Cisco 2950 Software Configuration Guide - Page 332
Configuring the Root Switch
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Configuring MSTP Features Chapter 15 Configuring MSTP Command Step 8 spanning-tree mode mst Purpose Enable MSTP. RSTP is also enabled. Step 9 Step 10 Step 11 end show running-config copy running-config startup-config Caution Changing spanning-tree modes can disrupt traffic because all spanning-tree instances are stopped for the previous mode and restarted in the new mode. You cannot run both MSTP and PVST+ or both MSTP and rapid PVST+ at the same time. Return to privileged EXEC mode. Verify your entries. (Optional) Save your entries in the configuration file. To return to the default MST region configuration, use the no spanning-tree mst configuration global configuration command. To return to the default VLAN-to-instance map, use the no instance instance-id [vlan vlan-range] MST configuration command. To return to the default name, use the no name MST configuration command. To return to the default revision number, use the no revision MST configuration command. To re-enable PVST+, use the no spanning-tree mode or the spanning-tree mode pvst global configuration command. This example shows how to enter MST configuration mode, map VLANs 10 to 20 to MST instance 1, name the region region1, set the configuration revision to 1, display the pending configuration, apply the changes, and return to global configuration mode: Switch(config)# spanning-tree mst configuration Switch(config-mst)# instance 1 vlan 10-20 Switch(config-mst)# name region1 Switch(config-mst)# revision 1 Switch(config-mst)# show pending Pending MST configuration Name [region1] Revision 1 Instance Vlans Mapped 0 1-9,21-4094 1 10-20 Switch(config-mst)# exit Switch(config)# Configuring the Root Switch The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A bridge ID, consisting of the switch priority and the switch MAC address, is associated with each instance. The switch with the lowest bridge ID becomes the root switch for the group of VLANs. To configure a switch to become the root, use the spanning-tree mst instance-id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root switch for the specified spanning-tree instance. When you enter this command, the switch checks the switch priorities of the root switches. Because of the extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switch to become the root for the specified spanning-tree instance. 15-14 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 78-11380-10
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