Routing Troubleshooting Process
Symptom: cannot reach a remote network, but local connectivity works. Traceroute shows where the path breaks. Identify the last responding router — check its routing table for the destination network. If the route is missing, the packet is dropped (implicit deny) or sent to the default route (which may be wrong).
Check routing tables: verify the destination network appears in the routing table of each router along the path. Verify the next-hop IP is correct and reachable. Verify the route source (connected, static, OSPF) — if dynamic route is missing, check routing protocol adjacency/neighbors.
Asymmetric routing: traffic flows one way on one path, returns on a different path. Can cause stateful firewall issues (return traffic doesn't match the session created by outbound traffic). Traceroute may show different paths in each direction. Check route metrics on each router.
Black hole routes: static routes pointing to a non-existent next hop — traffic is sent toward the route but silently dropped. Check that next-hop IPs are reachable from the routing device.
Routing Protocol Troubleshooting
OSPF adjacency not forming: check that both ends have matching OSPF area ID, MTU, hello/dead intervals, authentication settings, and network type (broadcast vs point-to-point). Mismatched settings prevent adjacency and route exchange.
Missing routes: OSPF not advertising a route — check that the network is included in OSPF configuration. Static route missing — verify configuration persisted (not just in running-config without 'copy run start'). Route filtered by distribute-list or prefix-list — check routing policy.
Route flapping: a route repeatedly appears and disappears from the routing table. Causes: physical link instability (the root cause), failing SFP transceiver, or routing protocol timer mismatch. Fix the physical issue — routing stability follows.