Cover by Gary A. Donahue

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Managing Spanning Tree

Spanning tree is enabled by default. To see its status, use the show spanning-tree command in IOS and NX-OS:

Cat-3550#sho spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0009.43b5.0f80
             Cost        23
             Port        20 (FastEthernet0/20)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     000f.8f5c.5a00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/13           Altn BLK 19        128.13   P2p
Fa0/14           Altn BLK 19        128.14   P2p
Fa0/15           Altn BLK 19        128.15   P2p
Fa0/20           Root FWD 19        128.20   P2p
Fa0/23           Desg FWD 19        128.23   P2p
[-text removed-]

The bolded text shows the priority and MAC address of the root bridge, as well as which port the switch is using to get there (this is the root port). This is very useful information when you’re trying to figure out where the root bridge is on a network. By running this command on every switch in the network, you should be able to map your connections and figure out which switch is the root.

Here is the output from the show spanning-tree command from a Nexus 7000 that is operating as the STP root:

NX-7K-1-Cozy# sho spanning

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     0026.9807.95c2
             This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext ...

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