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mediumospf~50 min

An OSPF neighbor won't come up

Three routers, OSPF area 0, but one adjacency is stuck. Two routers can ping each other yet never become neighbors. Diagnose why.

Scenario

Three routers should form a single OSPF area 0 domain in a triangle. Two of the adjacencies came up fine, but R2 and R3 are stuck: they never reach FULL, even though the link between them is up and they can ping each other across it.

Topology

  • R1 Lo0 1.1.1.1/32, R2 Lo0 2.2.2.2/32, R3 Lo0 3.3.3.3/32
  • R1 ↔ R2 10.0.12.0/30, R2 ↔ R3 10.0.23.0/30, R1 ↔ R3 10.0.13.0/30
  • Everything is meant to be in area 0.

Your job

Get all three pairs to FULL and make every loopback reachable from every router, without changing the addressing.

What "done" looks like

show ip ospf neighbor is FULL on every router for every neighbor, and every router's table has all three loopbacks.

Teaches: two routers only form an adjacency if their OSPF parameters agree, and the area on the shared link has to match on both ends.

What gets checked

Your solution is verified against each of these:

  • Every directly-connected router pair reaches FULL adjacency
  • Each router learns all three loopbacks (1.1.1.1, 2.2.2.2, 3.3.3.3) via OSPF
  • Every loopback is reachable from every router

Solve it in the browser lab

No setup, no install. Open a live lab: configure each device in the editor or its Cisco IOS terminal, run show/ping/traceroute (or test from the hosts), and watch the network react. The in-house engine grades your fix instantly and issues your proof the moment every check passes.

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Prefer your own lab?

  1. Build the fix locally. New to the tooling? See setting up your lab.
  2. Push your topology file, device configs, and any playbooks to a public repo (GitHub or GitLab).
  3. Submit the repo link. We review it by hand, confirm it works, and issue your proof page.
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