Link-State Database
What the exam tests
You must read LSP IDs, sequence numbers, checksums, lifetimes and flags, and explain how the LSDB differs per level.
Core rules
- Each router originates its own LSPs; the LSDB is built from all routers' LSPs for a given level.
- An LSP ID is system ID + pseudonode ID + fragment number. A zero pseudonode ID means a router LSP; a non-zero value means a pseudonode LSP.
- Key fields are sequence number, checksum, remaining lifetime, hold time, ATT bit, overload bit and IS type.
- Large databases are split into fragments; fragment zero carries the main information.
- L1 and L2 maintain separate databases.
- The overload bit tells SPF not to use the router as transit, while its directly connected prefixes remain reachable.
Worked example
LSP ID 1921.6800.1001.00-01 means system ID 1921.6800.1001, pseudonode ID 00 (the router itself), fragment 01. A pseudonode LSP for a LAN would look like 1921.6800.1001.01-00. If a newer LSP arrives with a higher sequence number, it replaces the older copy; a lower sequence number is rejected unless the current copy is being purged or has expired.
Traps
Mixing up pseudonode ID and fragment number; forgetting that sequence numbers wrap and can cause instability; ignoring remaining lifetime; assuming the overload bit removes the router entirely from reachability.