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 10.2.0.0 255.255.0.0 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:-
OSPF
router ospf 1
no discard route

EIGRP
interface serial0 ip summary-address eigrp 1 10.2.0.0 255.255.0.0 255

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