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.
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.
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.
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.
Real OSPF design rules: keep Area 0 stable and small, cap routers per area, use stub where you can, and make summarization a feature of your area boundaries.
RADIUS is the glue between a Catalyst switch and Cisco ISE in 802.1X. Here is how the Access-Request carries EAP, why UDP 1812/1813 matters, and where failures surface.