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Can we identify the root switch in a layer 2 from a non-root switch

Hi Everybody,

There was a question about identifying the root switch in a layer 2 network from a non-root switch on a CCNA exam simulator. I would like to know if there is a command on a switch that could identify the root hostname or if the only way to identify a root switch on a CCNA simulator is to identify the neighbors with show CDP neighbor, telnet to the neighbors and look for the root switch with show spanning-tree vlan 1.

Thanks for your help

Stephane

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Hall of Fame Super Gold

Re: Can we identify the root switch in a layer 2 from a non-root

sh spanning-tree summary

Re: Can we identify the root switch in a layer 2 from a non-root

Hi Everybody,

There was a question about identifying the root switch in a layer 2 network from a non-root switch on a CCNA exam simulator. I would like to know if there is a command on a switch that could identify the root hostname or if the only way to identify a root switch on a CCNA simulator is to identify the neighbors with show CDP neighbor, telnet to the neighbors and look for the root switch with show spanning-tree vlan 1.

Thanks for your help

Stephane

Hi Stephane,

Show spanning-tree root would be the best to just find the root, but it doesn't give you all the extra info you get from the show spanning-tree command.

Switch#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     001e.f6d6.e400
             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 1)
             Address     001e.f6d6.e400
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Desg FWD 3         128.56   P2p

Hope to Help !!

Ganesh.H

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Hall of Fame Super Silver

Re: Can we identify the root switch in a layer 2 from a non-root

Hello Stephane,

>> r is to identify the neighbors with show CDP neighbor, telnet to the neighbors and look for the root switch with show spanning-tree vlan 1

if you look at the show spanning-tree vlan 1 in Ganesh's post you can see that the root bridge is identified by its MAC address.

the show spanning-tree provides you also the root port that is the port on the best path to the root, following the root ports of switches between the first one and the root bridge you can find it.

Actually, CDP is of great help in identifying the neighobor switch connected to the root port.

The root switch hostname is not available as STP lies in OSI layer2 and the hostname is linked in DNS with an IP address (OSI layer3)

Hope to help

Giuseppe

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