Re: mls qos map policed-dscp

Narbik,

What do you mean "unless you change the mapping." There are default maps
that will make the COS when you trust DSCP. I have tried it! :-)

Bryan Bartik
CCIE #23707, CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com

On Sat, May 2, 2009 at 8:06 PM, Narbik Kocharians <narbikk_at_gmail.com> wrote:

> *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.
> > http://www.INE.com <http://www.ine.com/>
> > Toll Free: 877-224-8987
> > Outside US: 775-826-4344
> >
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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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