Network & Infrastructure / OSPF & Underlay Convergence
OSPF LSDB, RIB and FIB: prove where a route disappears
A troubleshooting team sees a destination LSA and concludes the network should forward correctly. The route may fail calculation, lose protocol preference or never reach hardware.
Environment and inputs: OSPFv2 IPv4 underlays, with OSPFv3 explicitly identified where used; timers and diagnostics vary by platform. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Compare the relevant LSA, calculated OSPF route, selected routing-table entry and hardware forwarding entry in the correct instance. Record next-hop resolution and installation errors at each transition.
Explain the result
The link-state database describes topology, not the final forwarding decision. Multiple protocols, route preferences and hardware resource limits can change what actually forwards packets.
Plan the fix
Correct the first failed transition and keep topology, route selection and hardware programming ownership separate. Avoid clearing the database before preserving the evidence that explains the failure.
Check the result and rollback
Verify destination-specific forwarding and the next-hop chain after correction. Revert if unrelated routes change, and retain before/after evidence rather than relying solely on a healthy neighbor count.