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OSPF picks a Router ID from manual config, then the highest loopback, then the highest physical IP. Here is the selection order, examples, and how to lock it down.

OSPF cost is Reference Bandwidth / Interface Bandwidth, and the default 100 Mbps reference turns every gigabit link into cost 1. Here is how to fix it properly.

OSPF has four network types: broadcast, point-to-point, NBMA, and point-to-multipoint. Each changes DR election and Hello behavior. Here is when to use which.

OSPF Hello and Dead timers must match on both sides or neighbors never form. Here is how to check them, change them safely, and tune for faster convergence.

Virtual links patch a discontiguous Area 0 by tunneling through a transit area. Configuration is simple, but treat them as a temporary fix, not a permanent design.

Stub, Totally Stubby, and NSSA areas each block different LSA types to shrink the LSDB. Here is the config, the verification output, and when to pick each one.