Spanning Tree root switch

Discussion in 'Cisco' started by John Carter, Jan 6, 2006.

  1. John Carter

    John Carter Guest

    I'm trying to understand importance of having a "high end" switch as
    the root switch. I see references of it here and there but haven't seen
    a solid document on it. I've spent quite some time googling spanning
    tree and root switches. I accidently ran into a situation where a low
    end "access" switch became the root and things still seem to be
    working and I'm trying to understand why and what the downsides are.
    Here is a little backgroud:

    I have a fairly simple network, with a potential for growth and hence
    the need for proper STP configuratoin. I'm trying to understand the
    worse case scenario for a misconfigured "root" switch. I understand
    that I can force the root switch to be the core switch by changing the
    priority and also understand the use root gaurd protection and other

    Here is what my network looks like:

    Switch (M) Core switch and also desired STP root, VLAN 20,30,40 )
    / |
    switch A(VLAN 20) switchB (VLAN 20) switch C (VLAN 30)

    Now it turns out that becuase of lower MAC ID, switch A become my root
    for VLAN 20.

    Here is what my STP diagram looks like for VLAN 20.

    Switch A (root)
    Switch M (Core)
    Switch B
    1. Things seem to be working with switch A as the root switch. Where
    should I look for spanning tree misconfiguration/bottlenecks? Any IOS
    command recommendations?
    2. What is the worse case scenario with having switch A as the STP
    3. Does the bottleneck only happen when spanning tree reconverges or
    would it affect layer 3 traffic in any way?

    John Carter, Jan 6, 2006
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  2. John Carter

    Make Guest

    1. Things seem to be working with switch A as the root switch. Where
    Switch M:
    spanning-tree vlan 20 root primary
    Make, Jan 7, 2006
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  3. John Carter

    anybody43 Guest

    I'm trying to understand importance of having a "high end" switch as
    I very much doubt that you need a "a "high end" switch as
    the root switch" however it is important that the root is at
    the centre of the network.

    Think of the "tree" bit of "Spanning-tree". This name
    was not chosen randomly as many seem to be:)

    Imagine a squirrel on branch A of a fir tree wants to get
    to Branch B.

    For most possible journeys in a tree the squirrel will
    head first of all towards the root then away from the root
    of the tree.

    (Clearly there are exceptions for some nearby branches)

    If your Root Bridge is on the network edge then there
    is the likelihood that a large proportion of your traffic
    will first head towards the network edge and then back towards
    the destination. This is not what most people want.

    The worst case (and this is not at all unlikely)
    is that most network traffic goes via the root
    bridge (switch). This may for example
    mean that your gigabit core links have no traffic at all and that
    the uplinks to a particular "access" switch carry all of the traffic.

    Get a network diagram and draw the Spanning tree on it
    and follow the traffic.

    A trivial case which makes the point is
    two core switches say with Gigabit between them,
    and a single access switch connected by 100M
    to both core switches.

    You have a server on one core switch
    and a tape-backup server on the other core switch.
    If the Access switch is ROOT then traffic
    between these two servers will go via the access
    switch. Probably not what is wanted.

    Good luck.
    anybody43, Jan 8, 2006
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