How to Configure Single-Area OSPF on Cisco Routers (Step-by-Step)
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.
Jaime
Latest posts — page 51
OSPF Router ID: What It Is and How to Configure It
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.
How OSPF Calculates Metric and Cost
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 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 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.