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Configuring BGP4
Figure 10.4 shows an example of a BGP4 confederation.
IBGP
Sub-AS 64513
Router D
AS 20
Router C
This BGP4 router sees all
traffic from Confederation 10
as traffic from AS 10.
Routers outside the confederation
do not know or care that the routers
are subdivided into sub-ASs within a
confederation.
Sub-AS 64512
Router B
Router A
IBGP
EBGP
EBGP
Confederation 10
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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