Viewing the change history of root bridges

The show spanning-tree root-history command allows you to display change history information (up to 10 history entries) for a specified root bridge in any of the following MSTP topologies:

  • Common Spanning Tree (cst):Provides connectivity in a bridged network between MST regions, STP LANs, and RSTP LANs.

  • Internal Spanning Tree (ist):Provides connectivity within an MST region for VLANs associated with the default Common and Internal Spanning Tree (CIST) instance in your network (VLANs that have not been mapped to an MST instance).

  • MST Instance (mst):Connects all static and (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

Displays the change history for the root bridge in the specified MSTP topology.

cst

Displays the change history for the root bridge of a spanning tree network, including MST regions and STP and RSTP bridges.

ist

Displays the change history for the root bridge in the IST instance of an MST region.

mst instance-id

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-tree root-history command 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-tree root-guard command.

Sample output of the show spanning-tree root-history command for different MSTP topologies

The following examples show sample output of the show spanning-tree root-history command for different MSTP topologies. 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.

Viewing show spanning-tree root-history CST output

Viewing show spanning-tree root-history IST output

Viewing show spanning-tree root-history MSTI output