Cisco WS-2960-24LC-S Software Guide - Page 342
Configuring a Secondary Root Switch, spanning-tree mst
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Configuring MSTP Features Chapter 16 Configuring MSTP The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning-tree primary root. Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time. Note After configuring the switch as the root switch, we recommend that you avoid manually configuring the hello time, forward-delay time, and maximum-age time through the spanning-tree mst hello-time, spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration commands. Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch. This procedure is optional. Step 1 Step 2 Command configure terminal spanning-tree mst instance-id root primary [diameter net-diameter [hello-time seconds]] Step 3 Step 4 Step 5 end show spanning-tree mst instance-id copy running-config startup-config Purpose Enter global configuration mode. Configure a switch as the root switch. • For instance-id, you can specify a single instance, a range of instances separated by a hyphen, or a series of instances separated by a comma. The range is 0 to 4094. • (Optional) For diameter net-diameter, specify the maximum number of switches between any two end stations. The range is 2 to 7. This keyword is available only for MST instance 0. • (Optional) For hello-time seconds, specify the interval in seconds between the generation of configuration messages by the root switch. The range is 1 to 10 seconds; the default is 2 seconds. Return to privileged EXEC mode. Verify your entries. (Optional) Save your entries in the configuration file. To return the switch to its default setting, use the no spanning-tree mst instance-id root global configuration command. Configuring a Secondary Root Switch When you configure a switch with the extended system ID support as the secondary root, the switch priority is modified from the default value (32768) to 28672. The switch is then likely to become the root switch for the specified instance if the primary root switch fails. This is assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become the root switch. 16-18 Catalyst 2960 Switch Software Configuration Guide OL-8603-04
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