OSPF summarisation

[poulterc85054]

afternoon guys 

just a theory based question here 

If a OSPF border router summarises all the routes to its inside interface that’s on a separate area 

when I do a show up route I can see the summarised routes from other areas /24 in this instance 

but even though I know the border router has summarised the routes To routers  in my area (area 4) I still see there addresses as a /32 and not the summarised /24 

but other routers in the network see them as a /24 

why is this please

[Giuseppe Larosa]

Hello @poulterc85054 ,

OSPF summarization at area border creates summary routes to be sent to other nodes either to the backbone area or to a non backbone area like area 4 in your case.

The OSPF summary route will be flooded on the target area until at least one component route exists.

On the other way the summarization provides benefits only to routers internal to area 4 in your case, the ABR node performing this route summarization cannot suppress or filter the received component routes for consistency (without any of them the summary route should be removed) and also because it is not the owner of those LSAs.

In addition remember that in area 0 all routers must have the same set of LSAs to build a loop free routing domain.

Remember also that each OSPF area has its own separate link state DB, the component routes are in area 0 LS DB the summary route is in area 4 DB. This is allowed and correct.

So what you see is normal and expected behaviuor on the ABR node (0, 4)

Hope to help

Giuseppe

[Richard Burts]

I am not clear what you are asking when you say "OSPF border router summarises all the routes to its inside interface that’s on a separate area". If I am understanding your situation you have a Border router that has an interface in area 4 (is this the inside interface that you mention?) and an interface in area 0. The Border router can be configured to summarize routes originating in area 4 to area 0 (and therefore to other areas). Routers in area 4 will continue to see the unsummarized routes while routers in other areas would see the summarized routes.