OSPF Multi-Area: ABR Area Mismatch and Route Repair

Bundle lab

Build and troubleshoot a compact multi-area OSPF design with a single ABR between area 0 and area 10. You will deploy OSPF, observe an adjacency failure caused by an area mismatch on one transit link, repair it, and validate that routes propagate end to end between two user LANs.

Learning objectives

• Implement OSPF on a multi-area topology with a correct ABR.
• Detect adjacency and routing failures caused by OSPF area mismatches.
• Restore stable routing adjacencies and inter-area route exchange.
• Validate OSPF neighbor state and route propagation end to end.

Troubleshooting focus

• No adjacency forms at all on one link: verify both ends are in the same OSPF area with show ip ospf interface brief. Area 0 against area 10 is a hard failure.
• A neighbor is stuck in INIT/2WAY: check for unidirectional loss, an incorrect network type, or mismatched MTU. Timers and authentication (if configured) must match.
• Neighbors are FULL but routes are missing: ensure LAN and Loopback networks are advertised, with wildcard masks neither too broad nor too narrow. A passive interface still advertises — it just forms no neighbors, as expected on user LANs.
• A transit link is down or misaddressed: show ip interface brief must show each transit up/up with correct /30 pairs. A /30 mismatch or shutdown looks like an OSPF problem but is L1/L2.
• Host tests still fail after routing looks good: confirm end-host IP/mask/gateway, check no host firewall drops ICMP, and traceroute to find the first unreachable hop.