VLAN Trunk Not Working? Check 802.1Q, Native VLANs, and Allowed VLANs

<!– wp:gfm-renderer/markdown {"content":"A trunk can be physically up while carrying only some VLANs—or none. Check both ends and work from Layer 1 toward VLAN policy before changing configuration.\n\n## 1. Confirm the link and trunk mode\nOn Cisco IOS/IOS XE, start with:\n\n“`text\nshow interfaces trunk\nshow interfaces gigabitEthernet1/0/1 switchport\nshow interfaces gigabitEthernet1/0/1\n“`\n\nConfirm the port is operationally trunking, uses the expected encapsulation where configurable, and has the intended native VLAN. A configured trunk mode alone does not prove the link is operationally trunking.\n\n## 2. Compare allowed VLANs\nThe VLAN must exist and be allowed on every trunk along the path. Compare the allowed list and the VLANs actually forwarding; do not assume a VLAN is carried just because it is allowed on one side.\n\n## 3. Check native VLAN consistency\nA native VLAN mismatch can place untagged traffic into different VLANs at opposite ends. Configure matching native VLANs unless the design explicitly requires otherwise, and investigate mismatch warnings.\n\n## 4. Check spanning tree and access ports\nUse `show spanning-tree vlan ` and confirm the expected port is forwarding for that VLAN. At the edge, verify the endpoint port is in the correct access VLAN.\n\n## 5. Validate safely\nTest one known endpoint in the affected VLAN, check its IP address and gateway, then inspect MAC learning with `show mac address-table vlan `. Make one change at a time and preserve a rollback path.\n\nReference: [Cisco VLAN configuration guide](https://www.cisco.com/c/en/us/support/docs/lan-switching/virtual-lans-vlan.html).\n\n**Summary:** Verify operational trunk state, VLAN existence, allowed VLANs, native VLAN agreement, and spanning-tree forwarding on the entire path.”,”defaultLanguage”:”en”,”showLanguageSwitcher”:false,”shikiTheme”:”github-dark”,”mermaidBgColor”:”transparent”,”codeLineNumbers”:”off”} /–>