OSPF: State Machine

Reference:

http://www.rfc-editor.org/rfc/rfc2328.txt (Page 66, section 9.1, Standards Track)

Routing TCP/IP Volume 1 page 348 – 349

1. Down: Initial state. No waiting timer set. All parameters set to initial state. No hello exchanges and hence no adjacency formed.

2. Loopback: The interface is looped back using hardware or software means. No data packets will be transmitted via this interface, however in order to test the integrity of this interface OSPF LSAs still advertise this as a single host route.

3. Waiting: Router will try to identify the DR and BDR in the network. No DR or BDR should be elected in this state while the waiting timer is still running. Applicable only to BC and NBMA network types.

4. Point-To-Point: Functional state. Applicable to point-to-point, point-to-multipoint and virtual links network types. Hello will be exchanged among neighbours in every HelloInterval and routers will attempt to form adjacency.

5. DR Other: Neither DR nor BDR. Will form adjacency only with DR and BDR if either one or both exist.

6. Backup: BDR will be promoted if DR fails. Adjacency is formed to every routers.

7. DR: Designated router will form adjacency to all routers. DR must originate Network LSAs which contains routes attached to the network node, route includes DR itself.