OSPF Concepts and Neighbor Formation

Distance-vector protocols tell neighbours which networks they can reach and how far away those are. OSPF does something different: each router describes its own links in a link-state advertisement, floods those LSAs to every router in the area, and every router assembles an identical link-state database — a full map of the area. Each router then runs the SPF (Dijkstra) algorithm on that map by itself to compute the shortest path to every destination.

Because everyone shares the same map and computes independently, OSPF converges quickly and avoids the loops distance-vector protocols risk during a change. Once neighbours are up they exchange database-description packets to compare databases, request the LSAs they are missing, receive them, and acknowledge. After that initial sync, updates are event-driven — a link change floods a new LSA — rather than periodic full-table dumps.

Notice what is not on that list: process ID, the Router ID value — which must be unique rather than matching — interface cost, and bandwidth. None of those ever blocks an adjacency, which is why a neighbour problem is never a metric problem.

R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:37    10.1.12.2       GigabitEthernet0/1
3.3.3.3           1   FULL/DR         00:00:34    10.1.23.3       GigabitEthernet0/2
4.4.4.4           0   2WAY/DROTHER    00:00:35    10.1.23.4       GigabitEthernet0/2

Neighbor ID is the peer's Router ID, not its interface address. Pri is the interface OSPF priority used in DR election. State has two parts — the adjacency state and the neighbour's role on that segment. Dead Time counts down and resets on every Hello; if it reaches zero the neighbour drops. Address is the peer's interface IP, which becomes the next hop for its routes, and Interface is the local port facing it.

Line by line: R1 is FULL with 2.2.2.2 and 3.3.3.3, so those databases are synchronised and their routes are trustworthy. The third neighbour sits at 2WAY/DROTHER with priority 0. That is not a fault — both R1 and 4.4.4.4 are DROTHERs on the 10.1.23.0 segment, and a priority of 0 even makes 4.4.4.4 ineligible for DR, so they intentionally stop at 2-WAY and rely on the DR to relay LSAs.

• Link-state: LSAs build an LSDB, then each router runs SPF on it independently.
• OSPF rides on IP protocol 89 and sends Hellos to 224.0.0.5; its AD is 110.
• Must match: hello and dead timers, area ID, subnet and mask, stub flag, authentication. Process ID need not.
• States run Down, Init, 2-Way, ExStart, Exchange, Loading, Full.
• FULL means synchronised databases; 2-WAY between two DROTHERs is normal.
• Stuck ExStart or Exchange means an MTU mismatch; stuck INIT means one-way Hellos.