OSPF Single-Area: How Neighbours Form

OSPF in one page

OSPF routers send hellos every 10 s (dead time 40 s) on Ethernet. Two routers become neighbours only if these match:

- Area ID, subnet and mask - Hello and dead timers - Authentication, and the interface MTU (for FULL)

States: Down → Init → 2-Way → ExStart → Exchange → Loading → Full. On Ethernet only the DR and BDR go FULL with everyone; DROTHERs stay 2-WAY with each other.

DR election: highest interface priority (default 1, 0 = never), then highest router ID. It is not pre-emptive — use clear ip ospf process after changing priorities.

Cost = reference bandwidth (100 Mbps) / interface bandwidth. Lower is better.

Useful commands: show ip ospf neighbor, show ip ospf interface brief, show ip route ospf, show ip protocols.