LSAs and Area Types
LSAs and Area Types
This domain tests which LSA types exist in which area type, and what an ABR or ASBR does with them. Nokia exams often frame it as "which LSAs will this router see".
LSA types
- Type 1 Router: one per router per area, lists links and costs.
- Type 2 Network: generated by the DR for a broadcast/NBMA segment.
- Type 3 Summary: inter-area prefix and ABR reachability, flooded by ABRs.
- Type 4 ASBR Summary: tells other areas where the ASBR is.
- Type 5 External: redistribution, flooded domain-wide except into stub/NSSA.
- Type 7 NSSA External: carried inside an NSSA and translated to type 5 by the translator ABR, normally the one with the highest router ID.
Area types
- Normal: 1, 2, 3, 4, 5.
- Stub: no type 5 or 4; the ABR injects a default type 3. No ASBR can exist inside.
- Totally stubby: also suppresses type 3, leaving only intra-area routes plus the default.
- NSSA: type 5 blocked, type 7 allowed, default optionally originated by the ABR. Totally NSSA additionally filters type 3.
Worked example
Area 1 is NSSA and redistributes a static into OSPF. The ASBR there originates a type 7. Area 0 sees a type 5 only because the translator ABR rewrote it; the forwarding address inside the type 7 is preserved so traffic can go directly to the ASBR's domain.
Traps that cost marks
- Expecting a type 5 inside a stub or NSSA - it will never be there.
- Forgetting type 4 is also blocked in stub areas.
- Assuming every NSSA ABR translates; by default only the highest router ID does.
- Redistributing into a stub area: the command succeeds but no external route appears.