Hellos, adjacencies and the link-state database
OSPFv2 runs over IPv4 using IP protocol 89. Routers use Hello packets to discover neighbors and check parameters such as area ID and timers. Routers that form a full adjacency synchronize the relevant link-state information and flood later changes.
On a broadcast network, the designated router and backup designated router become fully adjacent with the other participating routers. Two routers with neither role can remain in 2-Way state with each other while the network is healthy.
Areas, LSAs and path cost
An area has its own topology database. A Type 1 router LSA describes a router’s links in the area. On a transit broadcast network, the designated router originates a Type 2 network LSA describing the routers attached to the segment. Each router runs shortest path first (SPF) to find paths based on the sum of their link costs.
Area 0 is the backbone for traffic between areas. An area border router (ABR) advertises inter-area reachability with Type 3 summary LSAs. Routers in another area can learn a prefix and its cost without receiving all the LSAs describing that prefix’s local topology.
Try it in a lab
- Connect two OSPFv2 routers over a point-to-point transit link in area 0. Advertise a loopback from one router. Verify that the neighbor reaches Full and that the other router learns the loopback as an intra-area route.
- Change the transit area on only one end. Confirm that the connected IPv4 interfaces can still reach each other, while the OSPF adjacency drops and the learned route is removed.
- Restore the matching area and verify that the adjacency and route return.