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Catalyst 2960 Switch SoftwareConfiguration Guide
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Chapter15 Configuring STP
Configuring Spanning-Tree Features
If 128 instances of spanning tree are already in use, you can disable spanning tree on one of the VLANs
and then enable it on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global
configuration command to disable spanning tree on a speci fic VLAN, and use the spanning-tree vlan
vlan-id global configuration command to enable spanning tree on the desired VLAN .
Caution Switches that are not running spanning tree still forward BPDUs that they receive so that the other
switches on the VLAN that have a running spanning-tree instance can break loops. Therefore, spanning
tree must be running on enough switches to break all the loops in the network; for example, at least one
switch on each loop in the VLAN must be running spanning tree. It is not absolutely necessary to run
spanning tree on all switches in the VLAN. However, if you are running spanning tree only on a minimal
set of switches, an incautious change to the network that introduces another loop into the VLAN can
result in a broadcast storm.
Note If you have already used all available spanning-tree instances on your switch, adding another VLAN
anywhere in the VTP domain creates a VLAN that is not run ning spanning tree on that switch. If you
have the default allowed list on the trunk ports of that switch, the new VLAN is carried on all trunk ports.
Depending on the topology of the network, this could create a loop in the new VLAN that will not be
broken, particularly if there are several adjacent switches that have all run out of spanning-tree instances.
You can prevent this possibility by setting up allowed lists on the trunk ports of switches that have used
up their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many cases and
can make it more labor-intensive to add another VLAN to the network.
Spanning-tree commands control the configuration of VLAN spanni ng-tree instances. You create a
spanning-tree instance when you assign an interface to a VLAN. The spanning-tree instance is removed
when the last interface is moved to another VLAN. You can configure switch and port parameters before
a spanning-tree instance is created; these parameters are applied when the spanning-tree instance is
created.
The switch supports PVST+, rapid PVST+, and MSTP, but only one version can be active at any time.
(For example, all VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For
information about the different spanning-tree modes and how they interoperate, see the “Spanning-Tree
Interoperability and Backward Compatibility” section on page15-10.
For configuration guidelines about UplinkFast and BackboneFast, see the “Optional Spanning-Tree
Configuration Guidelines” section on page 17-10.
Changing the Spanning-Tree Mode.
The switch supports three spanning-tree modes: PVST+, rapid PVST+, o r MSTP. By default, the switch
runs the PVST+ protocol.
Beginning in privileged EXEC mode, follow these steps to change the spanning-tree mode. If y ou want
to enable a mode that is different from the default mode, this procedure is required.