A carrier VLAN (Virtual LAN) defines the scope of the STPD (Spanning Tree Domain), which includes the physical and logical ports that belong to the STPD and if configured, the 802.1Q tag used to transport Extreme Multiple Instance Spanning Tree Protocol (EMISTP) or Per VLAN Spanning Tree (PVST+) encapsulated bridge protocol data units (BPDUs).
See Encapsulation Modes for more information about encapsulating STP (Spanning Tree Protocol) BPDUs.
Only one carrier VLAN can exist in a given STPD, although some of its ports can be outside the control of any STPD at the same time.
If you configure EMISTP or PVST+, the STPD ID must be identical to the VLAN ID of the carrier VLAN in that STPD. See Specifying the Carrier VLAN for an example.
If you have an 802.1D configuration, we recommend that you configure the StpdID to be identical to the VLAN ID of the carrier VLAN in that STPD. See Basic 802.1D Configuration Example for an example.
If you configure Multiple Spanning Tree (MSTP (Multiple Spanning Tree Protocol)—IEEE 802.1Q-2003, formerly IEEE 802.1s), you do not need carrier VLANs for MSTP operation. With MSTP, you configure a Common and Internal Spanning Tree (CIST) that controls the connectivity of interconnecting MSTP regions and sends BPDUs across the regions to communicate the status of MSTP regions. All VLANs participating in the MSTP region have the same privileges. For more information about MSTP, see Multiple Spanning Tree Protocol.
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