show interfaces trunk is a Cisco IOS verification command for viewing trunk-port status, encapsulation, native VLAN, and VLAN forwarding information. It helps confirm that a port is trunking and identify which VLANs the trunk can carry. section 12.3.2 section 12.5
Read the output in these practical sections:
- Port details show the interface, administrative mode, encapsulation, operational status, and native VLAN. Mode on indicates a manually configured trunk, while status trunking confirms active trunk operation. section 12.3.2
- Vlans allowed on trunk lists the VLANs permitted by the port configuration. By default, every VLAN from 1 through 4094 is allowed. section 12.3.2 section 12.5
- Vlans allowed and active in management domain lists permitted VLANs that also exist on the switch. A VLAN cannot forward through the trunk if it does not exist locally. section 12.3.2
- The native VLAN appears in the port details. It is VLAN 1 by default, and native-VLAN mismatches can place untagged traffic into the wrong VLAN. section 12.3.2 section 12.5
Verifying a configured trunk
SW1# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/0 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/0 1-4094 Port Vlans allowed and active in management domain Gi0/0 1,10,20,30 This output shows an active 802.1Q trunk. All VLANs are permitted, but only VLANs 1, 10, 20, and 30 both exist and can be forwarded by the switch. section 12.3.2
