BGP Route Reflectors and Advertisement: Why doesn't the route reflector advertise the route?

[UncleJP]

A route reflector learned a route from a route reflector client. However, the route reflector does not advertise the route to other route reflector clients, because it doesn't have a way to reach that route itself.

It seems like the route reflector should advertise the route to the clients anyway. Why doesn't it advertise the route?

Any input is appreciated. 

[Giuseppe Larosa · Accepted solution]

Hello @UncleJP ,

the route reflector server has to first accept / validate the advertisement in its own BGP table.

Most of the times the issue is that the BGP next-hop is unknown = there is no route to reach it on the RRS.

Solutions are :

advertise in IGP subnets with eBGP peers using passive interface.

or

use neighbor x.x.x.x next-hop self   on the RR client to force a BGP next-hop attribute change to the IP address of the RR client

(the root cause is the BGP next-hop is left unchanged in IBGP for an advertisement coming from an eBGP peer)

To add more , a BGP RRS propagates only the best advertisement / best path and not all the received ones.

The iBGP multipath was later added to support this scenario.

However, route reflection is not a blind / uncontrolled operation the RRS has the responsability to check the validity of the BGP advertisement from its own point of view. It would make no sense to propagate advertisements that would be dropped / ignored by all other RR clients for the same reason that would make the advertisement not valid on the RRS itself.

Hope to help

Giuseppe