Cisco WS-C2960S-24PD-L Software Guide - Page 161

Chapter 6 Configuring the System Configuring STP Configuring STP Spanning Tree Protocol (STP) provides path redundancy while preventing undesirable loops in the network. Only one active path can exist between any two stations. STP calculates the best loop-free path throughout the network. Supported STP Instances You create an STP instance when you assign an interface to a VLAN. The STP instance is removed when the last interface is moved to another VLAN. You can configure switch and port parameters before an STP instance is created. These parameters are applied when the STP instance is created. You can change all VLANs on a switch by using the stp-list parameter when you enter STP commands through the CLI. For more information, refer to the switch command reference. The Catalyst 2912 XL, Catalyst 2924 XL, and Catalyst 2924C XL support only 64 STP instances and 64 VLANs. All other Catalyst 2900 XL switches and all Catalyst 3500 XL switches support 64 STP instances and 250 VLANs. Each VLAN is a separate STP instance. If you have already used up all available STP instances on a switch, adding another VLAN anywhere in the VLAN Trunking Protocol (VTP) domain creates a VLAN that is not running STP on that switch. For example, if 250 VLANs are defined in the VTP domain, you can enable STP on 64 of those VLANs. The remaining VLANs must operate with STP disabled. You can disable STP on one of the VLANs where it is running, 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 STP on a specific VLAN, and use the spanning-tree vlan vlan-id global configuration command to enable STP on the desired VLAN. For more information about VLANs, see Chapter 8, "Configuring VLANs." Caution Switches that are not running spanning tree still forward bridge protocol data units (BPDUs) that they receive so that the other switches on the VLAN that have a running STP instance can break loops. Therefore, spanning tree must be running on enough switches so that it can 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 STP 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 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 all have run out of STP instances. You can prevent this by setting allowed lists on the trunk ports of switches that have used up their allocation of STP instances. Setting up allowed lists is not necessary in many cases and makes it more labor-intensive to add another VLAN to the network. 78-6511-08 Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide 6-33