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.
Latest posts — page 48
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.
OSPF Neighbors Not Forming: Complete Troubleshooting Guide
OSPF neighbors not forming has a short list of root causes: connectivity, Hello/Dead mismatch, area, subnet, MTU, or authentication. Here is the full checklist with commands.
OSPF Redistribution: How to Inject Routes from Other Protocols
Redistributing static, connected, or EIGRP routes into OSPF creates Type 5 externals. Here is the syntax, E1 vs E2 metric types, and tag-based loop prevention.
OSPF Route Summarization: Strategy and Configuration
Use area range on ABRs for inter-area summaries and summary-address on ASBRs for externals. Here is the difference, the config, and the show command output.
How OSPF SPF Algorithm and LSDB Work
OSPF builds the LSDB, then runs Dijkstra (SPF) to calculate best paths. Here is what triggers partial vs full SPF and how to tune the throttle timers.
OSPF LSA Types Explained (Type 1 to 7) with Real LSDB Output
show ip ospf database prints five section headers and none of them says "Type 1". Here is every LSA type identified on real output from a three-router, two-area CML lab with an ASBR, on IOS XE 17.18.2, plus the route code each type ends up as.
OSPF Subnet Mask Mismatch: How to Troubleshoot and Fix
On broadcast networks, mismatched subnet masks leave OSPF stuck in Init or 2-Way. Here is the fast diagnosis with show ip ospf neighbor and the one-line fix.