OSPF NSSA def-info-or vs no-summary def route

Routing & Switching OSPF
1

Hello Renee,

Please help me getting this straight.

I have a big doubt. Why when we have a NSSA area in OSPF configured with default-information-originate command, basically the default route will be shown as external and when we have NSSA area configured with no-summary the default route is shown as IA route.

I know that when we have AREA x NSSA NO-SUMMARY, we are blocking LSA type 3, but it doesn t make sense.

I’m Waiting for your answer.

Thank you,

Catalin

2

Hi @neagucatalin2,

This is a great question and sorry for the delayed response.

  1. For a NSSA that is not Totally Stubby, there is no default route originated by the ABR by default, but I think you know that already :slight_smile:
  2. When we use the no-summary technique on the ABR, a network summary LSA (Type 3) is produced and this propagates into the NSSA and is seen as an inter-area (IA) route.
  3. When we use default-route-originate on the ABR, a type 7 LSA is generated, so these will be regarded as External Routes. (Remember we have to use a Type 7 as Type 5s are being filtered due to the Area being stubby.)

This provides a tool for ABRs to have a very nuanced level of influence for routers within the NSSA. Any router receiving two default routes will prefer an Inter-Area default route over an External default route.

Kind regards,
Jon