Diagnosing Cisco Trunk Port Mismatches Using IOS Trunk links connect switches and carry traffic for multiple VLANs using IEEE 802.1Q tagging. When trunk ports are misconfigured, VLAN traffic can fail between switches, causing widespread network connectivity problems. Common trunk mismatch issues include: Native VLAN mismatch One side configured as access instead of trunk Different encapsulation types Allowed VLAN lists not matching Step 1: Verify Trunk Status Check whether the interface is operating as a trunk: show interfaces trunk If the interface does not appear in the list, it is not trunking. Step 2: Verify Switchport Mode show interface gigabitEthernet0/1 switchport Look for: Administrative Mode: trunk Operational Mode: trunk If the port is in access mode: configure terminal interface gigabitEthernet0/1 switchport mode trunk Step 3: Check Native VLAN Native VLAN mismatches cause untagged traffic problems. show interfaces trunk Fix mismatch: configure terminal interface gigabitEthernet0/1 switchport trunk native vlan 10 Step 4: Verify Allowed VLAN List show interfaces trunk If a VLAN is missing: configure terminal interface gigabitEthernet0/1 switchport trunk allowed vlan add 30 Example Working Trunk Configuration interface GigabitEthernet0/1 description Uplink to Core Switch switchport trunk encapsulation dot1q switchport mode trunk switchport trunk native vlan 10 switchport trunk allowed vlan 10,20,30 Quick Troubleshooting Checklist Verify trunk status (show interfaces trunk) Confirm switchport mode Check native VLAN Verify allowed VLANs Inspect running configuration Final Thoughts Trunk mismatches are a very common source of VLAN communication failures. A quick verification of trunk status, VLAN permissions, and native VLAN settings can resolve most issues in minutes.