What would happen in a BGP route reflector cluster if the route reflector loses connection with the client but not the rest of the network?

If a BGP (Border Gateway Protocol) route reflector (RR) loses connection with one of its clients but remains connected to the rest of the network (including other clients, non-clients, and other route reflectors), the impact is localized to the communication between the RR and the affected client. Here's what happens in more detail:

1. Loss of Routes from the Client

• The route reflector stops receiving BGP updates from the disconnected client.

• Any prefixes originated by the client or learned through the client are withdrawn from the route reflector's BGP table.

• These withdrawn routes are also no longer advertised by the route reflector to other clients or external peers, leading to potential reachability issues for those prefixes.

2. No Redistribution to the Client

• The client stops receiving route updates from the route reflector.

• As a result, the client may lose access to routes that were only reachable via the route reflector (e.g., routes from other clients or external peers that the RR was redistributing).

3. Impact on Other Clients and the Network

• Other clients of the same route reflector will not be able to reach the prefixes learned only through the disconnected client.

• However, if redundant paths exist (e.g., the client is also peered with other RRs or iBGP peers), those paths may be used instead, minimizing the impact.

4. Route Flapping and Convergence

• The network may experience some route withdrawals and re-advertisements (flapping), especially if multiple paths existed via the lost client.

• BGP convergence will occur as alternative paths are recalculated.

5. Recovery

• Once the BGP session between the route reflector and the client is re-established:

  • Full route exchange resumes.

  • Prefixes are re-learned and re-advertised as appropriate.

Summary

The key impact of a BGP RR losing connection with a client is:

• Loss of client-originated routes network-wide.

• Client loses access to reflected routes.

• The impact is isolated, unless that client was a sole source for some critical routes.

To mitigate such issues, best practices include:

• Using multiple route reflectors for redundancy.

• Having direct iBGP sessions between critical routers.

• Implementing BFD (Bidirectional Forwarding Detection) for fast failure detection.