Cisco Catalyst 2960 Software Configuration Manual page 450

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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.

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

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

For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the

"Optional Spanning-Tree Configuration Guidelines" section on page

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.

Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE

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