Multiple Instance Spanning-Tree Operation

Configuring MSTP

Syntax: [no] spanning-tree instance < 1..16 > vlan < vid [ vid..vid ] > no spanning-tree instance < 1..16 >

— Continued

Note: The valid VLAN IDs that you can map to a specified MSTI are from 1 to 4094. The VLAN ID-to-MSTI mapping does not require a VLAN to be already configured on the switch. The MSTP VLAN enhancement allows you to preconfigure MSTP topologies before the VLAN IDs associated with each instance exist on a switch.

When you use preconfigured VLAN ID-to-MSTI topologies, ensure that MSTP switches remain in the same region by mapping all VLAN IDs used in the region to the same MSTIs on each regional switch.

When you upgrade switch software to release 13.x.x and later, the existing MSTP topology configuration is automatically saved. All existing VLAN ID-to-MSTI assignments are maintained on a switch for uninterrupted MSTP network operation.

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

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 IST regional root bridge provides the path to instances in other regions that share one or more of the same VLAN(s).

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 for the specified MST instance is:

(priority-multiplier) x 4096

For example, if you configure “5” as the priority-multiplier 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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