Introduction to Troubleshooting Trunks

In this article we will discuss Introduction to Troubleshooting Trunks, will make brief discussion on Introduction to Troubleshooting Trunks, In last article we discuss about Missing VLANs.

A common task of a network administrator is to troubleshoot trunk formation, or ports incorrectly behaving as trunk ports. Sometimes a switch port may behave like a trunk port even if it is not configured as a trunk port.

For example, an access port might accept frames from VLANs different from the VLAN to which it is assigned. This is called VLAN leaking. To troubleshoot issues when a trunk is not forming or when VLAN leaking is occurring, proceed as follows:

Step 1. Use the show interfaces trunkcommand to check whether the local and peer native VLANs match. If the native VLAN does not match on both sides, VLAN leaking occurs.

Step 2. Use the show interfaces trunk command to check whether a trunk has been established between switches. Statically configure trunk links whenever possible. Cisco Catalyst switch ports use DTP by default and attempt to negotiate a trunk link.

To display the status of the trunk, the native VLAN used on that trunk link, and verify trunk establishment, use the show interfaces trunk command.

CDP displays a notification of a native VLAN mismatch on a trunk link with this message:

*Mar 1 06:45:26.232: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/1 (2), with S2 FastEthernet0/1 (99).

Connectivity issues occur in the network if a native VLAN mismatch exists. Data traffic for VLANs, other than the two native VLANs configured, successfully propagates across the trunk link, but data associated with either of the native VLANs does not successfully propagate across the trunk link.

Native VLAN mismatch issues do not keep the trunk from forming. To solve the native VLAN mismatch, configure the native VLAN to be the same VLAN on both sides of the link.

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