OSPF External route summarization questions

[per.nielsen]

With summarization of external LSA 5

you configured that on the ASBR under the router ospf 1 with the summary-address command , so good so far :-)

you have the option of using a flag called not-advertise which then wont advertise the summarized routes ,

I cant understand why you would ever used that option , surely you are summariztion because you want to advertise this summary route ???

thanks

per

[pkhatri · Accepted solution]

Hi Per,

It does have limited use :-)

One instance in which that can come in useful is when you have redistributed routes into OSPF but have not applied a route-map to select exactly which routes are redistributed. In such a case, all routes from the source protocol will be injected into OSPF. If you subsequently decide that you don't want some of these routes in OSPF, you can use the summary-address command with the no-advertise option to filter out these unwanted routes.

Hope that helps.

Paresh

[pkhatri]

Hi Per,

I really should add that a more important usage of no-advertise is when using NSSAs. When you use not-advertise on an ABR that borders an NSSA area, that allows you to use those routes for local routing within the NSSA without leaking them out to other areas.

The routes will still be present and used within the NSSA area, but will not be advertised beyond it.

Paresh