Nokia NRS II Composite (OSPF): Link-State Database

Link-State Database

The LSDB domain tests whether you understand that every router in an area holds the same LSA set, how LSAs are aged and refreshed, and how to read the database on SR OS.

Core rules

• The LSDB is per area. An ABR holds a separate database for each attached area.
• Every LSA has a header: type, link-state ID, advertising router, age, sequence number and checksum. The triplet type/LSID/ advertising router uniquely identifies an LSA.
• A router only accepts a newer instance of an LSA: higher sequence number, or equal sequence with a higher checksum; otherwise it floods back the newer copy it already has.
• LSAs are refreshed at 1800 seconds and flushed at 3600 seconds (MaxAge). DoNotAge is used with demand circuits and graceful restart.
• On SR OS, show router ospf database lists LSA headers, show router ospf database extensive shows full body detail, and show router ospf database summarize gives counts per type.

Worked example

You see two router LSAs for the same advertising router with identical sequence numbers but different checksums. Only one can be current. The router with the higher checksum wins and the other is replaced; the loser's copy is not installed. This is why a checksum difference alone can look like a "duplicate" in show output.

Traps that cost marks

• Assuming the LSDB is identical across areas - it is only identical within an area.
• Reading the LSDB as the routing table: the LSDB is input, SPF output is show router ospf routes.
• Forgetting that type 2 network LSAs exist only on broadcast/NBMA segments, advertised by the DR.
• Comparing only sequence numbers and ignoring checksum on ties.