Cisco WS-C2960S-48LPD-L Software Guide - Page 316
Disabling Spanning Tree, Configuring the Root Switch
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Configuring Spanning-Tree Features Chapter 15 Configuring STP 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" section on page 15-9. 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. Beginning in privileged EXEC mode, follow these steps to disable spanning-tree on a per-VLAN basis. This procedure is optional. Step 1 Step 2 Step 3 Step 4 Step 5 Command configure terminal no spanning-tree vlan vlan-id end show spanning-tree vlan vlan-id copy running-config startup-config Purpose Enter global configuration mode. For vlan-id, the range is 1 to 4094. Return to privileged EXEC mode. Verify your entries. (Optional) Save your entries in the configuration file. 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 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 15-1 on page 15-4.) 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 system 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. 15-14 Catalyst 2960 Switch Software Configuration Guide OL-8603-04
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