Read routing-table decisions.
OSPF link-state operation and path calculation
OSPF is a link-state routing protocol. Routers exchange information about the network and calculate paths using cost. OSPFv2 is used for IPv4. An OSPF process ID is locally significant; the two routers do not have to use the same process ID to become neighbours.
A router ID is a 32-bit identifier written in dotted-decimal form. Give each router a unique router ID. An area groups OSPF information; the single-area beginner labs use area 0. The network command selects local interfaces to participate, rather than creating a new physical network.
OSPF adjacency and routing table verification
Check active interfaces and addressing, the area, compatible hello/dead timers and authentication settings. A passive interface can advertise its attached network but does not form OSPF neighbours there. Use show ip ospf neighbor, show ip ospf interface and show ip route to separate adjacency problems from route problems.
On broadcast networks, a designated router (DR) and backup DR reduce adjacency work. Two DROTHER routers can remain in 2-Way state normally, so “every neighbour must always be FULL” is incorrect. Point-to-point links use different adjacency behaviour. MTU mismatches can also prevent database exchange from finishing.
Verification commands
Use these viewing commands to inspect the idea in this lesson. Start in the environment shown above each example.
show ip ospf neighbor
show ip route ospfCommand reference · enter in the stated environment; output varies by device.
Practical exercise
Two directly connected routers ping each other, but one uses area 0 and the other area 1 on that link. Predict OSPF neighbour formation.
Show the answer and explanation
The area mismatch prevents the expected adjacency. Ping can work because it does not validate OSPF parameters. Make the link’s OSPF area consistent and inspect neighbours again.
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Technical references checked on 25 October 2026. These are original teaching explanations, not official Cisco course material. See how content and lab validation are described.
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