Re: 3650 COS/DSCP to queue mapping

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
>>
>>
>>Blogs and organic groups at http://www.ccie.net
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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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