OSPF Summarization - Null Route

In OSPF, the discard route is created automatically whenever a summary route is configured, there are two types of summary routes: Internal and External. When internal summary routes are configured, OSPF will inject an internal discard route, and when an external summary route is configured, the OSPF process will create an external discard route. The discard routes are created to stop forwarding loops.

R2(config)#router ospf 1
R2(configrouter)#no discard-route internal
The discard route that we are discarding is the result of summarizing the internal routes, therefore we need to specify internal.

R4(config)#router ospf 1
R4(configrouter)#no discard-route external
In the above command we are discarding the external discard-route that was created as a result of summarizing the external routes.