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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 14 Configuring STP
Configuring Spanning-Tree Features
Spanning-Tree Configuration Guidelines
If more VLANs are defined than there are spanning-tree instances, you can enable PVST+ or rapid
PVST+ on STP ports s in 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 15, “Configuring MSTP.”
If 128 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 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.
Tab le 14-3 Default Spanning-Tree Configuration
Feature Default Setting
Enable state Enabled on NNIs in VLAN 1.
Disabled on ENIs. (Not supported on UNIs)
For more information, see the “Supported Spanning-Tree
Instances” section on page 14-10.
Spanning-tree mode Rapid PVST+
Rapid PVST+ interoperates with PVST and PVST+.
MSTP is disabled.
Switch priority 32768.
Spanning-tree port priority (configurable on a per-interface basis) 128.
Spanning-tree port cost (configurable on a per-interface basis) 1000 Mbps: 4.
100 Mbps: 19.
10 Mbps: 100.
Spanning-tree VLAN port priority (configurable on a per-VLAN
basis) 128.
Spanning-tree VLAN port cost (configurable on a per-VLAN
basis) 1000 Mbps: 4.
100 Mbps: 19.
10 Mbps: 100.
Spanning-tree timers Hello time: 2 seconds.
Forward-delay time: 15 seconds.
Maximum-aging time: 20 seconds.