1 − OSPF external type 1.10. 2 subnets.13. S − static. R − RIP.63.255.211. But you need synchronization later for other issues.250.255. I − IGRP.250.213.0 [200/0] via 128. L2 − IS−IS level−2.15.0 [200/0] via 128.1. L1 − IS−IS level−1. Serial0 O 203. 00:01:07 B 195. 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.10.255.13.0 is directly connected. 00:14:15.63.200.255 is subnetted. * − candidate default Gateway of last resort is not set B 200. with a metric of 2000: RTA# hostname RTA . E − EGP i − IS−IS. EX − EIGRP external. 00:01:07 203.63.2.63.250.250. 00:01:07 B 192.213.213.255. Ethernet0 128. 00:12:37.250.213. 1 subnets O 203. 2 masks B 128.0.2.252 is subnetted.1. EX − EIGRP external.15.13. L1 − IS−IS level−1.255.0 is now reachable via OSPF. This problem is a synchronization issue. L2 − IS−IS level−2.252 is subnetted.213.0 255. 1 subnets O 128.0 and 195.0 255. 00:12:37. 00:12:37.

255.0. 00:00:14.252 [110/138] via 203.0 [110/2000] via 203.15.255. O − OSPF. 00:00:15.5 via IGP.2 remote−as 100 neighbor 203.15. EX − EIGRP external.255.255.1.1.0. Loopback1 C 203.0 [110/2000] via 203.13. L2 − IS−IS level−2.250. S − static. 2 subnets. 00:00:15.255.0.250.255 [110/75] via 203.14.255 area 0 network 128.0 mask 255.211.0 0.0.15.255.0 neighbor 128.250.208.1 255. Turn off synchronization on RTA so that RTA can advertise 203.0 is directly connected.255.250.15. E − EGP i − IS−IS. M − mobile. Serial0 O E2 195. Serial0 203.250.8 is directly connected.0 is variably subnetted.252 is subnetted. The OSPF distance is 110.255. 2 masks O E2 128. If you do not take this step.0 interface Ethernet0 ip address 203.213.255.1.1.0 [110/2000] via 203.10.0 255. ip subnet−zero interface Loopback0 ip address 203.0 is variably subnetted.250. Serial0 203. in order to reach next hop 192. Also. enable OSPF on serial 1 of RTB to make it passive. I − IGRP. 2 subnets C 203.0.250. Now. you need to go the other way .0.213.0 [110/2000] via 203.15. E2 − OSPF external type 2.13.200. Serial0 O E2 192.250.208. IA − OSPF inter area E1 − OSPF external type 1.0 [110/74] via 203.1. routing loops occur because. Serial0 O 203.15.255.0.255 area 0 router bgp 100 network 203. 00:00:15.1.255.250.0. B − BGP D − EIGRP. 00:00:14. L1 − IS−IS level−1. Serial0 128. 00:00:16. This step allows RTA to know about the next hop 192. bring up the RTB s1 interface to see what the routes look like.1.255.0 0. 2 masks O 203.15.250.213.250.255.10.250.10.15.15.255.255.10.0 255.250.213.13.63.63. 2 subnets. while the iBGP distance is 200. Keep synchronization off on RTB so that RTB can advertise 203.250. 00:00:15.255.0.250.15. Serial0 The BGP entries have disappeared because OSPF has a better distance than iBGP. R − RIP.0.0 255.13.250.213.15.250.15.14.0 [110/2000] via 203.250.1 255.2 update−source Loopback0 The routing table looks like this: RTB# show ip route Codes: C − connected.213.0 255. Serial0 O E2 203.15.250. This action is necessary because RTA does not synchronize with OSPF because of the difference in masks.15.1. 00:00:14.250.63.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 network 203.250.13.10.255.2 remote−as 200 neighbor 203.0.Serial0 O 128. * − candidate default Gateway of last resort is not set O E2 200.250.0 interface Serial0 ip address 128.5.208.41 255. This action is necessary on RTB for the same reason.41 255.255.250.

213.5 remote−as 300 neighbor 203.via eBGP.0 0.1 255.255.14.250.0.0 128.10.10. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 128.13.250.63.250.63.0 192.213. d damped.2 remote−as 200 neighbor 203.15.41 Status codes: s suppressed.5 0 100 0 300 i *>i195.0 neighbor 192.41 255.10.250.255.250.255 area 0 router bgp 100 no synchronization network 203.255.2 0 0 200 i *>i192.41 remote−as 100 The BGP tables look like this: RTA# show ip bgp BGP table version is 117.0 192.0.208. > best.255.0 0. e − EGP. i −internal Origin codes: i − IGP.0 interface Serial0 ip address 128.255.250.208.255.213.213.255.0.15. h history.250. local router ID is 203.6 255.0 interface Ethernet0 ip address 203.0 0.255 area 0 network 192.255.250.208.255.15.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 network 203.0.2 remote−as 100 neighbor 203.255.14.250.2 255.213.0.0 network 203.13.208.5 100 0 300 500 i * 128.252 interface Serial1 ip address 192.0.213.255 area 0 router bgp 100 no synchronization network 203.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive−interface Serial1 network 203.208.255.0.0 0.10.13.255.255.250.255.211.0. * valid.2 0 200 400 500 i .63.10.0.250.2 update−source Loopback0 RTB# hostname RTB ip subnet−zero interface Serial0 ip address 203. These are the new configurations of RTA and RTB: RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203.15.250.63.10.255 area 0 network 128.1 255.208.0 neighbor 128.13.

213.14. h history. You can learn partial routes from one of the ISPs and default routes to both ISPs. you receive partial routes from AS200 and only local routes from AS300. AS200 and AS300. the same major net.255. with attributes such as local preference.2 100 0 200 400 i * 192.211.252 router ospf 10 redistribute bgp 100 metric 2000 subnets passive−interface Serial0 .0 0 32768 i There are multiple ways to design your network to talk to the two different ISPs.255.0. when you talk to the two ISPs.208.255. *> 200.10. This situation can occur if you use the same pool of IP addresses.250. This configuration is the final configuration for all the routers: RTA# hostname RTA ip subnet−zero interface Loopback0 ip address 203.63.15.213.250.208.2 0 200 400 i *> 203.0 0.5 0 0 300 i *> 195.13.63.13. even though you have multiple points to the Internet. Both RTA and RTB generate default routes into OSPF.250.2 0 100 0 i RTB# show ip bgp BGP table version is 12.10.13. traffic from AS400 that has your network as the destination always comes in via RTA because of the shorter path.213. In the example. In this example.208. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.13.0. Potential asymmetry can occur if traffic that leaves RTA comes back via RTB.15.14. Another potential reason for asymmetry is the different advertised path length to reach your AS.15. In this way.213. or weight.0 interface Serial0 ip address 128.10. You can discover that all incoming traffic to your AS arrives via one single point.1 255.15.41 0 100 0 i *>i203. metric.200. your whole AS can look like one whole entity to the outside world.5 0 300 500 400 200 i *> 192.10.0 128. One way is to have a primary ISP and a backup ISP.0 0.10.0 203. 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. d damped.255.250. e − EGP.0 203.255. you have two different major nets when you talk to the two ISPs. you can balance outgoing traffic between the two ISPs.63.250.0.213.0.5 0 300 500 i *>i200.0 192.10. with RTB as the preference because of the lower metric.0.5 0 300 500 400 i *>i203.250.63.0 192. But. You can try to effect that decision. > best.208. * valid.0 128.250. In this case.250.10 Status codes: s suppressed. i −internal Origin codes: i − IGP.0 0. local router ID is 203.255.41 0 100 0 i *> 203.250.0 203.41 255. In the example.0 0 32768 i *>i203.250.0.13.0.0 128.200.1 255. there is nothing that you can do.0 0 32768 i *> 203. Perhaps one service provider is closer to a certain destination than another. Entry points to your network can occur via RTA or RTB.250.10. AS400 can have set the exit point to be AS200.10.208.2 0 100 0 200 i * 192.250.0 interface Ethernet0 ip address 203.14. Because of aggregation.

0 into the IGP domain.250. you can redistribute a static route to 0.0 neighbor 128.15.15.200.208.213.63.2 255.255.250.0 route−map setlocalpref permit 10 set local−preference 200 On RTA.213. This example also uses this command with Intermediate System−to−Intermediate System Protocol (IS−IS Protocol) and BGP.0.2 255. Also.10 255.0 0.255 area 0 ip classless RTB# hostname RTB ip subnet−zero interface Loopback1 ip address 203.255.255.0. network 203.0.0. For IGRP and EIGRP. there is an automatic redistribution into RIP of 0.255 area 0 network 192.2 remote−as 100 neighbor 203.14.13.6 255.250.6 0.0 0.250.1 255.15. Also.255.250.0.200.0 area 0 .0 interface Serial1 ip address 203.255.255. RTF# hostname RTF ip subnet−zero interface Ethernet0 ip address 203.0. use of the default−information originate command with OSPF injects the default route inside the OSPF domain.0.255.0 0.14.250. injection of the default information into the IGP domain occurs after redistribution of BGP into IGRP and EIGRP.0.2 update−source Loopback0 ip classless ip default−network 200.255 area 0 default−information originate metric 2000 router bgp 100 no synchronization network 203.208.255.0 network 203.255.255.0.0.250.250.10.255 area 0 network 128.0 0. For RIP.255.0.15. Also in this example.250.252 interface Serial0 ip address 203.2 remote−as 200 neighbor 128. network 200.0.63.252 router ospf 10 redistribute bgp 100 metric 1000 subnets passive−interface Serial1 network 203.0.0.255.213.0 is the choice for the candidate default.252 router ospf 10 network 203.250.0.2 route−map setlocalpref in neighbor 203.10.250.0. The ip default−network command enables you to choose the default.252 ! interface Serial1 ip address 192.0.15. without additional configuration. with IGRP and EIGRP.255. the local preference for routes that come from AS200 is set to 200.255.

6 remote−as 400 ip classless access−list 1 deny 195. e − EGP.10.250.41 remote−as 100 ! ip classless ip default−network 192. i − internal Origin codes: i − IGP.208.213. if other routes exist.63.208.0. which are the customers of the ISP.63.13.5 0 300 0 300 RTC# hostname RTC ip subnet−zero interface Loopback0 ip address 128.10.255. If the ISP refuses to do this task.0 ip as−path access−list 1 permit ^300$ route−map localonly permit 10 match as−path 1 set local−preference 300 For RTB. * valid.1 distribute−list 1 out neighbor 128. This value is lower than the local preference of 200.255 access−list 1 permit any On RTC.10. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 192.255.255.5 255. AS100 picks RTB for the local routes of AS300.15. Any other routes on RTB. In this way.0/16 and indicate the specific routes for injection into AS100.213.213. So RTA is the preference. you must filter on the incoming end of AS100. h history.63.250.0 192. default−information originate metric 1000 ! router bgp 100 no synchronization network 203. If you want to advertise the local routes and the neighbor routes.213.0 neighbor 128.5 route−map localonly in neighbor 203.0 neighbor 192.255.10 Status codes: s suppressed.5 remote−as 300 neighbor 192.1 remote−as 100 neighbor 128.0. local router ID is 203. d damped.0.213.211. use ^300_[0−9]*.10.252 router bgp 200 network 128. This value is higher than the local preference value of iBGP updates that come from RTA.255.213.208.63. the local preference for updates that come from AS300 is set to 300.213. you aggregate 128. Note: You only advertised the AS300 local routes.192 interface Serial2/0 ip address 128.208.15.208.0 0.63. which comes from RTA.250. Here is the output of the regular expression that indicates the AS300 local routes: RTB# show ip bgp regexp ^300$ BGP table version is 14.2 255. > best. .255.255. transmit internally with a local preference of 100.63. Any path information that does not match ^300$ drops.213.10.0.130 255.252 ! interface Serial2/1 ip address 128.

10.252 ! interface Serial0/1 ip address 192.1 255.0 0.211. in this case.255.192 ! interface Serial0/0 ip address 192.174 255.10. RTD# hostname RTD ip subnet−zero interface Loopback0 ip address 192.211.255.208.252 router bgp 300 network 192.6 remote−as 100 RTG# hostname RTG ip subnet−zero interface Loopback0 ip address 195.0.1 255.0 aggregate−address 195.255.10.192 interface Serial0 ip address 192.208.1 255.2 remote−as 400 ! ip classless access−list 1 permit 195.255.10.0.1 remote−as 500 neighbor 192.0.255.10.10.211.208.211.2 remote−as 300 neighbor 192.2 send−community neighbor 192.10.208.10.255.0.2 255.10. In this way.10.255.200.255.255.211.208.208.0 interface Serial0 ip address 195.10.255.10.208.208.211.10.255.5 255.2 255.10. RTD does not export that route to RTB.255.0 255.10.0 summary−only neighbor 192. However. RTE# hostname RTE ip subnet−zero interface Loopback0 ip address 200.255.10.0 neighbor 192.208.252 .255. RTB does not accept these routes anyway.2 route−map setcommunity out neighbor 195.0.255.252 interface Serial1 ip address 195.255.252 router bgp 500 network 195.211. You add a no−export community to 195.211.255.255.208.0 updates toward RTD.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.174 255.

2 to network 0.10.0 192. Ethernet0 C 203.0 192. 00:41:26 C 128.63.252 is directly connected. d damped.200.252 [110/74] via 203.208.213.255. RTF.0. S − static. IA − OSPF inter area E1 − OSPF external type 1.0 [110/1000] via 203.14.0 is directly connected. L2 − IS−IS level−2.0.0 128.0.213.250.255.0 203.0. EX − EIGRP external.255 [110/75] via 203.255.10.63.0.6 255.250.0.200. E2 − OSPF external type 2.2 0 100 0 i RTA# show ip route Codes: C − connected. L2 − IS−IS level−2. EX − EIGRP external. L1 − IS−IS level−1.208.213. B − BGP D − EIGRP.13. I − IGRP.2.255.10.2. Ethernet0 B 203.211.0.213. Serial0 O*E2 0.10.63. E − EGP i − IS−IS.255.63.0 is variably subnetted.0 is variably subnetted.0 aggregate−address 200.0.0 0 32768 i *> 203.250.208.5 remote−as 200 neighbor 195. and RTB: RTA# show ip bgp BGP table version is 21. 00:41:25.0.14.213.0.0/16 128.0 [200/0] via 203. 2 masks . B − BGP D − EIGRP.250.0 summary−only neighbor 128.255.200.0/0 [110/1000] via 203. E − EGP i − IS−IS.4 255. 2 subnets.250.15.0 [20/0] via 128.255.13.250. 00:41:25.14.208.15.213.250.250. ? − incomplete Network Next Hop Metric LocPrf Weight Path *> 128. * − candidate default Gateway of last resort is 203. IA − OSPF inter area E1 − OSPF external type 1.252 [110/138] via 203.2 0 200 0 200 i *>i192.250.0. Ethernet0 O 192.0 255.1 remote−as 500 ip classless RTE aggregates 200. Here are the final BGP and routing tables for RTA.2 200 0 200 400 i *> 203. 2 subnets.250. i − internal Origin codes: i − IGP.41 Status codes: s suppressed.10.0 is variably subnetted.0. * − candidate default Gateway of last resort is 128.14. O − OSPF.2 to network 200.14. 00:41:25 C 203.2. 00:02:38 RTF# show ip route Codes: C − connected.13.213.255.0 255.200. R − RIP. > best. * valid.0 0.250.200.255. Ethernet0/0 B* 200. 00:41:25. h history.255.14.0.255.10.255. Loopback0 203.0. 2 subnets.255.0 255.0.63.0 is variably subnetted.14. 3 masks O 203.15.255. M − mobile.2.10 255.2.250.213.2. S − static.5 0 300 0 300 i *> 200.15.208. Ethernet0 128.208.200. interface Serial1 ip address 128.0. E2 − OSPF external type 2.15.255.250.0 255.0 192. 00:41:25.213.0/16. R − RIP.0 0 32768 i *>i203.10.0.63. local router ID is 203.213.255. L1 − IS−IS level−1.63.15.250.0 0.0 255.2.250.255.0. O − OSPF.15.250.63.0 is directly connected.0.10. 3 subnets. e − EGP.15.0 255.0 [20/0] via 128.250. I − IGRP.213. 2 masks O E2 192. 2 masks B 128.252 router bgp 400 network 200.0 255. M − mobile.2. Ethernet0 O 203.250.

0 192.0.255. Loopback1 C 203.15. 01:12:11. > best.255.250. e − EGP.208.0.208.0/16 128.0 255.250.0. Serial1 C 203. 2 subnets. 00:46:55.252 [110/74] via 203. I − IGRP. d damped.255.15.255.0 255.255. Serial0 O 203. L1 − IS−IS level−1. Serial1 O 192.213.250.255.0.0 [110/2000] via 203.255. Ethernet0 O E2 203.208. S − static.0 [110/74] via 203.0 [110/1000] via 203.0 [110/1000] via 203.200.255.13. 2 masks O E2 128.0 is variably subnetted.0. 00:48:50.15.0 is variably subnetted.0 [110/2000] via 203.213. 01:12:09.213.10.2.213.0 255.0. E − EGP i − IS−IS.208. Ethernet0 O E2 200.1. 2 subnets.250.0 255.250.41 255.13. Serial1 203.255.0 is variably subnetted. Serial0 128.252 is directly connected. R − RIP. i − internal Origin codes: i − IGP.250. 2 masks O 203.15.250.63.255 [110/75] via 203. Serial1 Note: The RTF routing table indicates that the way to reach networks local to AS300.1. 2 subnets C 203.10.10. Ethernet0 128.250.1.41 0 100 0 i *> 203. 2 subnets.14.250. RTB# show ip bgp BGP table version is 14.5 to network 192.0.250.250.0 [110/2000] via 203.13.250. 01:12:09.14.250.10.255.250.0.250.15.255.63.1.15. local router ID is 203.250.250.0.10. 01:20:33.0 128.255.255 [110/65] via 203.2.250.255.0 is variably subnetted. B − BGP D − EIGRP.208. M − mobile.0 0 32768 i RTB# show ip route Codes: C − connected. Serial1 203.255.250.0. 2 masks O 203.1.2.1.208.0 255.250.0.41 0 100 0 i *>i203.255.250. 00:50:46 C 192.0 is variably subnetted.250.5 0 300 0 300 i *>i200.255. Serial0 203.252 is directly connected.0 * 192.41 255.250. L2 − IS−IS level−2.0 0. 01:20:33.0 203.255.10. is through RTA.0 is directly connected. Serial1 C 203.252 is subnetted.15. E2 − OSPF external type 2.15. 2 subnets.250.15. 2 masks O E2 128. h history. Serial0 .15. ? − incomplete Network Next Hop Metric LocPrf Weight Path *>i128.208.213. 2 subnets. EX − EIGRP external.250. such as 192.14.0 is variably subnetted.0 255.250.15.255 [110/11] via 203.0.250.10.213. 00:03:47.4 255. * − candidate default Gateway of last resort is 192.250. The gateway of last resort is set to RTB.4 255.1.255.0 is directly connected. 00:45:01.0 [110/2000] via 203.13. IA − OSPF inter area E1 − OSPF external type 1.208.200.252 [110/128] via 203.250.255.0.255.0 [110/2000] via 203. 01:15:40.2 0 200 0 200 i *> 192. O E2 192.0 0.2 200 0 200 400 i *>i203. the default that RTA advertises kicks in with a metric of 2000.0.1.255. O − OSPF.250.14.15.10 Status codes: s suppressed.0. 00:03:33. is through RTB.10 255.1.13.0.250.0 255.255.0. Ethernet0 203.0 [20/0] via 192. Ethernet0 O 128.14.13.8 is directly connected.14.0. such as 200.255. 2 subnets.10.15. 2 masks O 203. 2 masks B* 192.250.14.15.213.0 255.10.10. If something happens to the connection between RTB and RTD.2.0 255.13.0 203. 01:12:09.13.250.13. * valid.15.255. Ethernet0 O*E2 0.208.0.15. The way to reach other known networks.255.14.208.0 255.255.255.63. Serial0 O E2 203.208. 01:12:09.213.200.5.10.

Serial0 O E2 200. and technologies. The featured links are some of the most recent conversations available in this technology. Updated: Feb 13.0. All rights reserved. O 128.63. 00:08:33.255.1. and information about networking solutions.250.0 [110/2000] via 203.0 255. 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. 01:20:34.15.0. Serial0 NetPro Discussion Forums − Featured Conversations Networking Professionals Connection is a forum for networking professionals to share questions.250. Inc.0. suggestions.250.0 255.213. 00:05:42.200.15.15.255. Important Notices and Privacy Statement.0.0/0 [110/2000] via 203. Serial0 O*E2 0. 2008 Document ID: 26634 .1.252 [110/138] via 203.1.255. products.