BGP best path selection

Hi. 

 

I need a confirmation on this. what if a router that are RR server, receives a route from its client but have better route than that. in my lab, it selected just the best route that it has and advertise it to other clients; taht is, it did not advertised the routes that have been received from the RR clients to other clients (based on the RR rule), just because it has the better route. is it a BGP normal behavior?

this can be right but has side effects; for example, suppose that RR server advertised the routes received from the RR clients to other clients beside the route that it had before, and this route had a better next-hop address?

Comments

  • It is expected behaviour, as BGP advertises out only one best-path by default; route-reflection just allows to reflect ibgp paths in case those are selected as best.

    I don't see what is the problem with the next-hop.

  • I been studying this exact thing too recently. Brian describes this as being a trade-off for scaling BGP. So the RR only reflects HIS best path, and nothing else. Therefore, when the RR reflects a route from clientA to ClientB, it means ClientB doesn't make the most "informed" decision because it doesn't receive all of the possible information since the RR only advertised one route (the RR's best path). In this situation, it's sometimes preferred to have a partial iBGP mesh to the other routers so that if the best path selection gets to point 8 in the algorithm (best path is determined based on lowest IGP cost to the next hop), the router can pick the true best path according to the algorithm.

  • peetypeety ✭✭✭

    Correct.

    -------Real World Bulletin, disregard if you're studying for the exam-----

    A somewhat common trick to this problem, assuming you have different regions each with one or more RRs, is to enable 'no bgp client-to-client reflection' on the RRs, then keep a full mesh WITHIN the region. That way, your clients have fewer sessions than if you were still truly full-mesh, but more than if you stuck with traditional route reflection. The clients learn all local routes from their local neighbors, and (best) remote routes from their RRs.

     

    --------end Real World Bulletin---------

     

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