EIGRP "feedback"/loops with autosummary

Unanswered Question
Jul 24th, 2008


I am somewhat puzzled by EIGRP's summarization when it says that next hop is null 0. From BSCI book I understand that this technique, of forwarding packets for unknown subnets to a null interface, somehow suppose to prevent routing loops but I can't see how or why. The book is somewhat brief about this and google wasn't really of any help either.

Can somebody please explain how this is suppose to prevent routing loops?



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Giuseppe Larosa Fri, 07/25/2008 - 03:45

Hello Assaf,

packets with a destination interface null0 are silently discarded by the router.

So the router that generates the EIGRP summary route adds a routing entry for same prefix to null0.

If later the router receives packets destined for un unknown subnet within the summary route IP prefix it can discard it.

This avoids to send the packet to some other router with the risk to receive it back (for the EIGRP summary route) that would be a routing loop: a packet going out and back between two routers.

It avoids to send in the network packets that will be discarded when their TTL becomes 0.

Hope to help



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