[c-nsp] iSCSI, port buffers, and small switches

[Nick Hilliard]

On 26/07/2011 21:47, Geert Nijs wrote:
> On tip i have is also the following: although the C3750E switch is a 'shared
> bufffer switch', each ASIC does have memory on-board. Normally 4 GE ports
> are using 1 ASIC.
> (you can see the mapping with "sh platform pm port-asic" or something like
> that).

I spent a long time looking one day, and eventually found the following URL:

> http://www.cisco.com/en/US/docs/solutions/Enterprise/Video/tpqoscampus.html#wp1062179

"Additionally, these platforms provide (minimally) 750 KB of receive 
buffers and (up to) 2 MB of transmit buffers for each set of 4 ports."

So, this indicates that there is a shared buffer pool per ASIC with <= 2M 
transmit buffers for egress traffic.  As this is a store-n-forward switch, 
this is really very little.  And as pointed out before, if you enable "mls 
qos" the default setting is to carve this up into 4, leaving you with a 
default of <= 512k for class-default per 4 ports.  Little wonder people 
drop packets on these boxes!

It would help if Cisco were more forthcoming about their switch buffer 
sizes.  Unfortunately, the received wisdom is: "if you have to ask, the 
buffers are too small for your requirements".

Other vendors (Juniper, Brocade, etc) are much more up-front about their 
kit's capabilities in this regard.

Nick