Re: mls qos map policed-dscp

*if I have interface with just mls qos trust dscp, if port receives packet
with for example DSCP 46, does it remark also COS field when it sends packet
to anoter switch over trunk.*
*NO,* it does NOT, when you enter "mls qos trust dscp", it ONLY trusts the
DSCP value and it has NOTHING to do with CoS. UNLESS YOU change the mapping.
That was your question and this is your answer. If in doubt, please try it,
IOS WILL ALWAYS TELL YOU THE TRUTH.

*Does mls qos map policed-dscp remark DSCP and COS field or just DSCP?*

*NO,* it does NOT touch COS field at all. Unless its configured to do so.

This is without twisting word around.
On Sat, May 2, 2009 at 1:40 PM, Petr Lapukhov
<petr_at_internetworkexpert.com>wrote:

> Hi,
>
> In general, Catalyst switches implement "synchronous" marking logic:
> i.e. all potential
> QoS labels (L3,L2) are kept synchronized using the DSCP-to-CoS,
> CoS-to-DSCP and IP Prec-to-CoS
> global mapping tables. This is pretty natural if you think of
> "uniform" QoS marking model.
> In some cases it is possible to modify just one QoS marking point,
> keeping the other intact.
>
> See below for more details.
>
> > Does mls qos map policed-dscp remark DSCP and COS field or just DSCP?
>
> Yes, CoS is modified according to the DSCP-to-CoS mapping table (in
> both 3550 and 3560).
> You may avoid CoS modification while policing in the 3550 model by
> using the configuration
> similar to the following:
>
> mls qos cos policy-map
> !
> policy-map POLICE_INBOUND
> class ICMP
> trust dscp
> set cos 2
> police 64000 16000 exceed-action policed-dscp-transmit
>
> Here you trust DSCP values and set CoS at the same time. Setting CoS
> directly in policy-map in only
> possible in the 3550 model when enabled by the special global-mode
> command. The 3560 does
> not provide such flexibility, and most of the time it is not needed.
>
> > if I have interface with just mls qos trust dscp, if port receives packet
> > with for example DSCP 46, does it remark also COS field when it sends
> packet
> > to anoter switch over trunk.
>
> Yes, by default CoS is modified according to the DSCP to CoS mapping table.
> In the 3550 model you may avoid this behavior by enabling special CoS
> pass-thru mode:
>
> interface FastEthernet 0/4
> mls qos trust dscp pass-through cos
>
> (there is symmetric DSCP pass-thru mode, configured using the command
> "mls qos trust cos pass-through dscp")
>
> In the 3560 model, it is not possible to disable CoS rewrite. More
> than that, you have to set
> CoS values for non-IP packets by using the "set dscp" command, as there is
> no "set cos" commad. Still, you can disable DSCP rewriting while trusting
> CoS
> (mls qos trust cos) by entering the global command "no mls qos rewrite ip
> dscp"
>
> HTH,
> --
> Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
> petr_at_INE.com
>
> Internetwork Expert, Inc.
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-- 
Narbik Kocharians
CCSI#30832, CCIE# 12410 (R&S, SP, Security)
www.MicronicsTraining.com
www.Net-Workbooks.com
Sr. Technical Instructor
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