Viewing the change history of root bridges

Syntax:

show spanning-tree root-history vlan vlan-id

Displays the last 10 root bridge changes on a specified VLAN configured with RPVST+. Included are the timestamp and Root Bridge ID recorded at each root bridge change.

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 VLAN 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 a port connected to the device from being selected as the root port in a topology, use the spanning-tree root-guard command.

switch# show spanning-tree root-history vlan 20

 Status and Counters - RPVST Root Changes History

  VLAN ID                : 20
  Root Changes Counter   : 53
  Current Root Bridge ID : 32768:0024a8-d13a40

 Root Bridge ID      Date       Time
 ------------------- ---------- --------
 32768:0024a8-d13a40 05/04/2012 21:54:11
     0:001185-c6e500 05/04/2012 21:54:07
 32768:0024a8-d13a40 05/04/2012 16:41:11
     0:001185-c6e500 05/04/2012 16:41:11