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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.