3560G/E and 3750G/E Buffers

The size of the buffers in the 3560/3750 platforms is commonly referred to as inadequate and small, but it's rare that the buffer size is listed.  I've found one document that lists it:

Catalyst 3560G/3750G and 3560-E/3750E

[SNIP]these platforms provide (minimally) 750 KB of receive buffers and (up to) 2 MB of transmit buffers for each set of 4 ports.[SNIP]


  1. Thanks for finding and posting this information.

    Reading on, we find: "These buffers can be allocated, reserved, or dynamically borrowed from a common pool, on a port-port, per-queue basis, depending on the administrative configurations chosen."

    I'm guessing that "port-port" should have been "per-port".

    ...but the "borrowed from a common pool" bit confuses me. I thought that one of the most important distinctions between these platforms and their predecessor was the common pool: available on 3550, and fixed-buffer-per-port on the 3560.

    Can you help untangle my understanding here?

  2. In these platforms, you allocate output buffers via the queue-set configuration:

    Switch#show mls qos queue-set
    Queueset: 1
    Queue : 1 2 3 4
    buffers : 25 25 25 25
    threshold1: 100 200 100 100
    threshold2: 100 200 100 100
    reserved : 50 50 50 50
    maximum : 400 400 400 400

    The 'reserved' is how much you're keeping for that port (read: dedicated), the remaining is returned to the common pool.

    I will have a blog about buffer tuning in the next week or two that will go over tweaking these values.

    Here is the config guide for tuning these values:

    "The switch uses a buffer allocation scheme to reserve a minimum amount of buffers for each egress queue, to prevent any queue or port from consuming all the buffers and depriving other queues, and to decide whether to grant buffer space to a requesting queue. The switch decides whether the target queue has not consumed more buffers than its reserved amount (under-limit), whether it has consumed all of its maximum buffers (over-limit), and whether the common pool is empty (no free buffers) or not empty (free buffers). If the queue is not over-limit, the switch can allocate buffer space from the reserved pool or from the common pool (if it is not empty). If there are no free buffers in the common pool or if the queue is over-limit, the switch drops the frame. "