Specifications

9-12
Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 9 Configuring STP
Configuring Spanning-Tree Features
If 255 instances of spanning tree are already in use, you can disable spanning tree on STP ports in 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.
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 devices 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+ and MSTP, but only one version can be active at any time. (For example,
all VLANs run 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 9-10.
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.
Enabling Spanning Tree on an ENI
By default, spanning tree is enabled on all NNIs on the router and disabled on ENIs. Beginning in
privileged EXEC mode, follow these steps to enable spanning tree an ENI:
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify an interface to configure, and enter interface configuration
mode.
Step 3
no shutdown Enable the port, if necessary.