iSCSI traffic running across Cisco 3750 Switches

Unanswered Question
Mar 13th, 2009

My customer has a small shop with 2 servers runnig iSCSI to a SAN device. They are looking for a switch recommendation and I would like to use a pair of Cisco 3750's, to take advantage of the VSS technology for redundancy,L3 and some other Core requirements, but I am concerned about performance.

I thought my other option is to use 3750E's but concerned about the added costs.

I have this problem too.
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Michael Brown Fri, 03/13/2009 - 12:42

I believe that both models are wire rate switches, so either should work fine.

- Mike

dvaggalis Wed, 07/01/2009 - 10:44

the 3750's share an 768K ingress buffer between the ports on the same ASIC, so my understanding is that the 3750 or the 3560, are not true wire rate switches.

dvaggalis Wed, 07/01/2009 - 10:46

Another questions is if this limitation effects the (4) SFP ports or just the 10/100/1000 copper ports?

Joseph W. Doherty Wed, 07/15/2009 - 04:37

The fabric and pps ratings for the 3750Gs don't support wire-rate for more than 16 gig ports. (Max performance for 3750G models is 38.7 Mpps and 32 Gbps fabric; for 3750-E it's 101.2 Mpps and 128 Gbps fabric [NB: pps is enough, slightly insufficent fabric bandwidth for 48 port models - similar 4948 offers 102 Mpps, 136 Gbps].)

Another performance limitation of the 3750s (and to lessor extent the 3750-Es) is stack ring bandwidth. As best I can tell, the 32 Gbps is really dual 8 Gbps duplex (dual 16 Gbps duplex for -Es). An important distinction between the original StackWise technology and the later StackWise+, the former puts a copy of all traffic on the stack, the latter suppresses unnecessary unicast. The former also requires the sender to remove the traffic from the stack ring, the latter the destination removes the traffic. (I.e. the "+" technology, really is plus.)

For really, really demanding performance, a stack ring isn't the same as a chassis fabric (e.g. 4500s), and within a single switch, the lower end switch models, they can't always provide wire-rate for all their ports. However, the real question is whether you need this performance in a small shop even though iSCSI is being used.

In other words, its rare to see all ports demanding full bandwidth, so a stack of 48 port 3750Gs migtht work just fine for your customer if the actual need doesn't require more than the device can supply.

In similar situations, I present the customer with such facts. Based on what the expected load is, device "A" might work fine, but it can't guarantee performance beyond a certain level. If customer wants the capability for more performance, for growth or "just to be safe", can do too, here's your options (and extra cost) for that too.

BTW, if SAN devices can support 10gig, then you'll need something better than the 3750G since the model with a single 10gig port has been discontinued.

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