[c-nsp] Increasing hold-queue to alleviate microbursts with small hardware queues

John Neiberger jneiberger at gmail.com
Fri Aug 17 10:41:19 EDT 2012

On Fri, Aug 17, 2012 at 8:36 AM, Phil Mayers <p.mayers at imperial.ac.uk> wrote:
> On 17/08/12 15:29, John Neiberger wrote:
>>  > Can you be more specific here? >Where would you shape?
>> I was wondering if an outbound shaping policy on the 1g links would
>> smooth out the peaks of those bursts prior to them hitting the small
>> hardware queue. I'm just kind of thinking out loud.
> Not really. Shaping happens at the output of the queue, so it can't help you
> with the queue overflowing.
> On the 6500, packets are
>  1. received
>  2. put into an ingress queue
>  3. forwarding lookup takes place
>  4. packets are moved across the fabric into an egress queue
>  5. egress queueing takes place
> Steps 1-3 don't really "block" because the fabric is non-blocking. So the
> ingress queueing is largely useless on this platform. So, your only room for
> dealing with output speed/load mismatches is in the egress queue.
> You'd need to ingress shape (which is not possible) or egress shape on the
> upstream box. And shaping is not widely available.
> TBH, you either need a box with much bigger buffers, or move to a 10g port.

Thanks. That's really what I figured, but I was hoping to find some
creative way to alleviate the issue since in many cases we can't
upgrade the link speed or hardware, at least in the short term. That's
definitely going to be my recommendation in the long term.

Thanks again!

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