Determining the Root Bridge Switch Spanning Tree Protocol

Looking at the above figure we can determine that this SW1 is not the root bridge.

  1. The Address fields do not match, if this was the root bridge both mac address fields would match
  2. The STS (status) field is another give away, a root switch does not have any ports being blocked BLK
  3. Role is another sign, a root switch would not list another root switch in its connected interfaces.
This is the root bridge as we can see the address fields match and all ports are in FWD
Another way to tell would be to add the priority and the MAC address together creating the BID

Which is the root bridge?
a.) 32769:000b.be2c.5180
b.) 32769:000f-90e2-25c0

a is the answer because the MAC address is lower

Other commands that are useful for determining the root bridge are

SW1#show spanning-tree vlan (vlan number)

SW1#show spanning-tree root

SW1#show span vlan 1 bridge

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