Spanning-Tree Configuration Guidelines - Cisco Catalyst 2950 Software Configuration Manual

Chapter 13 Configuring STP
Configuring Spanning-Tree Features

Spanning-Tree Configuration Guidelines

If more VLANs are defined in the VTP than there are spanning-tree instances, you can enable PVST+
or rapid PVST+ on only 64 VLANs. If the number of VLANs exceeds 64, we recommend that you enable
the MSTP to map multiple VLANs to a single spanning-tree instance. For more information, see the
Chapter 14, "Configuring MSTP."
If 64 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 specific VLAN, and use the spanning-tree vlan
vlan-id global configuration command to enable spanning tree on the desired VLAN.
Switches that are not running spanning tree still forward received BPDUs so that the other switches on
Caution
the VLAN with 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 determine 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.) For
information about the different spanning-tree modes and how they interoperate, see the "Spanning-Tree
Interoperability and Backward Compatibility" section on page 13-10.
For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the
"Optional Spanning-Tree Configuration Guidelines" section on page 15-14.
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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