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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
OL-13270-01
Chapter17 Configuring STP
Configuring Spanning-Tree Features
Disabling Spanning Tree
Spanning tree is enabled by default on VLAN 1 and on all newly created VLAN s up to the spanning-tree
limit specified in the “Supported Spanning-Tree Instances” section on page 17-10. 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 t he t op ology, excessive traffic and indefinite
packet duplication can drastically reduce network performance.
Beginning in privileged EXEC mode, follow these steps to disable spanning-tree on a per-VLAN basis.
This procedure is optional.
To re-enable spanning-tree, use the spanning-tree vlan vlan-id global configuration command.
Configuring the 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 pri orit y fr om the defau lt 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 to4096 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 Table17-1 on page 17-5.)
Note The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the
root switch is less than 1.
Note If your network consists of switches that both do and do not support the extended sy stem ID, it is unlikely
that the switch with the extended system ID support will become the root switch. The extended system
ID increases the switch priority value every time the VLAN number is greater than the priority of the
connected switches running older software.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 no spanning-tree vlan vlan-id For vlan-id, the range is 1 to 4094.
Step3 end Return to privileged EXEC mode.
Step4 show spanning-tree vlan vlan-id Verify your entries.
Step5 copy running-config startup-config (Optional) Save your entries in the configuration file.