Configuring BGP4

Figure 10.4 shows an example of a BGP4 confederation.

Confederation 10

 

AS 20

Sub-AS 64512

 

 

IBGP

 

 

Router A

Router B

 

 

 

EBGP

EBGP

 

 

Sub-AS 64513

 

This BGP4 router sees all

 

 

 

 

traffic from Confederation 10

IBGP

 

as traffic from AS 10.

 

 

 

 

Routers outside the confederation

 

 

do not know or care that the routers

Router C

Router D

are subdivided into sub-ASs within a

confederation.

 

 

Figure 10.4 Example BGP4 confederation

In this example, four routing switches are configured into two sub-ASs, each containing two of the routing switches. The sub-ASs are members of confederation 10. Routers within a sub-AS must be fully meshed and communicate using IBGP. In this example, routers A and B use IBGP to communicate. Routers C and D also use IBGP. However, the sub-ASs communicate with one another using EBGP. For example, router A communicates with router C using EBGP. The routers in the confederation communicate with other ASs using EBGP.

Routers in other ASs are unaware that routers A – D are configured in a confederation. In fact, when routers in confederation 10 send traffic to routers in other ASs, the confederation ID is the same as the AS number for the routers in the confederation. Thus, routers in other ASs see traffic from AS 10 and are unaware that the routers in AS 10 are subdivided into sub-ASs within a confederation.

Configuring a BGP Confederation

Perform the following configuration tasks on each BGP router within the confederation:

•Configure the local AS number. The local AS number indicates membership in a sub-AS. All BGP routers with the same local AS number are members of the same sub-AS. BGP routers use the local AS number when communicating with other BGP routers within the confederation.

•Configure the confederation ID. The confederation ID is the AS number by which BGP routers outside the confederation know the confederation. Thus, a BGP router outside the confederation is not aware and does not care that your BGP routers are in multiple sub-ASs. BGP routers use the confederation ID when communicating with routers outside the confederation. The confederation ID must be different from the sub- AS numbers.

•Configure the list of the sub-AS numbers that are members of the confederation. All the routers within the same sub-AS use IBGP to exchange router information. Routers in different sub-ASs within the confederation use EBGP to exchange router information.

To configure a routing switch to be a member of a BGP confederation, use one of the following methods. The procedures show how to implement the example confederation shown in Figure 10.4.

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