3.7.e Implement and troubleshoot scalability

3.7.e [i] Route-reflector, cluster

BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when there are many iBGP speakers. Instead of configuring a confederation, another way to reduce the iBGP mesh is to configure a route reflector.

When the route reflector receives an advertised route , depending on the neighbor, it takes the following actions:

A route from an external BGP speaker is advertised to all clients and non-client peers.

A route from a non-client peer is advertised to all clients.

A route from a client is advertised to all clients and non-client peers.

Hence, the clients need not be fully meshed.

To configure a route reflector and its clients, use the following command in router configuration mode:

Router( config-router)# neighbor ip-address | peer-group-name route-reflector-client

Whenever an IBGP route is reflected (propagated to another IBGP peer), the route reflector appends two optional, non-transitive attributes to the BGP route:

If the route does not have the Originator ID attribute (it has not been reflected before), the router ID of the IBGP peer from which the route has been received is copied into the Originator ID attribute.

If the route does not have the Cluster list attribute, it’s added to the route.

The value configured with the bgp cluster-id router configuration command (or the router ID of the route reflector if the cluster-id is not configured) is prepended to the Cluster list attribute.

Route reflector does not change or remove any other attributes of the reflected routes (even non-transitive attributes), ensuring that the iBGP routes are not changed within the autonomous system.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4022-4025).  . Kindle Edition.

http://blog.ipspace.net/2013/10/can-bgp-route-reflectors-really.html