Packet Types and Neighbor State Machine
Packet Types and Neighbor State Machine
This domain checks whether you can read a neighbor state and name the packet exchange that produced it. It is the most common source of "why is my adjacency stuck" questions.
The five packet types
- Type 1 Hello: discovery, keepalive, DR election and parameter checking. Carries area ID, RID, hello/dead timers, priority, DR/BDR and the neighbor list.
- Type 2 Database Description (DBD): header summary of the LSDB. Master/slave is negotiated with the Init/More/ Master bits; only the master polls.
- Type 3 Link State Request: asks for specific LSA headers seen in DBDs.
- Type 4 Link State Update: carries the requested LSAs, and also flooded changes.
- Type 5 Link State Acknowledgment: confirms receipt of updates.
States in order
Down, Attempt (NBMA only), Init, 2-Way, ExStart, Exchange, Loading, Full. ExStart/Exchange are where MTU and duplicate-RID problems appear; Loading ends when all requested LSAs arrive.
Worked example
A neighbor sits in Init. R1 receives R2's hellos, but R2's hello does not list R1's router ID. That means R2 is not hearing R1 - check one-way filtering, multicast suppression, or a wrong area/authentication on R1's side. Init is a receive-path symptom, not a timer problem.
Traps that cost marks
- Stuck in ExStart: almost always MTU mismatch or duplicate router ID.
- Stuck in Exchange: master/slave disagreement or DBD retransmission.
- Stuck in 2-Way on a point-to-point link: a real fault, since p2p must reach Full.
- Hello 10 / dead 40 on broadcast and p2p, 30 / 120 on NBMA and point-to-multipoint.