EIGRP Summary Route & NULL 0

EIGRP Summary Route & Null 0

 As many of us learned and done many labs and seen same thing in real time also. Whenever we configure summary route in EIGRP ,It creates a summary route in own router pointing towards Null 0. Many of you will be having question why?

Everybody knows answer also…But then also some confusion. Here I am trying to explain in simple way.

As EIGRP is having flexibility to configure summary route anywhere in network. No restrictions like OSPF (only ABR and ASBR). That is add on advantage for a network administrator to manage his network.

As we configure summary route in particular interface. That means we want to advertise a summary address to that neighbor instead of full routing table. As we configure Summary route on our router(originating router) the neighboring router will receive a summary address route pointing towards our router. But same time in our router one summary route will be created pointing towards Null 0.

Imagine the Packet flow, As the request will generate for a specific network which comes under summary address it will come to our router(originator) and if that time that concern network is unreachable then what will happen, Packet will process like -First it will look the longest prefix (exact route) and longest prefix is not available. Then packet will go to summary route and as summary route is pointing towards Null 0 interface so the packet will be dropped.

Suppose if the Summary route is not configured, then what will be situation. The packet will go to somewhere else where it should not go (May be from default route).

So in EIGRP, Null 0 works as Loop prevention Mechanism.

I think now it’s very clear that, In EIGRP the summary route pointing towards Null0 is a mechanism of Loop prevention in network.