Cisco WS-C3560E-48PD-SF Command Reference - Page 602

spanning-tree extend system-id Chapter 2 Catalyst 3560 Switch Cisco IOS Commands spanning-tree extend system-id Use the spanning-tree extend system-id global configuration command to enable the extended system ID feature. spanning-tree extend system-id Note Though visible in the command-line help strings, the no version of this command is not supported. You cannot disable the extended system ID feature. Syntax Description This command has no arguments or keywords. Defaults The extended system ID is enabled. Command Modes Global configuration Command History Release 12.1(19)EA1 Modification This command was introduced. Usage Guidelines The switch supports the IEEE 802.1t spanning-tree extensions. Some of the bits previously used for the switch priority are now used for the extended system ID (VLAN identifier for the per-VLAN spanning-tree plus [PVST+] and rapid PVST+ or as an instance identifier for the multiple spanning tree [MST]). The spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN or multiple spanning-tree instance. Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For more information, see the "spanning-tree mst root" and the "spanning-tree vlan" sections. If your network consists of switches that do not support the extended system ID and switches that do support it, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches. 2-570 Catalyst 3560 Switch Command Reference 78-16405-05