Cisco Systems 3750E manual 18-14

Chapter 18 Configuring STP

Configuring Spanning-Tree Features

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 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

aspanning-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.) In Catalyst 3750-E-only and mixed switch stacks, 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 18-11.

For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the “Optional Spanning-Tree Configuration Guidelines” section on page 20-12.

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.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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