OSPF neighbors progress through 8 states from Down to Full, and understanding the packet exchange at each step is the key to troubleshooting adjacency problems. A guide to OSPF neighbor states with real packet captures.
Router IDs, ABRs, ASBRs, stub areas, LSAs, costs, and adjacencies: the OSPF vocabulary is large. A plain-English reference to the key terms and concepts every network engineer working with OSPF should know.
Interface-based OSPF configuration (ip ospf [process] area) skips wildcard masks and makes per-port membership obvious. Here is when to use it and how.
A full R1/R2/R3 single-area OSPF lab with IP plan, router ospf config, and the show commands that prove neighbors are Full and routes are installed.
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.
One internal router, one area, stepped through stub, totally stubby and NSSA on CML IOS XE 17.18.2. The LSDB and routing table at every stage show which LSAs survive, which routes vanish, and the Type-7 to Type-5 translation proved on the far side of the ABR.
OSPF supports plain-text, MD5, and SHA authentication at the interface or area level. Here is how to configure each, why MD5 is the production default, and how to verify.
Multi-area OSPF across Area 0, 10, and 20 with two ABRs. Full R1 to R5 config plus show commands to confirm inter-area routes and ABR behavior.