[c-nsp] ingress vs egress queues

[Phil Mayers]

On 04/03/11 10:59, Nick Hilliard wrote:
> On 04/03/2011 09:25, Phil Mayers wrote:
>> Genuinely curious to hear real-world use-cases for the ingress
>> queueing on
>> 6500/7600.
>
> Ingress queueing is used extensively for cut-thru fabrics.

Interesting, but not really what I had in mind - 6500/7600 are not cut-thru.

Now I'm curious though - how on earth can you queue on a cut-thru 
switch? If you queue, you're storing the packet and are no longer cut-thru?

Do you have a pointer to any docs on this?

I'm guessing that even a cut-thru switch has to receive "enough" of the 
header to switch/route the packet, and apply whatever QoS classification 
so it's sort of a "store & forward lite"; then once the output port is 
"known", if a packet is not currently being transmitted you jump to 
cut-thru mode. If a packet is being transmitted you have to queue, but 
how is that different to egress queueing?

Interesting...