OSPF Notes – Neighbor States
My prediction about covering network types was wrong. I’m going to puke out some information about neighbor states for now. As is always the case, corrections are welcome.
Down : No hellos have been received from this router.
Attempt : This state only applies to manually-configured neighbors on an NBMA network. In this state, a router has sent unicast hellos to the neighbor but has not received any back from it.
Init : The router has received hellos, but none of the hellos have the router’s RID included as a known neighbor.
2way : The router has received hellos with its RID included. This means the other router has received the hellos from this router, so they now have 2-way communication going. The DR and BRD is elected at the end of this stage.
ExStart: When a router grows up and starts to have feelings for other routers, it enters the ExStart state to have further relations with a neighbor. This is where the master/slave relationship and the initial sequence numbers are established.
Exchange : Once we know who wears the pants in this relationship, the master sends over a DBD with it’s LSAs listed. In response, the slave does the same so that both routers have all the LSA headers they both know.
Loading : This is where the LSRs and LSUs flow. If a router need the full LSA from the neighbor, it sends an LSR, and the neighbor should send an LSU in response.
Full : After the LSR/LSU exchange, the routers should both be in sync, so they each send an LSAck to the other to confirm.
As a bonus, here’s a nifty little animation showing neighbor states.
https://masto.ai/@aconaway
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