Re: 3560 : Doubt for QOS

Hi Pradeep

There is a good explanation here
http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_50_se/configuration/guide/swqos.html#wp1331378

"If DSCP transparency is enabled by using the no mls qos rewrite ip dscp
command, the switch does not modify the DSCP field in the incoming
packet, and the DSCP field in the outgoing packet is the same as that in
the incoming packet."

"Regardless of the DSCP transparency configuration, the switch modifies
the internal DSCP value of the packet, which the switch uses to generate
a class of service (CoS) value that represents the priority of the
traffic. The switch also uses the internal DSCP value to select an
egress queue and threshold."

And here (for 3750ME but likely to still apply)
http://www.ciscosystems.co.nz/en/US/docs/switches/metro/catalyst3750m/software/release/12.1_14_ax/release/notes/OL464603.html#wp64894

"Even though the QoS configuration, including the DSCP input and output
queue threshold maps, affects the internal DSCP value of the packet, the
DSCP value of the packet at the ingress and the egress is the same. "

"If you enable the DSCP transparency feature, the switch does not change
(rewrite) the DSCP field of an IP packet at the ingress and egress
interfaces. During QoS processing, the switch modifies the CoS value of
the packet based on the internal DSCP value and DSCP-to-CoS map"

I take this all to mean that if you are trusting CoS the internal DSCP
value will be based on the COS-DSCP map but the packet will leave the
switch with the original DSCP value but with a CoS value derived from
the internal DSCP value. This could be labbed up to test to be sure.

Ivan

Pradeep Malik wrote:
> 3560 :Doubt for QOS
>
>
> Let's say if following is config :
>
> mls qos
>
> int f0/0
> mls qos trust cos
> no mls qos rewrite ip dscp
> ......
> end
>
>
>
> My question is that will switch change the dscp of the incoming packets
> depending on COS-DSCP map ??
>
>
> Any suggestions are welcome.
>
> Tks !

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Received on Sun Jun 28 2009 - 21:51:42 ART