Confederations Cont

© 2002, Cisco Systems, Inc. All lights reserved.

Cisco CCIE Prep v1.0—Module 8-4!

© 2002, Cisco Systems, Inc. All lights reserved.

Cisco CCIE Prep v1.0—Module 8-4!

In this scenario, AS200 is running a common IGP. Each router is advertising its loopback interface. AS200 is sub-divided into to mini-AS's 65345 and 65016.

When customers are multihomed to a single Internet Service Provider (ISP), the ISPs assign private Autonomous System (AS) numbers in order to conserve AS numbers. These private AS numbers come from the range 64512 to 65535.

A confederation is a technique for reducing the iBGP mesh inside the AS. In the diagram, AS 200 consists of multiple BGP speakers (although there might be other routers that are not configured for BGP). Without confederations, BGP would require the routers in AS 200 to be fully meshed. That is, each router would need to run iBGP with each of the other routers. Specifically:

■ You use confederations to divide the AS into multiple sub-AS's and assign the sub-AS's to a confederation

■ Each sub-AS is fully meshed, and iBGP is run among its members

■ Each sub-AS has a connection to the other sub-AS's within the confederation

Remember, even though the sub-AS's have eBGP peers to AS's within the confederation, they exchange routing updates as if they were using iBGP.

Now look at the configuration for R3:

R3 R3 R3 R3

(config)# router bgp 65345

(config-router)# bgp confederation identifier 200 (config-router)# bgp confederation peers 65016 (config-router)# network 3.3.3.0 mask 255.255.255.0

R3(config-router)# neighbor R3(config-router)# neighbor R3(config-router)# neighbor R3(config-router)# neighbor

172.16.23.2 remote-as 100 172.16.45.5 remote-as 65345 172.16.70.4 remote-as 65345 172.16.134.1 remote-as 65016

Analyze this configuration to see exactly what is happening.

R3(config)# router bgp 65345

This command is stating that R3 is part of AS 65345.

R3(config-router)# bgp confederation identifier 200 R3(config-router)# bgp confederation peers 65016

These commands work in conjunction with the router bgp 65345 command. They are stating that this router is actually part of overall AS 200, but the sub-AS is 65345. That means you will perform iBGP peering with other routers in sub-AS 65345, not AS 200.

Other sub-AS's of AS 200 are defined with the bgp confederation peers command. In this case, only sub-AS 65016 is defined. If there were other sub-AS's they would also need to be defined here.

R3(config-router)# network 3.3.3.0 mask 255.255.255.0

This command states R3 will be advertising network 3.3.3.0 to its BGP peers.

R3(config-router)# neighbor 172.16.23.2 remote-as 100 R3(config-router)# neighbor 172.16.45.5 remote-as 65345 R3(config-router)# neighbor 172.16.70.4 remote-as 65345 R3(config-router)# neighbor 172.16.134.1 remote-as 65016 These commands define your neighbors.

Neighbor 172.16.23.2 is part of AS 100 an eBGP neighbor. Normal BGP processes occur with this neighbor.

Neighbors 172.16.45.5 and 172.16.70.4 are part of the mini-AS and standard iBGP processing occurs here.

Neighbor 172.16.134.1 is part of mini-AS 65016 R3's confederation peer. Even though you have an eBGP neighbor relationship and it is part of AS 200's confederation, normal iBGP processing will occur.

Confederations (Cont.)

* Routers 3, 4, and 5 are configured for the same confederation

* A full BGP mesh is no longer necessary

Rl(config)# router Rl(config-router) # Rl(config-router) # Rl(config-router) # Rl(config-router) # Rl(config-router) #

bgp 65016

bgp confederation identifier 200 bgp confederation peers 65345 network 1.1.1.0 mask 255.255.255.0 neighbor 172.16.16.6 remote-as 65016 neighbor 172.16.134.3 remote-as 65345

R6(config)# router R6(config-router) # R6( config-router) # R6(config-router) # R6(config-router)# R6(config-router) #

bgp 65016

bgp confederation identifier 200 bgp confederation peers 65345 network 6.6.6.6 mask 255.255.255.0 neighbor 172.16.16.1 remote-as 65016 neighbor 172.16.134.3 remote-as 65345

© 2002, Ci

co Systems, Inc. All rights reserved.

Cisco CCIE Prep v1.0—Mo

ule 8-49

You can examine these two other configurations, specifically R6 and Rl.

Notice that since R6 does not peer with a router in sub-AS 65345 the bgp confederation peers command is not required. Placing it in the configuration will not affect BGP. This is also true for R4 and R5 in AS 65345.

Since Rl peers with sub-AS 65345, the confederation peer statement is required here. Finally, view what the BGP table looks like on Rl.

Rl#show ip bgp

BGP table version is 13, local router ID is l.l.l.l

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Rl#show ip bgp

BGP table version is 13, local router ID is l.l.l.l

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

Origin

codes: i -

- IGP, e

-

EGP, ?

- incomplete

Network

Next

Hop

Metric

LocPrf

Weight

Path

*> l.

l

l

.0/24

0.0.

0.

0

0

32768

i

*> 2 .

2

2

.0/24

l72.

l6

.23.2

0

l00

0

(65345)

l00

*> 3 .

3

3

.0/24

l72.

l6

. l34.3

0

l00

0

(65345)

i

*> 4 .

4

4

.0/24

l72.

l6

.70.4

0

l00

0

(65345)

i

*> 5 .

5

5

.0/24

l72.

l6

.45.5

0

l00

0

(65345)

i

*>i6 .

. 6 .

6

0/24

l72.

l6

.l6.6

0

l00

0

I

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