Routing Troubleshooting

Watch first

Plain-English explanation

Routing problems feel chaotic until you use a fixed order: interface up? Connected route present? Routing table entry for destination? Next-hop reachable? ACL/firewall? Remote return path?

Skip a step and you chase ghosts — especially when ping fails because the reply has no route home.

Deep dive

Top-down routing checklist:

Physical/Data Link — interface up/up, correct VLAN, ARP works to next-hop
Local config — IP/mask correct, no typo in static
Routing table — specific route or valid default? LPM correct?
Next-hop test — ping/traceroute to next-hop, not only final dest
Protocol — OSPF neighbor FULL? Static with correct AD?
Return path — asymmetric routing, missing route on far side
Policy — ACL, PBR (awareness) blocking

Common patterns:

| Pattern | Likely cause | |---------|--------------| | Ping gateway fails | Host config, VLAN, switch port | | Ping gateway OK, remote fails | Missing route or wrong next-hop | | Traceroute stops at hop N | Router N lacks forward or return route | | Intermittent | Floating routes, dual paths, flapping adjacency |

Step-by-step — worked example

Symptom: PC A cannot reach server S across two routers R1→R2.

PC A pings gateway R1 — fails → fix host mask/gateway or ARP first
Gateway OK — ping R2 interface on link — fails → check R1 route to R2 link subnet
R1 has route to S via R2 — ping R2 as next-hop — OK
Ping S from R1 — fails, from R2 — OK → problem on R2→S or S itself
R2 missing return to A's subnet → add route or enable OSPF on R2 LAN

Fix return path, not only forward.

Commands to know

Routing troubleshooting toolkit

show ip interface brief
show ip route
show ip route [destination]
show ip ospf neighbor
ping [next-hop]
traceroute [destination]
show running-config | section router

Troubleshooting

Meta-tip: Change one thing at a time. Reloading configs randomly creates new failures.

Use the routing table trainer to build speed reading tables under pressure — Part 1 gate includes this.

Exam relevance

Exam trap

Successful ping to a host does not prove return path works if ICMP echo-reply is filtered — but for CCNA routing scenarios, assume ICMP unless ACL mentioned.

Part 1 gate

Week 5 gate retests subnetting, routing tables, and packet paths — review Weeks 1–5 before attempting.

Practice checklist

Run full checklist on a deliberately broken three-router lab
Document each hop in traceroute with matching routing table line
Fix a missing return route scenario without touching forward config
Score 80%+ on routing table trainer
Explain asymmetric routing in plain English

You can ping a next-hop router but not the final destination behind it. What should you check first on the next-hop?

Traceroute stops at router R2. R1 has a correct route. What is a likely issue?

Learning objectives