Specifications

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Catalyst 2950 and Catalyst 2955 Switch Command Reference

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Chapter 2 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

Usage Guidelines In Release 12.1(9)EA1 and later, the switches support the 802.1T spanning-tree extensions, and 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 [PVST] or an instance identifier for the multiple spanning tree

[MST]). In earlier releases, the switch priority is a 16-bit value.

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. With earlier

releases, spanning tree used one MAC address per VLAN to make the bridge ID unique for each VLAN.

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 running older software.

Release Modification

12.1(9)EA1 This command was first introduced.