show interfaces trunk
Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.
show interfaces trunkWhen to Use This Command
- Verify which interfaces are configured as trunks and their operational status after configuring trunking.
- Check the allowed VLAN list on a trunk to ensure only desired VLANs are permitted.
- Troubleshoot VLAN connectivity issues by confirming trunk encapsulation and mode.
- Review pruning information to see which VLANs are pruned on a trunk link.
Command Examples
Basic trunk status display
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Gi0/2 desirable n-802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-1005 Gi0/2 1-1005 Port Vlans allowed and active in management domain Gi0/1 1-1005 Gi0/2 1-1005 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1-1005 Gi0/2 1-1005
Line 1-2: Shows trunk ports Gi0/1 and Gi0/2 with mode 'on' and 'desirable', encapsulation 802.1q, status 'trunking', native VLAN 1. Line 4-5: Lists VLANs allowed on each trunk (1-1005). Line 7-8: Shows VLANs that are allowed and active in the management domain. Line 10-11: Shows VLANs in spanning tree forwarding state and not pruned.
Trunk with specific allowed VLANs
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 10 Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30-40 Gi0/2 1-1005 Port Vlans allowed and active in management domain Gi0/1 10,20,30-40 Gi0/2 1-1005 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 10,20,30-40 Gi0/2 1-1005
Gi0/1 has native VLAN 10 and only allows VLANs 10,20,30-40. Gi0/2 is a default trunk allowing all VLANs. The output confirms the restricted VLAN list on Gi0/1 and shows that only those VLANs are active and forwarding.
Understanding the Output
The output is divided into four sections. The first section shows each trunk port, its administrative mode (on, off, desirable, auto, nonegotiate), encapsulation (802.1q or ISL), operational status (trunking or not-trunking), and native VLAN. The second section lists the VLANs allowed on each trunk (configured with 'switchport trunk allowed vlan'). The third section shows VLANs that are both allowed and active in the VLAN database. The fourth section shows VLANs that are in spanning tree forwarding state and not pruned (via VTP pruning). Key things to check: ensure status is 'trunking', encapsulation matches the other end, native VLAN matches on both sides, and allowed VLANs include the necessary VLANs. A common issue is a native VLAN mismatch, which can cause connectivity problems. Also, if a VLAN is not in the allowed list, it will not be forwarded across the trunk.
CCNA Exam Tips
Remember that 'show interfaces trunk' shows only trunk ports; use 'show interfaces switchport' for all switchport details.
The native VLAN must match on both ends of a trunk; a mismatch can cause VLAN hopping or connectivity issues.
If a trunk is not forming, check the mode: one side must be 'trunk' or 'dynamic desirable' to form a trunk.
The 'Vlans allowed and active' section excludes VLANs that are shutdown or not created in the VLAN database.
Common Mistakes
Assuming all VLANs are allowed on a trunk without checking the allowed VLAN list.
Forgetting that the native VLAN is implicitly allowed even if not in the allowed list.
Misinterpreting 'Vlans in spanning tree forwarding state' as all VLANs are forwarding; some may be blocked by STP.
Related Commands
show interfaces switchport
Displays the administrative and operational status of a switch port, including VLAN membership, trunking mode, and access VLAN, used to verify VLAN configuration and port security settings.
show vlan
Displays the current VLAN configuration on the switch, including VLAN IDs, names, status, and ports assigned to each VLAN, used to verify VLAN creation and port assignments.
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