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How can the total amount of port buffer memory be determined for Cisco switches?

Cisco doesn't seem to publish this data anywhere as part of the hardware specifications, but the information seems crucial for determining whether or not a switch is adequate for iSCSI SAN traffic.

I see various values shown in the output of the show buffers command, but I'm not quite understanding how to turn this into usable data for comparing switches.

I'm particularly interested in comparing the Catalyst 2960 and 3750 models, specifically to help deter someone from using an inadequate switch for iSCSI traffic. Any thoughts on other factors that play into switch usability are welcomed, though I'm particularly interested in port buffer space data here.

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  • Check out the Dell 5500 powerconnects for iSCSI. They have big buffers, and automatically QoS properly as Jason below has suggested and they have built-in stacking over HDMI that has enough bandwidth to suit most people's needs. On a budget, they're not badly priced either, but I'd ONLY use them as a storage fabric if I could help it. Jul 20, 2013 at 16:23
  • Well familiar with the PowerConnect 55xx series - I definitely prefer the 62xx series and better for iSCSI. I'm more on a hunt for solid data supporting that 29xx series isn't fit for storage networks where performance is a concern. Thanks for the input!
    – JimNim
    Jul 20, 2013 at 20:09
  • Please don't use a 2K or 3K for iSCSI data center traffic :) The other problem you'll have on non-data-center class switches (outside of small per-asic shared buffers) is head of line blocking, in which a congested port can affect traffic destined for non-congested interfaces. You want a switch with virtual output queuing to mitigate this behavior. Nexus 5500 being a good choice, but I gather you're mostly just trying to prove to a customer not to use a 2K for iSCSI traffic? Have them ask their local Cisco Systems Engineer - I certainly wouldn't abide! Jul 21, 2013 at 20:00
  • Be aware the 6200 series does not have the larger port buffers that the 5500 has. they used to recommend the old 5000s but the 5500s are a pretty big step up. Jul 22, 2013 at 3:43
  • and just for good measure, I've seen 3750s fall on their face time and again in regards to iSCSI, the new 3800s are supposed to shore that up a bit. I'm less familiar with Nexus for this purpose. Jul 22, 2013 at 3:43

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The information on buffer sizes on the 29xx 37xx switches is not published. I certainly wouldn't use either switch in a data center where you need deep buffers as they simply don't have them - they're not designed for data center use - they are user access switches.

Here's a good discussion on a similar topic that may help on the decision making process:

http://www.networking-forum.com/viewtopic.php?t=23060

Whatever you do end up doing, you'll want to tune your egress queue buffers if maximum queue size is important to you. In general that's a 3 step process

1) Ensure your iSCSI traffic is getting mapped in to a unique queue (based on DSCP or COS value)

2) Configure the particular queue to be strict tail drop (set all drop thresholds to 100)

3) Increase the available buffers to that queue (starving to some degree the other queues) as well as increase its access to the shared pool.

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