You can read about the architecture here:
http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/white_paper_c11-462176.html
I'll give you my understanding of it -- I appreciate any corrections if
I miss the mark on something.
>>I don't know whether the packets are buffered on input or on output.
Both. Each port has a set of 416 virtual output queues (on the 5020 --
don't know if this is true for the 5010 or if there are half as many). A
VOQ is, essentially, a queue for each egress port. In other words, on
ingress, a packet is put into one of 416 queues (52 egress ports * 8
queues -- one for each 802.1p CoS). Congestion on one egress port
doesn't impact traffic destined for other ports.
Internally, the packets are moved around at greater than 10Gb speed
(+20%), so there is egress buffering as well. This allows multiple
packets to be queued up and sent out at 10Gb rate without interruption
and is also used for flow control buffering.
>>per-port buffers...quite a bit smaller than on other products
480KB per port shared between per-CoS ingress and egress buffers. Most
are assigned to ingress, but I don't know the ratio.
There is also buffering on the fabric itself, though I'm not entirely
sure what its impact is in this scenario (I think it's primarily just
used as an optimization to increase throughput).
James
Nick Hilliard wrote:
On 06/11/2009 21:35, Gert Doering wrote:
Out of curiosity: how does it cut-through if it has to multiplex
multiple
ports, as in: packets coming in on port A and B and leaving on C? As
soon as two packets overlap (time-wise) on A and B, you can't do
cut-through...
The switch has per-port buffers; from what i remember, quite a bit
smaller than on other products, as the unit is cut-through. You also
need these buffers when you're operating 1G ports in store-n-forward
mode. I don't know whether the packets are buffered on input or on
output.
Nick
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