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This Question is Answered 2 Helpful Answers available (2 pts)
1285 Views 2 Replies Latest reply: Apr 26, 2012 10:39 AM by Martin RSS

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spanning tree port priority ?

Apr 26, 2012 7:28 AM

SUP 117 posts since
Feb 2, 2012

IN this network diagram R3 is root switch and in switch R1 f1/0 is root port and f1/1 is in blocking state.

Now if I want to make port f1/1 in switch R1 as root port I have to change (lower) the port priority of f1/1 on switch R2.

 

why changing the port priority of f1/1 on switch R2 only  doesn'tmake it root port ?

 

why it is calculating the port priority of R2 .

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  • CiscoLoco - CCNP 956 posts since
    Feb 11, 2009
    Currently Being Moderated
    1. Apr 26, 2012 8:22 AM (in response to SUP)
    Re: spanning tree port priority ?

    When R1 is deciding which port to block it will look to see which one of its ports "received" the best information.  If cost is equal the tie breaker will go down to port priority.  Lower is better.  So when you change the port priority of R2 f1/1, R1's fa1/1 receive a better metric than the one its receiving on Fa1/0 so it decides to forward out Fa1/1 and block Fa0/0

  • Martin 13,075 posts since
    Jan 16, 2009
    Currently Being Moderated
    2. Apr 26, 2012 10:39 AM (in response to SUP)
    Re: spanning tree port priority ?

    Every thing in STP is based on sender port or sender BID;

    that just the way things are.

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