Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Syntax: spanning-tree [e] < port-list> priority < priority-multiplier>

This command sets the priority for the specified port(s) for the IST (that is, Instance 0) of the region in which the switch resides. The “priority” component of the port’s “Port Identifier” is set. The Port Identifier is a unique identifier that helps distinguish this switch’s ports from all others. It consists of the Priority value with the port number extension— PRIORITY:PORT_NUMBER. A port with a lower value of Port Identifier is more likely to be included in the active topology. This priority is compared with the priorities of other ports in the IST to determine which port is the root port for the IST instance. The lower the priority value, the higher the priority. The IST root port (or trunk) in a region provides the path to connected regions for the traffic in VLANs assigned to the region’s IST instance.

The priority range for a port in a given MST instance is 0-240. However, this command specifies the priority as a multiplier (0 - 15) of 16. That is, when you specify a priority multiplier of 0 - 15, the actual priority assigned to the switch is:

(priority-multiplier) x 16

For example, configuring “5” as the priority multiplier on a given port in the IST instance for a region creates an actual Priority setting of 80. Thus, after you specify the port priority multiplier for the IST instance, the switch displays the actual port priority (and not the multiplier) in the show spanning-tree instance ist or show spanning-tree < port-list> instance ist displays. You can view the actual multiplier setting for ports in the IST instance by executing show running and looking for an entry in this format:

spanning-tree < port-list> priority < priority-multiplier>

For example, configuring port A2 with a priority multiplier of “2” in the IST instance, results in this line in the show running output:

spanning-tree A2 priority 2

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