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JimNim
Asked: 2013-07-19 06:51:45 +0800 CST

Cisco Port Buffer Memory

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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.

performance

1 Answers

  1. Best Answer

    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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