OSPF learned the route but it is not in the routing table. Walk through AD conflicts, passive interfaces, distribute-lists, and next-hop reachability step by step.
Two OSPF routers with the same Router ID corrupts the LSDB. Here is how duplicates happen, the log messages to look for, and how to fix them without a full OSPF restart.
When one router uses plain text and the other uses MD5, adjacency fails silently. Here is how to spot an OSPF authentication mismatch in debug and line up both sides.
OSPF stuck in ExStart or Exchange is almost always an MTU mismatch. Here is what the debug looks like, why it happens during DBD exchange, and how to fix it.
Different area IDs on either side of a link means no OSPF adjacency. Here is how to confirm the mismatch with show and debug, and the network statement fix.
OSPF neighbors not forming has a short list of root causes: connectivity, Hello/Dead mismatch, area, subnet, MTU, or authentication. Here is the full checklist with commands.
Redistributing static, connected, or EIGRP routes into OSPF creates Type 5 externals. Here is the syntax, E1 vs E2 metric types, and tag-based loop prevention.
Use area range on ABRs for inter-area summaries and summary-address on ASBRs for externals. Here is the difference, the config, and the show command output.
OSPF builds the LSDB, then runs Dijkstra (SPF) to calculate best paths. Here is what triggers partial vs full SPF and how to tune the throttle timers.
OSPF uses seven LSA types (1 through 7) to describe routers, networks, summaries, and externals. Here is what each one carries, who generates it, and where it floods.
On broadcast networks, mismatched subnet masks leave OSPF stuck in Init or 2-Way. Here is the fast diagnosis with show ip ospf neighbor and the one-line fix.
Real OSPF design rules: keep Area 0 stable and small, cap routers per area, use stub where you can, and make summarization a feature of your area boundaries.
