Open Shortest Path First, the link-state IGP behind most enterprise networks. Articles tagged OSPF cover neighbor relationships, LSA types, areas, DR/BDR election, and Cisco IOS XE configuration.
On multi-access OSPF networks, the DR and BDR centralize LSA exchange so routers do not flood 45 sessions across 10 neighbors. A practical guide to what the DR and BDR do and why OSPF design depends on them.
Put 200 routers in one OSPF area and SPF, LSDB size, and flooding all break down. Areas fix scalability by isolating LSAs, SPF runs, and flooding. A plain-English guide to OSPF area design and why Area 0 is special.
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.