There is a formula that you can calculate how many sessions is needed in network. Let’s say that you have 10 routers and that you have configure peer sessions with all routers in AS.
N*(N-1)/2
N represent total number of routers in AS. For example let’s say that we have 10 routers, we come up to 45 sessions that must be configured.
There is tool that help as to reduce number of needed sessions in network and that is route reflector.
Route reflection allows creating peer relationships only with designated route reflector router.
In that case you will need only IBGP peer session between route reflector clients and selected route reflector router.
Router reflector and router reflector clients must be in the same cluster.
You can have more than one router reflector router under same AS, also to enable connectivity for router reflector client and nonclient routers under AS, route reflector router must be full meshed with all routers in AS.
