Special 802.1Q Trunk Considerations

The Per VLAN Spanning Tree+ (PVST+) protocol is automatically enabled on 802.1Q trunks. This protocol ensures interoperability between Cisco devices that implement one spanning-tree instance per VLAN (PVST) and devices that implement one spanning tree for all VLANs in the network (specified in the IEEE 802.1 standard). By default, the switch forwards untagged traffic with the native VLAN configured for the port.

Disabling STP on the native VLAN of an 802.1Q trunk or disabling STP on any VLAN in the network can cause STP loops. We recommend that you leave STP enabled on the native VLAN of an 802.1Q trunk or disable STP on every VLAN in the network (see Configuring Spanning Tree Protocol).