Multiple Instance Spanning-Tree Operation

Troubleshooting an MSTP Configuration

MST Instance (mst): Connects all static and (starting from release 13.x.x) dynamic VLANs assigned to a multiple spanning-tree instance.

Syntax: show spanning-tree root-history <cst ist mst <instance-id>>

This command displays the change history for the root bridge in the specified MSTP topology.

The cst parameter displays the change history for the root bridge of a spanning-tree network, including MST regions and STP and RSTP bridges.

The ist parameter displays the change history for the root bridge in the IST instance of an MST region.

The mst <instance-id> parameter displays the change history for the root bridge in an MST instance, where <instance-id> is an ID number from 1 to 16.

Use the show spanning-treeroot-historycommand to view the number and dates of changes in the assignment of a root bridge. Possible intrusion into your MST network may occur if an unauthorized external device gains access to a spanning tree by posing as the root device in a topology. To prevent an MST port connected to the device from being selected as the root port in a topology, use the spanning-treeroot-guardcommand.

The following examples show sample output of the show spanning-tree root- history command for different MSTP topologies. Note that in each example, the root bridge ID is displayed in the format:

<priority:mac-address>

Where:

<priority> is the MSTP switch priority calculated for one of the following:

The IST (regional) root switch using the spanning-tree priority command

An MSTI root switch using the spanning-tree instance priority command

<mac-address> is the MAC address of the root (bridge) switch.

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