Cisco 2950 Software Configuration Guide - Page 333

Chapter 15 Configuring MSTP Configuring MSTP Features If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own priority 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 14-1 on page 14-4.) Note Catalyst 2950 switches running software earlier than Cisco IOS Release 12.1(9)EA1 do not support the extended system ID. Catalyst 2950 switches running software earlier than Cisco IOS Release 12.1(9)EA1 do not support the MSTP. 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. Note 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 by using the spanning-tree mst hello-time, spanning-tree mst forward-time, and the spanning-tree mst max-age global configuration commands after configuring the switch as the root switch. 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]] 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 15. • (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. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 15-15