Setting the bridge priority for a region and determining the root switch


spanning-tree priority priority-multiplier

Every switch running an instance of MSTP has a Bridge Identifier, which is a unique identifier that helps distinguish this switch from all others. The switch with the lowest Bridge Identifier is elected as the root for the tree.

The Bridge Identifier is composed of a configurable priority component (2 bytes) and the bridge's MAC address (6 bytes). You can change the priority component provides flexibility in determining which switch will be the root for the tree, regardless of its MAC address.

This command sets the switch (bridge) priority for the designated region in which the switch resides. The switch compares this priority with the priorities of other switches in the same region to determine the root switch for the region. The lower the priority value, the higher the priority. If there is only one switch in the region, then that switch is the root switch for the region. The root bridge in a region provides the path to connected regions for the traffic in VLANs assigned to the region's IST instance. 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 priority as a multiplier (0 - 15) of 4096. That is, when you specify a priority multiplier value of 0 - 15, the actual priority assigned to the switch is: (priority-multiplier) x 4096

For example, with 2 as the priority-multiplier on a given MSTP switch, the Switch Priority setting is 8,192.


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