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

[Lobo]

Thanks for the explanation Brad.  Makes sense to me now.

Jose

Brad Henshaw wrote:
>  
> 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
>