Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

OSPF discard route

There is a new command introduced which is ospf discard route internal/external . Can anyone tell the usefull ness of this command . I searched the web it does not clearly explain the need for it.

Thanks in advance


Re: OSPF discard route

The point of the command is to prevent the installation of the route to null0 when a summary is created on the local router. Generally, when you configure an area range, ospf will install a very low administrative distance route to null 0 in the routing table to prevent routing loops. This command allows you to override that behaviour. It's only applicable to some one situation that I know of. Suppose you have this:



Now, router 1 would like to advettise just a default to rtouers 2 and 3. The easy way to get this default out there as an internal si to build a summary range, assuming that router 1 is an abr. Well, when you configure this summary range, ospf will install a 0/0 to null0 in the routing table. Now, assume you are also learning a 0/0 from some other router in area 0--the 0/0 built for the summary range will beat this route, and make router 1 lose it's default.

You could simply allow the 0/0 through router1 to routers 2 and 3, but this might not work, since you could lose your default at router 1 without losing connection to the rest of the network. This command should allow you to send a 0/0 to routers 2 and 3 without disturbing a local 0/0 you are learning from elsehwere.

Be very wary of this command, however, since it should be _very_ easy to create a routing loop using it.


Re: OSPF discard route


Thanks for the answer. According to your explanation this feature is usefull only for abr means discard route internal is usefull.

But what about discard route external, which scenario will that be used. By configuring this along with summary-address, will it also install a null 0 route into routing table .

Thanks in advance