Wednesday, December 5, 2012

What is Discard Route or NULL0 Route?

What is Discard Routes and how does it help to stop layer 3 routing loops? As we know the fact that loops are not only originates in layer2 network but could also originate in layer 3 networks. We have various mechanisms to prevent layer 3 routing loops with spit horizon and route poisoning.

Would like to understand; IS there any kind of other scenario where in routing loop could exist? See the below infographic

As depicted above, routing loop could occur whenever a summary route is announced towards the peer. This loop could be stopped by creating a manual discard route pointing towards null interface.

IP Route null0

OSPF, IS-IS and EIGRP automatically creates discard route whenever summary is performed on the routers.

Below are the configurations for disabling discard routes:-
router ospf 1
no discard route

interface serial0 ip summary-address eigrp 1 255

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