Two unit 3750 stack: one cable or two?

Discussion in 'Cisco' started by sphealey, Jun 29, 2005.

  1. sphealey

    sphealey Guest

    I am creating a two unit stack consisting of one 3750-24TS and one
    3750G-24T. Should I use one stackwise cable or two? Creating a "ring"
    of two units seems odd, but I get the impression from the documentation
    (nowhere spelled out that I have been able to find, but that is
    probably my inability to successfully search that both
    stackwise ports must be used to achieve full stackplane capacity.

    Advice appreciated.

    sphealey, Jun 29, 2005
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  2. Yes, You should use two as this will create *ring*. Just connecting
    3750 by one cable will create point-to-point connection and capacity
    will be halved.
    =?ISO-8859-2?Q?=A3ukasz_Bromirski?=, Jun 29, 2005
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  3. I have a (2) unit stack and I only use one cable. I haven't heard that
    the stacks have to be in a ring configuration. I would assume that even
    if the stack is in a ring, it will only use 1 path at a time.

    Spencer Teran, Jun 29, 2005
  4. No, every link is 16Gbit/s, so both together give 32Gbit/s.

    Here You can read about it:

    ....under section titled "Bidirectional Flow":

    "To efficiently load balance the traffic, packets are allocated between
    two logical counter-rotating paths. Each counter-rotating path
    supports 16-Gbps of traffic for a total of 32 Gbps. The egress queues
    calculate path usage to ensure that the traffic load is equally


    When a break is detected in a cable, the traffic immediately loops back
    across the single remaining 16-Gbps path to continue forwarding."

    So if You use only one cable, for the StackWise You'll operate in
    "broken" mode.

    Please note, switch configuration guide mentions correct two units
    connection model - here, in figure 5-1:

    And to quick check it without going to CLI:

    "After adding or removing stack members, make sure that the switch
    stack is operating at full bandwidth (32 Gbps). Press the Mode
    button on a stack member until the Stack mode LED is on. The
    last two port LEDs on the switch should be green. Depending on the
    switch model, the last two ports are either 10/100/1000 ports or
    small form-factor pluggable (SFP) module ports. If one or both of
    the last two port LEDs are not green, the stack is not operating at
    full bandwidth."

    Cisco ships StackWise cable with every 3750, so if You have two
    units You should have two cables to properly connect them together.
    =?ISO-8859-2?Q?=A3ukasz_Bromirski?=, Jun 29, 2005
  5. sphealey

    sphealey Guest

    Thank you! I could not find the diagram for 2 switches in the
    configuration guide - must have been "Cisco documentation blindness".

    sphealey, Jun 30, 2005
  6. Thank you very much for the helpful advice. I added the second cable to
    my stack.


    Spencer Teran, Jun 30, 2005
  7. Thank you very much for the helpful advice. I added the second cable to
    my stack.


    Spencer Teran, Jun 30, 2005
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