Trunking issues are usually associated with incorrect configurations. When configuring VLANs and trunks on a switched infrastructure, the following types of configuration errors are the most common:
- Native VLAN mismatches - Trunk ports are configured with different native VLANs. This configuration error generates console notifications, and causes control and management traffic to be misdirected. This poses a security risk.
- Trunk mode mismatches - One trunk port is configured with trunk mode off and the other with trunk mode on. This configuration error causes the trunk link to stop working.
- Allowed VLANs on trunks - The list of allowed VLANs on a trunk has not been updated with the current VLAN trunking requirements. In this situation, unexpected traffic or no traffic is being sent over the trunk.
If an issue with a trunk is discovered and if the cause is unknown, start troubleshooting by examining the trunks for a native VLAN mismatch. If that is not the cause, check for trunk mode mismatches, and finally check for the allowed VLAN list on the trunk. The next two pages examine how to fix the common problems with trunks.