Go through the OSPF LSA types 1 to 7. Who generates each one and how far does it flood?
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Type 1 router LSA from every router, inside its area. Type 2 network LSA from the DR, inside its area. Type 3 summary from ABRs, describing other areas' prefixes. Type 4 ASBR summary from ABRs, saying how to reach an ASBR. Type 5 external from an ASBR, flooded everywhere except stub areas and NSSAs. Type 6 is multicast OSPF, unused. Type 7 is the NSSA external, which the NSSA ABR translates into Type 5.
Go deeper
The table I draw on the whiteboard
| Type | Name | Originated by | Flooding scope | Link state ID |
|---|---|---|---|---|
| 1 | Router | Every router, one per area it is in | Its area | Router ID |
| 2 | Network | DR of a multi-access segment | Its area | DR's interface IP |
| 3 | Summary (network) | ABR | Into each attached area | Destination prefix |
| 4 | ASBR summary | ABR | Into areas that do not contain the ASBR | ASBR router ID |
| 5 | AS external | ASBR | Whole domain except stub, totally stubby, NSSA | External prefix |
| 6 | Group membership | MOSPF | Not supported by Cisco | n/a |
| 7 | NSSA external | ASBR inside an NSSA | That NSSA only | External prefix |
How they fit together
Types 1 and 2 are the map of one area. Type 3 is "this prefix is reachable through me at this cost", regenerated by every ABR at each area boundary. Type 5 names the ASBR as advertising router, so a router in another area needs a Type 4 to find its cost to that ASBR.
In show ip ospf database
Type 1 is "Router Link States", Type 2 "Net Link States", Type 3 "Summary Net Link States", Type 4 "Summary ASB Link States", Type 5 "Type-5 AS External Link States" and Type 7 "Type-7 AS External Link States".
They will ask next
What are Types 9, 10 and 11?
Opaque LSAs with link-local, area and AS flooding scope. Type 10 carries MPLS traffic engineering and segment routing information. Type 9 is used for graceful restart.
Does an ABR pass a Type 3 from one area unchanged into the next area?
No. Each ABR originates new Type 3 LSAs with itself as advertising router and its own cost. That regeneration is why inter-area routing behaves like distance vector.
Traps
- Saying Type 3 LSAs carry topology. They carry a prefix and a cost, nothing about the links behind it.
- Saying the ASBR generates Type 4. The ABR does.