BGP Cluster-ID

Background Information

1. Route-reflectors reflect iBGP updates back to other iBGP peers so we don't have to create a full mesh between all iBGP peers.  
2. That means that the other iBGP peers only peer up with the route-reflectors and not each other.
3. This also means that if a route-reflector dies, then the iBGP peers will not have any updates from each other.
4. To protect against the failure of the route-reflector, we can create a redundant route-reflector.  You peer up all iBGP routers to the two redundant route-reflectors.
5. When you have multiple route-reflectors, cluster-id is used to detect routing information loop.  If a route-reflector receives an update with its own cluster-id, the update will be dropped.
6. By default, route-reflectors will use its router-id as the cluster-id.
7. When route-reflectors are configured to have the same cluster-id, they will drop updates from each other.  This means that if an adjacency fails somewhere between a route-reflector and another iBGP router, that route-reflector will no longer have prefixes advertised by the iBGP router because it will not accept the other route-reflectors updates.