Re: 3650 COS/DSCP to queue mapping

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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> >
> >
> >
>
>
>
> --
> 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 - 21:25:21 ART