Re: 3560 : Doubt for QOS

Hi Ivan,

Tks for your explaination. Config guide is very helpful. But I found below
paragraph bit tricky .... :)

"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."

Cheerz,
PM

On Sun, Jun 28, 2009 at 3:21 PM, Ivan Walker <ivan_at_itpro.co.nz> wrote:

> 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 !

Blogs and organic groups at http://www.ccie.net
Received on Sun Jun 28 2009 - 15:46:50 ART