Troubleshooting Flows

Step-by-step checks for the faults that turn up on every network: the swapped switch you cannot reach, the EtherChannel that will not bundle, HSRP that does not fail over, OSPF, EIGRP and BGP that will not come up, NAT that works only sometimes. Each step gives the command, what healthy looks like, and the usual causes when it does not.

How to work any ticket

Ask what changed

Most faults follow a change. Check show logging | include CONFIG.I and show archive log config all.

Bottom up

Cable and port, then VLAN and trunk, then addressing and routing, then services. Do not debug BGP over a broken trunk.

Prove each layer

Run the check, compare it with what good looks like, and only then move up.

Compare with a baseline

Paste a known-good capture and the current one into the Change Checker.

Fix one thing at a time

Change, test, note it. Two changes at once and you will not know which one fixed it.