[c-nsp] 3560 mtu miss-match causing output drops

[Peter Rathlev]

On Sun, 2010-03-14 at 06:41 +1000, mb at adv.gcomm.com.au wrote:
> Yes - they are outdiscards:
[...]
> The drops only occur when there is egress data - Minimal traffic(i.e
> 10Mb/sec) I see counters increase

The OutDiscards appear when the traffic is too bursty. Since the egress
buffers on the platform are terribly small, "bursty" is this case might
not need to be very bursty at all.

> > If the ISP link is 100 Mbps, the other links are 1 Gbps and traffic
> > is primarily in their direction you might've run into the much
> > discussed-about buffering on the 3560 platform.
> 
> Yes - As stated above, it is only egress traffic that appears to cause
> the drops - but we start to see the drops with only minimal traffic
> running over the link?

I've never found out exactly how large the egress buffers are, but I've
heard it's around 64 packets (* 1500 bytes ~ 100 KB). I that case a
sustained gigabit feed to a 100 Mbps port (i.e. 900 Mbps excess) can be
buffered for ~ 1 ms before the switch starts to drop. (Unlees my quick
calculations are wrong of course.)

-- 
Peter