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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 20 Configuring STP
Information About Configuring STP
Path Cost
The spanning-tree path cost default value is derived from the media speed of an interface. If a loop
occurs, spanning tree uses cost when selecting an interface to put in the forwarding state. You can assign
lower cost values to interfaces that you want selected first and higher cost values that you want selected
last. If all interfaces have the same cost value, spanning tree puts the interface with the lowest interface
number in the forwarding state and blocks the other interfa ces.
Spanning-Tree TimersSpanning-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 128 VLANs on the switch. The remaining VLANs operate with spanning tree
disabled. However, you can map multiple VLANs to the same spanning-tree instances by using MSTP.
For more information, see Chapter 21, “Configuring MSTP.”
If 128 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 speci fic VLAN, and use the spanning-tree vlan
vlan-id global configuration command to enable spanning tree on the desired VLAN .
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 run ning 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.
Tab l e 20-4 Spanning-Tree Timers
Variable Description
Hello timer Controls how often the switch broadcasts hello messages to other switches.
Forward-delay timer Controls how long each of the listening and learning states last before the interface begins
forwarding.
Maximum-age timer Controls the amount of time the switch stores protocol information received on an interface.
Transmit hold count Controls the number of BPDUs that can be sent before pa using for 1 second.