[c-nsp] DSCP Trust on 67xx cards

[Phil Mayers]

On 01/05/2011 12:59 PM, Robert Hass wrote:
> I'm a little confused regarding DSCP Trust support at 67xx linecards
> at 6500 platform. Documentation says that COS Trust should be used.
> Same shows me sh int capability. Exception is WS-X6708-10GE card where
> DSCP is also supported.

The important thing to remember is the different between ingress and 
egress queueing.

On most 67xx series cards, ingress queueing can only be done on CoS. 
Internal DSCP can still be derived from "real" DSCP, and mapped to 
egress CoS which is used for egress queueing.

On newer cards (6708, 6716) ingress queueing can be done on DSCP. Final 
egress queueing can also be done directly on internal DSCP rather than 
on the derived CoS, allowing >8 levels of granularity.


>
> Slot 1 - module WS-X6748-GE-TX, Sup720-3B, 6509-E, IOS 12.2(33)SXI5.
>
> #sh int gi1/1 capa | inc QOS
>    QOS scheduling:        rx-(1q8t), tx-(1p3q8t)
>    QOS queueing mode:     rx-(cos), tx-(cos)
>
> But I configured without problem on this port DSCP Trust as below:
>
> # sh run int gi1/1
>
> interface GigabitEthernet1/1
>   ...
>    mls qos trust dscp
> !
>
> Will it work ? Or it just parser mistake and I must configure "mls qos
> trust cos" ?

It will work - the real DSCP will be copied to the internal DSCP and 
honoured for egress QoS. Whether it does what you expect will depend on 
whether you're expecting to do ingress queueing or not.

FWIW, ingress queueing is not of much use on 67xx series linecards on 
this platform since the fabric is not blocking.

I highly recommend googling for "qos srnd" and giving it a careful read. 
It is very illuminating for QoS on the catalyst platforms.