Configuring confederations is optional and should only be implemented to reduce the interior border gateway protocol (IBGP) mesh inside an AS. An AS can be logically divided into smaller groupings called sub-confederations and then assigned a confederation ID (similar to an autonomous system number). Each sub-confederation has fully meshed IBGP and connections to other ASs outside of the confederation.
The sub-confederations have EBGP-type peers to other sub-confederations within the confederation. They exchange routing information as if they were using IBGP. Parameter values such as next hop, metric, and local preference settings are preserved. The confederation appears and behaves like a single AS.
Confederations have the following characteristics:
A large AS can be sub-divided into sub-confederations.
Routing within each sub-confederation is accomplished via IBGP.
EBGP is used to communicate between sub-confederations.
BGP speakers within a sub-confederation must be fully meshed.
Each sub-confederation (member) of the confederation has a different AS number. The AS numbers used are typically in the private AS range of 64512 to 65535.
To migrate from a non-confederation configuration to a confederation configuration requires a major topology change and configuration modifications on each participating router. Setting BGP policies to select an optimal path through a confederation requires other BGP modifications.
There are no default confederations. Router confederations must be explicitly created. Figure: Confederation configuration shows an example of a confederation configuration.
