EIGRP summarization caveat

July 24, 2008 at 4:14 pm | Posted in EIGRP, Routing | Leave a comment

WB1-Ver5-Task 5.11

By default when advertizing a summary route in EIGRP (and also OSPF, and BGP), router automatically installs a route to Null0 to match the summary. This is to prevent the router from forwarding traffic for destination inside the summary that it does not have a longer match for.

However in certain design scenario this could be undesirable behavior. To resolve this, we can implemen one of these options:

    • Keep the default admin distance of 5 for summary route, but add another route with distance of 1-4, e.g. a static route, to override the route to null.
    • Change the default distance of 5 to 255, meaning the router will not install the route to null0

      This is difference from summarization in RIP. RIP does not installs a route to null, which may create a “route feedback”, which may make the network unstable (reachblility is on and off). More specifically, the router may receives a route from neighbor, without knowing that the orignator is itself. The router then advertize this “”new route” with a new RIP metric, creating a route-advertisment loop. The route eventually become a dead route, due to infinte metric, and is then removed from the routing table. When the original router removes it from the routing table, the cycle starts again with the summarize route being advertized with original metric of 1. To guard again this route feedback behaviour, we can implement one of the following options:

        • Set a static route, pointing to null on the router that originate default route.
        • Filter this route from being learnt from its neighbor, or
        • Advertize the summary route to ALL its neighbors.
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