[c-nsp] 3750G vs. Nexus for a SAN

[James Slepicka]

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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