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

Phil Mayers p.mayers at imperial.ac.uk
Fri Aug 17 10:36:29 EDT 2012

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.

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