Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Syntax: spanning-tree instance < 1..16 > priority < 0 .. 15 >

This command sets the switch (bridge) priority for the designated instance. This priority is compared with the priorities of other switches in the same instance to determine the root switch for the instance. The lower the priority value, the higher the priority. (If there is only one switch in the instance, then that switch is the root switch for the instance.) The root bridge in a given instance provides the path to connected instances in other regions that share one or more of the same VLAN(s). (Traffic in VLANs assigned to a numbered STP instance in a given region moves to other regions through the root switch for that instance.)

The priority range for an MSTP switch is 0-61440. However, this command specifies the instance priority as a multiplier (0 - 15 ) of 4096. That is, when you specify an instance pri­ ority value of 0 - 15, the actual priority assigned to the switch for the specified MST instance is:

(instance-priority-value) x 4096

For example, if you configure “5” as the priority for MST Instance 1 on a given MSTP switch, then the Switch Priority setting is 20, 480 for that instance in that switch.

Note: If multiple switches in the same MST instance have the same priority setting, then the switch with the lowest MAC address becomes the root switch for that instance.

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