DSCP value on 3750 switches - What is d1 and d2?

[Difan Zhao]

Hi experts,

This is maybe a dumb question... However I can't seem to find answer anywhere. For example a dscp-cos map on the switch is like this (auto QoS is used)

   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

I used to think that d1 must be the decimal value of the first 3 bits in the DSCP value and d2 be the decimal value of the next 3 bits. So for example 2:2 will be 010:010, which is AF21 or decimal 18.

However if it's true then d1 and d2 can't exceed 7 since 3 bits of binary can only be decimal 7 at max. But in the map on the switch, seems to me that the d2 can be 9!

Then let's say that d1 and d2 each represent 4 bits in the entire ToS byte... however they should each go to up to 15...

So I guess d1 and d2 are just numbers on tens place and ones place of the decimal value of the entire DSCP field (6 bits), correct? For example AF21 will be 1:8? I think it must be it but I just want somebody to confirm... Thanks!

[cadet alain · Accepted solution]

Hi,

You take d1 and d2 which makes d1d2 and you get the dscp values from 0 to 63 and at the intersection in the table you get the CoS mapping corresponding to this DSCP value.

e.g  DSCP 40 gives a Cos of 5

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

Regards.

Alain.

[Difan Zhao]

Thank you Alain! that's what I thought too!

Do you have a good document which explains what each variable means for the output queue sets? I can't really understand the settings which are applied by the Auto-Qos about the egress queues. I know that it's not recommended to tune those values but I just want to have a fully understanding of the mechanism or even algorithms behind it...

Thanks,

Difan

[cadet alain]

Maybe you can take a look here:  http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_se/configuration/guide/swqos.html

Regards.

alain.