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Configuring STP
Information About Configuring STP
Default Spanning-Tree Settings
Disabling Spanning Tree
Spanning tree is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit specified
in the Supported Spanning-Tree Instances, page 322. Disable spanning tree only if you are sure there are no loops in the
network topology.
Caution: When spanning tree is disabled and loops are present in the topology, excessive traffic and indefinite
packet duplication can drastically reduce network performance.
Root Switch
The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting
of the switch priority and the switch MAC address, is associated with each instance. For each VLAN, the switch with the
lowest bridge ID becomes the root switch for that VLAN.
To configure a switch to become the root for the specified VLAN, use the spanning-tree vlan vlan-id root global
configuration command to modify the switch priority from the default value (32768) to a significantly lower value. When
you enter this command, the software checks the switch priority of the root switches for each VLAN. Because of the
extended system ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will cause this
switch to become the root for the specified VLAN.
If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the
specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch
priority value as shown in Table 37 on page 317.)
Table 39 Default Spanning-Tree Settings
Feature Default Setting
Enable state Enabled on VLAN 1.
Spanning-tree mode PVST+. (Rapid PVST+ and MSTP are
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 Mb/s: 4.
100 Mb/s: 19.
10 Mb/s: 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 Mb/s: 4.
100 Mb/s: 19.
10 Mb/s: 100.
Spanning-tree timers Hello time: 2 seconds.
Forward-delay time: 15 seconds.
Maximum-aging time: 20 seconds.
Transmit hold count: 6 BPDUs
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