OSPF Network Types Explained (Broadcast, Point-to-Point, NBMA)
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
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.
Latest posts — page 4
OSPF Timers: Hello and Dead Intervals Explained
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.
OSPF Virtual Links Configuration
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.
OSPF Stub Area Configuration: Stub, Totally Stubby and NSSA, Proved in the LSDB
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 Authentication Configuration (Plain Text and MD5)
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.
Configuring Multi-Area OSPF on Cisco Routers
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.
How to Advertise a Default Route in OSPF
The default-information originate command injects 0.0.0.0/0 as a Type 5 LSA and turns the router into an ASBR. Here is the syntax, the always option, and verification.
OSPF Routes Not Appearing in Routing Table: Troubleshooting
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.
Fixing Duplicate OSPF Router ID: Flapping Neighbors and Dead Timers
Flapping OSPF adjacencies usually come down to a duplicate router ID or an expiring dead timer. Real CML captures on IOS XE 17.18.2 show the neighbor-table tell, the fix, and why the duplicate-router-ID log never fired.
OSPF Authentication Mismatch Troubleshooting
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 MTU Mismatch: Neighbor Stuck in EXSTART, and Both Fixes
A neighbor parked in EXSTART means the DBD exchange is failing, and IP MTU mismatch is almost always why. Captured on CML with IOS XE 17.18.2: the asymmetric stuck states, the debug line that names the cause, a recipe to reproduce it, and both fixes proven all the way to FULL.
Fixing OSPF Area Mismatch Issues
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.