[c-nsp] mitigate output drops on Cisco 2960G platform

[Peter Rathlev]

On Fri, 2013-09-20 at 19:56 +0300, Martin T wrote:
> I have three WS-C2960G-24TC-L switches which all have about ten
> servers connected. Each switch has a single 1GigE uplink. Those 1GigE
> uplink ports on all switches experience occasional OutDiscards.
...
> Looks like the only way to modify port ASIC egress buffers on 2960G
> are with MLS(Multi-Layer Switching) QOS. 2960G family seems to have
> four output queues:

You can somewhat mitigate the drops by enabling QoS and adjusting the
taildrop thresholds to as much as possible:

mls qos
mls qos queue-set output 1 threshold 1 3100 3100 100 3200
mls qos queue-set output 1 threshold 2 3100 3100 100 3200
mls qos queue-set output 1 threshold 3 3100 3100 100 3200
mls qos queue-set output 1 threshold 4 3100 3100 100 3200

This is assuming all ports are in queue-set 1 (default) and you're using
a sufficiently new IOS version. Older versions like 12.2(25) can only
use up to 400 for the threshold value instead of 3200. It seems that
12.2(35) and foward uses 3200.

And don't listen to people saying it's because it's a cheap switch. The
flagship 3750 models have exactly the same problems. :-) Do complain to
your SE about it though.

-- 
Peter