Re: 3650 COS/DSCP to queue mapping

joe do you have a blog ? i would love to see if you have covered Qos in it
:-

On Mon, Sep 3, 2012 at 4:25 PM, Matt Eason <matt.d.eason_at_gmail.com> wrote:

> Hi Guys,
>
> Thanks for taking the time to reply to my email, really appreciate it. I
> will delve into the labs and see what I find.
>
> What you both said makes sense.
>
> Joe, with all the mappings things could get pretty crazy :)
>
> Matt
>
> On Fri, Aug 31, 2012 at 12:34 AM, Joe Astorino <joeastorino1982_at_gmail.com
> >wrote:
>
> > Hi Matt,
> >
> > Anthony answered your question simply and correctly, but I just wanted
> > to add some things that helped me understand this. Like Anthony said,
> > whatever you trust is basically how the switch determines the queue at
> > a high level, but at a deeper level there are a few different mappings
> > going on. Let's assume you trust CoS. You would have:
> >
> > - CoS to DSCP mapping INTERNAL to the switch
> > - DSCP to CoS mapping INTERNAL to the switch
> > - CoS to output queue mapping
> >
> > The point I am making is that even though a frame comes in with a
> > particular CoS value, that value COULD change internally based on the
> > internal COS-DSCP and DSCP-COS, and the frame COULD be queued based on
> > the value derived from the internal mappings and not on the original
> > value. Let's look at some example output for a second
> >
> > Here are some mapping tables for cos-dscp, dscp-cos and cos-output-q
> > on a 3750 switch. Note these are probably not default values because
> > this is a production switch.
> >
> > Here is the COS to DSCP mapping:
> >
> > switch#sh mls qos maps cos-dscp
> > Cos-dscp map:
> > cos: 0 1 2 3 4 5 6 7
> > --------------------------------
> > dscp: 0 8 16 24 32 40 48 56
> >
> > Here is the DSCP to CoS mapping
> >
> > switch#sh mls qos map dscp-cos
> > Dscp-cos map:
> > d1 : d2 0 1 2 3 4 5 6 7 8 9
> > ---------------------------------------
> > 0 : 00 00 00 00 00 00 00 00 01 01
> > 1 : 01 01 01 01 01 01 02 02 02 02
> > 2 : 02 02 02 02 03 03 03 03 03 03
> > 3 : 03 03 04 04 04 04 04 04 04 04
> > 4 : 05 05 05 05 05 05 05 05 06 06
> > 5 : 06 06 06 06 06 06 07 07 07 07
> > 6 : 07 07 07 07
> >
> > Finally, here is the CoS to output queue mapping
> >
> > switch#sh mls qos map cos-output-q
> > Cos-outputq-threshold map:
> > cos: 0 1 2 3 4 5 6 7
> > ------------------------------------
> > queue-threshold: 2-1 2-1 3-1 3-1 4-1 1-1 4-1 4-1
> >
> >
> > Let's just look at CoS 3 for example. We see that CoS 3 is mapped to
> > DSCP 24. In turn DSCP 24 is mapped right back to CoS 3 in the DSCP to
> > COS mapping. In turn, CoS 3 is put into output queue 3, threshold 1.
> > Fine. So in this case, it comes in as CoS 3 and is queued based on
> > CoS 3 because we trust CoS and because the DSCP-COS mapping is sort of
> > "synced". But...what if you went in and mucked with the DSCP-COS
> > mapping internally such that DSCP 24 was no longer mapped back to CoS
> > 3? What if it was re-mapped to CoS 5 ?
> >
> > So you COULD have the frame come in as CoS 3 ...internally we go CoS 3
> > --> DSCP 24, then DSCP 24 to CoS 5 then queued based on CoS 5
> >
> > These are intricate details, but when you are studying for the lab, I
> > think it is important to get to the dirty details! Best of luck and I
> > hope this helps you out.
> >
> >
> >
> > On Thu, Aug 30, 2012 at 10:45 AM, Anthony Sequeira
> > <terry.francona_at_gmail.com> wrote:
> > > Hi Matt!
> > >
> > > What an AWESOME question. While the documentation does not make it
> clear,
> > > the value that you trust on ingress, in your example, CoS, is the
> marking
> > > that is used in the appropriate default queue mapping table on the
> egress
> > > port.
> > >
> > > Anthony Sequeira, CCIE, CCSI, VCP
> > > http://www.stormwind.com
> > > Twitter: @compsolv
> > > Facebook: http://www.facebook.com/compsolv
> > >
> > >
> > >
> > > On 8/29/12 9:11 PM, "Matt Eason" <matt.d.eason_at_gmail.com> wrote:
> > >
> > >>Hi Guys,
> > >>
> > >>
> > >>
> > >>Can you help clarify the following. If I have a switchport configured
> on
> > a
> > >>3560 to trust CoS inbound, that cos value is then mapped to an internal
> > >>DSCP value via the COS>DSCP map. That s fine.
> > >>
> > >>
> > >>
> > >>Does this switch then determine the output queue from the original CoS
> > >>value or the internal DSCP value which was assigned by the switch? I
> see
> > >>both a DSCP>Output queue map and a COS>Output queue map exists.
> > >>
> > >>
> > >>
> > >>Thanks,
> > >>
> > >>
> > >>
> > >>Matt
> > >>
> > >>
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> > >>
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> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> > --
> > Regards,
> >
> > Joe Astorino
> > CCIE #24347
> > http://astorinonetworks.com
> >
> > "He not busy being born is busy dying" - Dylan
>
>
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Received on Mon Sep 03 2012 - 19:50:54 ART