Gigastack GBICs and STP hops

Discussion in 'Cisco' started by Pete Mainwaring, Jul 11, 2003.

  1. I know this question has been raised before, but I couldn't find a
    definitive answer anywhere.

    If we install a stack of 2950s connected using Gigastack GBICs in a
    1Gb HDX cascade configuration, does the stack count as one Spanning
    Tree hop or does each individual switch still count as one hop?

    The Cisco datasheet in Gigastack GBICs sort of suggests that the whole
    stack counts as a single hop:-

    "Customers can initially deploy the low-cost GigaStack GBIC to create
    a 1-Gbps independent stack backplane in a cascade configuration."

    I'm taking the word BACKPLANE to mean exactly that.

    However, my suspicion is that it will turn out that each switch counts

    We are going ahead with this configuration anyway, so I will be able
    to answer the question myself in a few weeks time. I just wondered if
    anyone could give me an answer before implementation. (I will post my
    findings anyway).
    Pete Mainwaring, Jul 11, 2003
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  2. Pete Mainwaring

    Chris Marva Guest


    Assume g0/1 is the uplink on the lead switch, and g0/2 is the stack. If
    g0/1 sees the root for a vlan at a path cost of 8, then
    all subsequent switches in the stack will see the root at a cost of 12. The
    only L2 hop you pick up is between g0/1 and g0/2 on
    the lead switch.
    The documentation does specify different topologies, depending on
    whether you set up for fast convergence or normal convergence. For fast
    convergence, all switches in the stack are connected together, and the stack
    runs at half duplex, and
    you don't pick up additional L2 hops.
    For normal convergence, they specify hooking up the lead switch g0/2 to
    g0/1 of the second switch, g0/2 of the second to
    g0/1 of the third, etc. These links will run at full duplex, but you pick up
    L2 hops at every switch.
    If you are using the fast convergence mechanisms, you have to stick with
    the first method. I guess the size of the stack matters,
    as I don't think you'd really want to hook up a shared bus if there are 7 or
    8 switches in the stack. Just my $.02, hope it helps,

    Chris Marva, Jul 11, 2003
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