Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

Syntax: spanning-tree < port-list> point-to-point-mac < force-true force-false auto >

This parameter informs the switch of the type of device to which a specific port connects.

Force-True(default): Indicates a point-to-point link to a device such as a switch, bridge, or end-node. Force-False: Indicates a connection to a hub (which is a shared LAN segment).

Auto: Causes the switch to set Force-False on the port if it is not running at full duplex. (Connections to hubs are half-duplex.)

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

MSTP uses this parameter to determine the port(s) to use for forwarding. The port with the lowest priority number has the highest priority. The range is 0 to 240, and is configured by specifying a multiplier in the range of 0 -

15.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, if you configure “2” as the priority multiplier on a given port, then the actual Priority setting is 32. Thus, after you specify the port priority multiplier, the switch displays the actual port priority (and not the multiplier) in the show spanning-treeor show spanning-tree < port-list> displays.

You can view the actual multiplier setting for ports 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 “3” results in this line in the show running output:

spanning-tree A2 priority 3

Syntax: spanning-tree < port-list> root-guard

MSTP only. When a port is enabled as root-guard, it cannot be selected as the root port even if it receives superior STP BPDUs. The port is assigned an “alternate” port role and enters a blocking state if it receives superior STP BPDUs. The BPDUs received on a port enabled as root-guardare ignored. All other BPDUs are accepted and the external devices may belong to the spanning tree as long as they do not claim to be the Root device.

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