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16-14
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 16 Configuring STP
Configuring Spanning-Tree Features
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 running 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 spanning-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.) All stack
members run the same version of spanning tree. For information about the different spanning-tree modes
and how they interoperate, see the “Spanning-Tree Interoperability and Backward Compatibility”
section on page 16-11.
For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the
“Optional Spanning-Tree Configuration Guidelines” section on page 18-12.
Caution Loop guard works only on point-to-point links. We recommend that each end of the link has a directly
connected device that is running STP.