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.
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 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.
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.
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.
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.
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 has a short list of root causes: connectivity, Hello/Dead mismatch, area, subnet, MTU, or authentication. Here is the full checklist with commands.
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.
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.