A “dead” switch port is usually one of a small number of causes: err-disabled state, wrong VLAN assignment, a duplex mismatch, or a genuinely faulty cable — each has a distinct, quick check.
Common Symptoms
- Port link light is off or amber
- Device connected to the port gets no IP address
- Port was working, then suddenly stopped
Possible Causes
- Port placed into err-disabled state (often from a security violation or loop detection)
- Port assigned to the wrong VLAN
- Speed/duplex mismatch with the connected device
- Faulty cable or SFP module
Before You Begin
Check the port status first with a simple show command before touching any configuration — this tells you immediately if it’s err-disabled versus just administratively down.
Method 1 — Check port status
Run show interface status
Use the command below to see whether the port is up, down, or err-disabled, and which VLAN it’s in.
Method 2 — Clear an err-disabled port
Shut and re-enable the port
If err-disabled, shut the interface, wait a few seconds, then no shut it — see the commands below.
Method 3 — Confirm VLAN and duplex settings
Check VLAN assignment
Confirm the port’s access VLAN matches what’s expected for that device.
Check speed/duplex
Set both ends to auto-negotiate, or match them manually — a mismatch commonly causes intermittent, not total, failure.
Method 4 — Swap the cable/SFP to isolate hardware
Test with a known-good cable
Swap in a cable known to work, and if possible, test the same device on a different switch port.
Useful Commands
show interface statusnshow interface gigabitEthernet0/1interface gigabitEthernet0/1nshutdownnno shutdownRepeated err-disable events on the same port are a symptom, not just an annoyance — investigate the underlying cause (loop, security violation, or failing cable) rather than only clearing the error each time.
Frequently Asked Questions
What causes a port to go err-disabled?
Common triggers include a detected network loop (spanning-tree), a port-security violation (unexpected MAC address), or repeated link flapping — the switch disables the port automatically to protect the network.
Conclusion
Checking interface status before making changes narrows a dead port down to one of a handful of causes in under a minute, avoiding unnecessary cable or hardware swaps.
Related Articles
