How do you troubleshoot switch trunk port issues in an inter-VLAN routing setup?

Verify the trunk is active with show interface trunk, and if the port is missing from the output, check its configuration with show running-config interface.

Common trunk problems:

1. Port not in trunk mode — configured as access instead of trunk
2. Port is administratively down — shutdown was applied
3. VLAN (Virtual Local Area Network) not allowed on trunk — the switchport trunk allowed vlan list excludes a needed VLAN
4. Native VLAN mismatch — trunk ends have different native VLANs (causes intermittent issues)

Verification steps:

S1# show interface trunk

This shows all active trunk ports. If the expected port doesn't appear, it's not trunking.

S1# show running-config interface fa0/5

Check if switchport mode trunk is configured and if there's no shutdown.

Key detail: In a router-on-a-stick setup, the trunk to the router is the single point of failure. If this trunk goes down, all inter-VLAN routing stops because all traffic must flow through that one link.

Go deeper:

• Troubleshoot Inter-VLAN Routing (Cisco Press) — misconfigured/shut trunks as the top cause, confirmed with show interfaces trunk.