Spanning-Tree Extend System-Id - Cisco Catalyst 3550 Command Reference Manual

spanning-tree extend system-id

spanning-tree extend system-id
Use the spanning-tree extend system-id global configuration command to enable the extended system
ID feature.
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(8)EA1
Usage Guidelines In Cisco IOS Release 12.1(8)EA1 and later, Catalyst 3550 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 plus [PVST+] and rapid 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
and the
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.
Catalyst 3550 Multilayer Switch Command Reference
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spanning-tree extend system-id
Modification
This command was first introduced.
"spanning-tree vlan" sections.
Chapter 2 Cisco IOS Commands
"spanning-tree mst root"
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