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TOPIC: Troubleshooting STP

Troubleshooting STP 7 years 9 months ago #28291

  • matrixr6
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Hi, I am wondering say if there are 4 switches (2950) connected together and there are 2 vlans and they all in the same VTP domain. My question is how will I find out which is the root bridge? in vlan 1( for example) if I have access only to one switch and that switch is a VTP client( no telnet access to the rest of the bridge) . I issue show spanning-tree vlan 1 and it lists the root bridge ID and and the trunk port info etc. This is just a packet tracer setup that I have and been using for my CCNA exam study. How can I tell which is the root switch for that VLAN 1 by looking at the bridge ID ??(by the way I do know the root switch for VLAN 1 by going to that switch and issue the show spanning-tree vlan 1 , it say " this bridge is the root".
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Re: Troubleshooting STP 7 years 9 months ago #28303

  • S0lo
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How can I tell which is the root switch for that VLAN 1 by looking at the bridge ID ??

The root bridge (switch) is always the one that has the least valued bridge ID.

A bridge ID is built up by joining the MAC address of that bridge with it's priority value. So if two switches have say:

Switch 1: Priority 32772, MAC: 0005.5E02.B830
Switch 2: Priority 32772, MAC: 0005.4001.0311

Then Switch 2 will be the root bridge because although they both have the same priority but switch 2 has a lower MAC address. If we have this:

Switch 1: Priority 31000, MAC: 0005.5E02.B830
Switch 2: Priority 32772, MAC: 0005.4001.0311

This time swtich 1 will be the root bridge since it has a lower priority, although Switch 2 has a lower MAC. Priority supersedes MAC.
Studying CCNP...

Ammar Muqaddas
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Re: Troubleshooting STP 7 years 9 months ago #28337

  • matrixr6
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Hi solo Thanks for your reply. I still have one more question. say there is 4 switch connected to each other via trunk. SW1,SW2,SW3,SW4 connected in the same physical order and say for example SW2 is the root bridge. I have access ONLY to SW4 and I issue the command sh spanning-tree vlan 1 it tells me the root bridge ID and the local bridge ID(bridge id of sw4) as well for vlan 1 . now I know what the root bridge ID is but how will I know that of SW2 is the root for vlan 1 ? although I know what the root bridge ID is. I hope this is not confusing . I saw this question in a semsim CCNA exam prep software. they had the above said diagram and they had a display of sh spanning-tree vlan 1 command issued from sw4 and the question was which is the root switch...is it sw1 or sw2 or sw 3 so I recreated that in my packet tracer and played with it but I could tell what the root bridge ID was but I had to go to SW2 ans issue the above said command and found out that that is the root switch
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Re: Troubleshooting STP 7 years 9 months ago #28382

  • Patiot
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show vlan, sh cdp neighbors should have a clue
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Re: Troubleshooting STP 7 years 9 months ago #28414

  • matrixr6
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Thx guys
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