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743 Views 4 Replies Latest reply: Jan 12, 2012 8:23 AM by Elvin Arias RSS

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MSTP

Jan 11, 2012 4:02 PM

Manuel Emilio Arias Soto 6 posts since
Apr 7, 2009

So, i was reading something about MSTP and i see that MSTP has like a distance vector behavior, in the sense that a BPDU, can not be sent from the root bridge 20 hops away in a given region, so if i that is true and im not wrong thats mean that can not be more than 20 switches in a given region, right ?

  • Jimmy Sands 644 posts since
    May 7, 2009
    Currently Being Moderated
    Re: MSTP

    While I can not speak from real world experience, even labbing it up would require 20 switches, I did find this(below) which infers your 20 number is correct

     

    Configuring the Maximum-Hop Count

    Beginning in privileged EXEC mode, follow these steps to configure the maximum-hop count for all MST instances:

     

    Command

    Purpose

    Step 1 

    configure terminal

    Enter global configuration mode.

    Step 2 

    spanning-tree mst max-hops hop-count

    Specify the number of hops in a region before the BPDU is discarded, and the information held for a port is aged.

    For hop-count, the range is 1 to 40; the default is 20.

    Step 3 

    end

    Return to privileged EXEC mode.

    Step 4 

    show spanning-tree mst

    Verify your entries.

    Step 5 

    copy running-config startup-config

    (Optional) Save your entries in the configuration file.

  • Elvin Arias 1,837 posts since
    Mar 12, 2010
    Currently Being Moderated
    Re: MSTP

    Is not that you can't have more than 20 switches on your domain. Think about the RIP logic, can you have more that 16 routers on a RIP domain? Sure you can, but depends of the topology. The case of MSTP is that have a default of 20 hops, but you can configure it until 40 hops away from the root.

     

    HTH,

     

    Elvin

  • Vijay Swaminathan 491 posts since
    Aug 29, 2008
    Currently Being Moderated
    3. Jan 12, 2012 7:33 AM (in response to Elvin Arias)
    Re: MSTP

    Does it mean that Max Age is not applicable for MST? How is this hop count different from max Age??

     

    -Vijay

  • Elvin Arias 1,837 posts since
    Mar 12, 2010
    Currently Being Moderated
    4. Jan 12, 2012 8:23 AM (in response to Vijay Swaminathan)
    Re: MSTP

    What's the logic behind the hop count? The hop count is going decreasing in a hop by hop basis (20 is the default value) until the zero/null value is reached, after this happens the information is aged out, so the switch will declare itself as the root for the STP domain which in MSTP terminologies will declared the root for the IST or the root for the instance 0.

     

    But is the max_age still used? Yes, is used for backward compatibilty with older STP versions (i.e, 802.1D/MST, PVST, PVST+, rapid-PVST+).

     

    Post any doubts,

     

    Elvin

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