[c-nsp] Cat3560E - insufficient buffers for microbursts?

[Marian Ďurkovič]

On Mon, Mar 02, 2009 at 10:41:26AM +0100, sthaug at nethelp.no wrote:
> >   Closer investigation showed, that for some reason, Cat3560E is doing
> > significantly less output buffering than all other Cisco access switches.
> > While Cat2960G, Cat3550, Cat3560G and Cat3750G happily buffer any
> > microburst upto 100 full size (1500 byte) packets, Cat3560E only buffers
> > upto 64 full size packets and starts dropping beginning with 65-th packet.
> > This was rather bad surprise, since the Cat3560E is supposed to have
> > 10GE uplinks - and with 10:1 or 100:1 speed downshift from uplink into
> > access ports decent buffering is vital for proper operation.
> 
> We experienced the same problem with ME3400 switches. It has extremely
> small buffers by default - the queue size is in units of 256 byte, and
> default queue size is only 48 of these. We got around this by configuring
> shaping (which is possible on the ME3400) and setting queue-limit 256 in
> the policy-map. The problem seems to be resolved in 12.2(46), note the
> following point from the release notes:
> 
> > CSCsm80634 
> >
> > The default queue limit for QoS has been changed from 48 to 160
> > (256-byte) packets.
> 
> I wouldn't be surprised if 3560E has similar behavior (but I haven't
> checked).

Yes, looks like it's indeed wrong IOS defaults. More buffering could be
achieved on 3560-E by enabling qos, and increasing the thresholds:

mls qos
mls qos queue-set output 1 threshold 2 1400 1400 100 1400

This restores the ability to buffer 100 full-size packets on 3560-E.

Anyway, there doesn't seem to be any CLI knob to tune this when qos is 
disabled and the default buffer size (64 full-size packets) is too small.
We do not want to enable qos, as it most probably reserves some packet
memory for unused queues and lowers total available buffer space.


    With kind regards,

        M.