BGP route reflectors

Route Reflector rules 

A BGP router implementing route reflector functionality propagates BGP routes according to these rules: Locally originated routes and routes received from EBGP neighbors and selected as best routes are propagated to all BGP peers (internal and external). Routes received from an IGBP peer that is not a route-reflector client and selected as best routes are propagated to all EBGP peers and all IGBP peers configured as route-reflector clients. Routes received from a route-reflector client and selected as best routes are propagated to all BGP peers (internal and external). 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.  

Loop Detection and Avoidance The BGP routers implementing original route reflector RFC (RFC 1966) use the following mechanisms to detect route-reflector-related loops: If the Originator ID in an incoming IBGP route update is equal to the BGP router ID, the update is ignored (reflected route was propagated back to its originator). If the Cluster ID of a route-reflector appears in the Cluster list attribute of an incoming IBGP update, the update is ignored (route reflector loop). The route reflector functionality was introduced in IOS release 11.1. It’s therefore relatively safe to assume that every Cisco router encountered in a production network supports it. Although the mechanisms specified in RFC 1966 ensure loop-free IBGP operation regardless of the actual topology of the IBGP sessions, RFC 4456 added new route selection rules that improve the actual convergence within an AS and reduce the amount of BGP updates propagated across the AS: Routes with shorter Cluster list attribute are preferred. This rule ensures that all routers select routes with minimum number of reflections, significantly reducing the amount of unnecessary BGP updates (remember: only best routes selected by a route reflector are reflected to its clients). The Originator ID attribute of a reflected route should be used as the Router ID attribute when comparing otherwise identical IBGP routes, ensuring the stability of route selection across AS regardless of the route reflectors.

Ref:http://wiki.nil.com/BGP_route_reflectors