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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 13 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.
If you have already used all available spanning-tree instances on your switch, adding another VLAN
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 STP port (an NNI or ENI with STP enabled) to a VLAN. The
spanning-tree instance is removed when the last port 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
.
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.
Changing the Spanning-Tree Mode.
The switch supports three spanning-tree modes: PVST+, rapid PVST+, or MSTP. By default, the switch
runs the rapid PVST+ protocol on all NNIs and ENIs on which spanning tree is enabled.
Beginning in privileged EXEC mode, follow these steps to change the spanning-tree mode. If you want
to enable a mode that is different from the default mode, this procedure is required.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
spanning-tree mode {pvst | mst |
rapid-pvst}
Configure a spanning-tree mode on STP ports on the switch.
•
Select pvst to enable PVST+.
•
Select mst to enable MSTP (and RSTP). For more configuration
steps, see
Chapter 14, “Configuring MSTP.”
•
Select rapid-pvst to enable rapid PVST+ (the default setting).
