Switching · Switching Basics

Interface troubleshooting

Distinguish administrative, physical, duplex and protection failures before changing a port.

Independent lab examples; selected output and frame summaries are illustrative. Port naming and command support vary by switch and software release.

Network and objective

Use an isolated access switch; counter names and media diagnostics vary by model.

H1 connects to SW1 Gi1/0/1 in VLAN 10. H1 cannot reach H2. Start at the local access interface before changing VLAN, routing or authentication policies.

Physical links are solid; device roles and relevant links are labeled.

How it works

Administratively down means local shutdown is configured. Notconnect suggests no usable physical link. Err-disabled means a protection mechanism disabled forwarding; its logs and cause must be investigated before recovery. An up/up result still does not prove the correct VLAN or host policy.

Read speed, duplex, CRC/input errors and output drops over a test interval. CRC errors can indicate damaged media or incompatibility; they are not a unique signature of duplex mismatch. Queue drops can reflect congestion without a bad cable. Compare both ends and the baseline counters.

Configure or inspect

show interfaces status
show interfaces GigabitEthernet1/0/1
show logging

Verify the result

Illustrative output · selected lines

SW1#show interfaces status
Port      Status        Vlan  Duplex Speed
Gi1/0/1   disabled      10    auto   auto
SW1#show interfaces GigabitEthernet1/0/1
GigabitEthernet1/0/1 is administratively down, line protocol is down
SW1#show logging
%LINK-5-CHANGED: Interface GigabitEthernet1/0/1, changed state to administratively down

The demonstrated cause is local shutdown, not an inferred cable fault. Re-enable only this intended lab endpoint, then check negotiated mode and a known exchange.

Failure and recovery

If the port becomes err-disabled after recovery, stop cycling it. Locate the triggering event (for example BPDU guard or port security), remove the condition and apply the documented recovery policy.

configure terminal
interface GigabitEthernet1/0/1
 no shutdown
end
show interfaces status

Key Takeaways

Administrative and physical failures are different states.
Err-disable has a triggering cause to investigate.
Read both ends’ speed and duplex.
Counter deltas are more useful than isolated cumulative totals.
Physical success does not prove VLAN or application correctness.