Finding root bridge exam question.

[surfbum99]

Hi,

In my CCNA exam I had another question that asks you in a topology of say 5 switches which one is the root bridge, but there wasn't any MAC addresses or Priorities displayed all you had was a console into one of the switches.

Is it possible to find out which switch is the root bridge just from the command line??

regards,

Alistair.

 

[KiscoKid]

Yes you can find out from the CLI.

If an IOS switch, you issue the 'show spanning-tree' command.

If a CATOS switch, you issue the 'show spantree' command.

Each of these commands will identify the MAC of the root bridge.

 

[surfbum99]

Excellent!! Many thanks, for this.

Have you got any clues on my other switch question, what happens to VLANS if a switch gets taken out of action?

 

[surfbum99]

THanks for this,

I did a "show spanning-tree"

It comes up with every vlan which is configured. It seems that each VLAN has its own root id and bridge id. should this be the case? It also just gives you a MAC address.

 

[techkiwi]

There are different types of spanning-tree that Cisco switches can run.

If you enter the command "show spanning-tree summary" it will show you what mode the switch is in. Sounds like your one is using Per-VLAN Spanning Tree (PVST) where it runs a separate spanning-tree instance for each VLAN. Another mode is Multiple Spanning Tree (MST) which from memory is designed to lessen the impact of running one spanning-tree instance per VLAN when you have a large number of VLANs.