Two unit 3750 stack: one cable or two?

Question

I am creating a two unit stack consisting of one 3750-24TS and one3750G-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 isprobably my inability to successfully search  that bothstackwise ports must be used to achieve full stackplane capacity.Advice appreciated.sPh

Łukasz Bromir · Accepted Answer

Yes, You should use two as this will create *ring*. Just connecting3750 by one cable will create point-to-point connection and capacitywill be halved.

Spencer Teran · Answer

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.

Cheers, Spencer

?ukasz Bromirski wrote:

Łukasz Bromir · Answer

No, every link is 16Gbit/s, so both together give 32Gbit/s.

Here You can read about it:

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...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 partitioned.

[...]

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:

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And to quick check it without going to CLI:

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Quote: "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.

sphealey · Answer

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

Spencer Teran · Answer

Thank you very much for the helpful advice.  I added the second cable to my stack.Thanks,Spencer?ukasz Bromirski wrote:

Spencer Teran · Answer

Thank you very much for the helpful advice.  I added the second cable to my stack.Thanks,Spencer?ukasz Bromirski wrote:

Two unit 3750 stack: one cable or two?

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