how to set OSPF NSSA on routers

[Majid Jalinousi]

Hi

There are two router 7606 connected together, and OSPF enabled between them. 

Router 1 is in the backbone area and connected to router 2 through area 10, and their configurations are like below

Router 1:

redistribute static subnets route-map Redist

area 10 nssa no-summary

area 10 network 10.0.0.0 0.255.255.255 area 10

"other commands omitted for clarity"

Router 2:

area 10 nssa no-summary

area 10 network 10.20.60.10 0.0.0.0 area 10

"other commands omitted for clarity"

when I google searched for NSSA scenario I found you only need to set no-summary on ABR and no other routers. but in this scenario no ASBR has set no summary too.

it's only kind of recommendation or this configuration is fault?

[Peter Paluch]

Hi,

The no-summary keyword only has effect on ABRs, and therefore it does not make sense to configure it on internal routers. The no-summary command tells the ABR to not inject any LSA-3 representing intra-area and inter-area OSPF routes in other areas into this particular area. Internal routers are never in the position of seeing internal OSPF routes from other areas as they are not connected to any other area except their own. Thus, configuring the no-summary on internal routers is a waste of effort even though it is not a configuration error per se.

So the examples on the internet that show the no-summary to be configured on ABRs only are correct. Configuring the no-summary on all routers in an area is not an error and won't do any harm but often shows a lack of understanding OSPF operations and its configuration.

Best regards,
Peter

[Majid Jalinousi]

Thanks for your response,

I have another question, as I saw in OSPF configuration, reason for using Totally NSSA is sending  just a default route toward the routers on NSSA area rather than individual routes, but when I saw output of "show ip route" in router 2, I could see all of individual routes.

But I have to say there was a static route " ip route 0.0.0.0 0.0.0.0 gig 1/0" in router 2 too, does this static route cause of this behavior? if it was, why would we use Totally NSSA?

[Peter Paluch · Accepted solution]

Hi,

A Totally NSSA, or more properly called, NSSA Totally Stubby, prevents routes from other areas from leaking into a NSSA-TS area - that is the "Totally Stubby" property (not seeing any other OSPF routes besides your own intra-area routes). The Not-So-Stubby property means that you can still perform redistribution, something that ordinary stubby and totally stubby areas do not allow.

So on Router2, it is normal to see OSPF routes but the only routes you can see on an internal router in a NSSA-TS area are intra-area routes (O), NSSA Type 1 (N1) and NSSA Type 2 (N2). You will not see any External Type 1/2 routes (E1/E2) and the only inter-area (O IA) route you may see is the default route.

Keep in mind that area types influence what one area knows about other areas. However, inside an area of any type, you will still see all internal intra-area routes.

If you have a static default route on Router2 then you will not see the OSPF-learned default route because the static route is preferred.

Best regards,
Peter

[Majid Jalinousi]

Thanks a lot, I got it.