5 Secret Cisco Commands for Broken Ports

The Cisco switch port is up, ping works, and the config looks right, but the network is still broken. These five Cisco IOS troubleshooting commands expose cable faults, interface errors, bad optics, spanning-tree changes, and failed 802.1X or MAB authentication: 1. `test cable-diagnostics tdr interface` and `show cable-diagnostics tdr interface` test the copper cable from the switch and estimate how far away the fault is. 2. `show interfaces counters errors` scans the entire switch for ports with actively climbing errors. 3. `show interfaces transceiver detail` exposes DOM readings so you can catch dirty fiber, weak receive power, and failing optics before the link drops. 4. `show spanning-tree detail` identifies the port causing repeated topology changes and random network slowdowns. 5. `show authentication sessions interface fa0/2 details` shows why an 802.1X or MAB device can be physically connected but still Unauthorized. On newer IOS XE, try `show access-session interface fa0/2 details`. If you're early in your networking career and stuck on a "but ping works" ticket, these commands can help you stop blaming the wrong box. If you've got a Cisco troubleshooting command that people sleep on, drop it in the comments. 00:00 When everything looks fine 01:18 Find cable faults with TDR 04:01 Scan every port for errors 06:25 Catch dying optics with DOM 08:35 Find the spanning-tree flapper 11:33 See why authentication failed 14:07 Quick recap