Stub/NSSA Areas and Route Redistribution
Stub/NSSA Areas and Route Redistribution
This domain combines area-type design with injection: what an ABR advertises into a stub or NSSA, and how redistribution enters OSPF from other protocols.
Core rules
- A stub area blocks type 5 and type 4. The ABR originates a default (type 3) so internal routers still reach the rest of the domain.
- Totally stubby suppresses type 3 summaries as well, leaving intra-area routes plus the default.
- An NSSA blocks type 5 but allows type 7, so an ASBR can exist inside. The ABR can originate a default into the NSSA; the highest-RID translator ABR converts type 7 to type 5 for area 0.
- Redistribution on SR OS is done with
exportpolicy under the OSPF instance, matching routes and setting the external metric and type. Without a policy that matches, nothing is redistributed. - External type 1 should be used when the external metric should add to internal cost; type 2 when the external metric should dominate.
Worked example
A static default is redistributed into an NSSA. The ASBR emits it as a type 7 with metric 20, type 2. Area 0 receives it as a type 5 from the translator ABR. Because type 2 metrics ignore internal cost, every area 0 router sees 20, even though the ASBR is several hops away.
Traps that cost marks
- Redistributing into a stub area: no external route can appear, by definition.
- Forgetting the export policy, then blaming OSPF for the missing route.
- Expecting an NSSA to translate type 7 without an ABR - translation happens only at the ABR.
- Summarising with an area range that does not cover the component prefixes.