[c-nsp] 3750 switch dropping packets when trust dscp enabled

[Brad Henshaw]

Lobo wrote:

> I setup a traffic generator to send 95Mbps of traffic with DSCP EF
> (46) across the different switches but when it hit the 3750, the
> egress traffic was only ~4Mbps.

> After reading up a bit, I found a command "srr-queue bandwidth shape" 
> that I could apply to the interfaces.  After adding that command with
> all 0s for the queues I was then able to receive all 95Mbps of
traffic.  
> I noticed that the default values for that command are 25 0 0 0.

> Is this something that I'm supposed to do if I just want to trust the
> DSCP markings and not overwrite them on the 3750s?

This isn't a marking issue, it's a shaping issue. As you discovered, 
Queue 1 (to which DSCP 46 is mapped) is shaped to 1/25th of the port
capacity which is 4Mbps. This was causing your traffic to be
rate-limited
to 4Mbps (not remarked).

The shape command you entered effectively disables shaping and all
queues
will operate in shared mode.

You might want to think about the implications of permitting an
uncontrolled quantity of DSCP 46 traffic into your network, either now
or
at a later date.

Regards,
Brad