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14793 Views 4 Replies Latest reply: Mar 22, 2011 2:11 PM by DelVonte RSS

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Spanning tree root primary question

Mar 21, 2011 12:53 PM

AndrewY 3 posts since
Dec 4, 2010

Hi,

 

I am hoping that someone can please help me with an answer here. I guess my question has multiple questions...

 

1) If i have 2 switches, lets say switch A and switch B and Switch A is the current root bridge with priority 0 on vlan 1

If I go to to switch B and type, "spanning-tree vlan 1 root primary"  what will happen?

 

On packet tracer, the default STP priority for Switch B is 32,768. After typing in the "spanning-tree vlan 1 root primary" it drops the priority to 24,576.

 

2) Isn't the way that root primary works, is that it sets the priority for that vlan to 4096 lower than the current root bridge, effectively making it the new root bridge?

 

3) If I continue to type spanning-tree vlan 1 root primary over and over, it continues to drop the priority

 

32,768

24,576

16,384

8,192


etc.. eventually, bugging out to like 4515151521521 priority. Is this a packet tracer bug? what should happen when you continue to type "spanning-tree vlan 1 root primary" on the same switch? does it lower the priority again and again? or should it stay.

 

 

Thanks in advance

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    1. Mar 21, 2011 1:32 PM (in response to AndrewY)
    Re: Spanning tree root primary question

    The spanning-tree vlan x root primary command will reduce the priority down to 4096 (when done enough times), and then it is up to the MAC addresses to decide on the winner (if the spanning-tree vlan x root command was issued over and over and over again on all the switches).

     

    Another option would be to use the command spanning-tree vlan x priority 0, and that would force that to be winner (if no one else had that same priority and a lower MAC address).

     

    Best wishes,

     

    Keith

  • Currently Being Moderated
    2. Mar 21, 2011 2:19 PM (in response to AndrewY)
    Re: Spanning tree root primary question

    I think Packet tracer does not know how to handle it. Eventually if the priority is 0, then it will be a tie, and just as Keith stated above it will be based on lower MAC.

     

    I tested this in PT and it does give a large number when you enter the spann vlan 1 root primary command and another switch already has a priority of 0. So it looks like a problem with PT.

     

    DelVonte

  • Currently Being Moderated
    4. Mar 22, 2011 2:11 PM (in response to AndrewY)
    Re: Spanning tree root primary question

    The spanning-tree vlan 1 root primary command does not override the election process. All it does is determine the priority of the current root, and lower the priority of the switch where the command was entered to a value of 4096 lower than the current root priority. This normally results in the switch where t he command was entered becoming the root.

     

    In the case of a 0 priority on the current root, the command will result in both switches having a priority of 0, and the one with the lowest MAC wil become the root.

     

    HTH

    DelVonte

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