[c-nsp] DSCP Trust on 67xx cards

[Peter Rathlev]

On Wed, 2011-01-05 at 13:59 +0100, 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.
> 
> 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)

This is CoS *scheduling*, i.e. in what queue do I place a packet.

> But I configured without problem on this port DSCP Trust as below:
> 
> # sh run int gi1/1
> 
> interface GigabitEthernet1/1
>  ...
>   mls qos trust dscp
> !

This is DSCP *trust*.

> Will it work ? Or it just parser mistake and I must configure "mls qos
> trust cos" ?

You can trust DSCP fine. When packets arrive on the interface, the
switch ignores the CoS value (in the p-bits in the 802.1Q header) and
uses the DSCP value from the IP header.

If you trust CoS instead, the switch ignores the DSCP value and
translates the CoS value (via cos-dscp-map) to an internal DSCP value
for further decisions.

When using DSCP trust and CoS scheduling, the dscp-cos-map decides what
queue packets end up using.

In short: The "trust" defines classification (inbound) and marking
(outbound), the "scheduling" defines scheduling/queueing.

I hope I'm not too unclear. :-)

-- 
Peter