Ping & Traceroute as Diagnostic Patterns

ping + traceroute answer “which layer is broken?” — read the patterns, don’t guess: Symptom Diagnosis ping gateway fails wrong IP/mask — host can’t even reach the local router gateway ✅ but 8.8.8.8 ❌, traceroute dies at hop 0 missing/wrong default gateway ping 8.8.8.8 ✅, ping google.com ❌ DNS — the network is fine ping […]

ICMP — The Error Messages That Explain the Network

ICMP is Layer 3 error and diagnostic messaging — not TCP/UDP, and not a transport for data. Type Meaning You meet it in 0 / 8 Echo reply / request ping 3 Destination unreachable codes: 0 net, 1 host, 3 port, 4 fragmentation needed (PMTUD) 11 Time exceeded every traceroute hop (TTL hits 0) Reading […]

STP Troubleshooting

Most STP problems reduce to missing BPDUs or bad links. Symptom Likely cause / tool Blocked port unexpectedly forwarding missing BPDUs → Loop Guard should have held it Up/up but one-way traffic unidirectional link → UDLD Duplex mismatch errors fix duplex settings MAC table flapping suspect a loop show spanning-tree show spanning-tree inconsistentports debug spanning-tree […]