DSCP 4 ?

atef yamin · ‎11-25-2011

hi

I am studying for cisco exam and am confused when it comes to this value. I have extensively read about the DSCP values and from my understanding a DSCP value of 4 means 000 100 (ECN). how can this value be even valid ? it doesnt have a class selector value and only drop precedence. following is a copy of config in cisco docs for Catalyst 3750:



Switch(config)# ip access-list 101 permit ip any any
Switch(config)# ipv6 access-list ipv6-any permit ip any any
Switch(config)# class-map cm-1
Switch(config-cmap)# match access-group 101
Switch(config-cmap)# exit
Switch(config)# class-map cm-2
Switch(config-cmap)# match access-group name ipv6-any
Switch(config-cmap)# exit
Switch(config)# policy-map pm1
Switch(config-pmap)# class cm-1
Switch(config-pmap-c)# set dscp 4
Switch(config-pmap-c)# exit
Switch(config-pmap)# class cm-2
Switch(config-pmap-c)# set dscp 6
Switch(config-pmap-c)# exit
Switch(config-pmap)# class class-default
Switch(config-pmap-c)# set dscp 10
Switch(config-pmap-c)# exit
Switch(config-pmap)# exit
Switch(config)# interface G0/1
Switch(config-if)# switch mode access
Switch(config-if)# service-policy input pm1

Can someone please elaborate on this please?

Peter Paluch · ‎11-26-2011

Hello,

 I have extensively read about the DSCP values and from my understanding  a DSCP value of 4 means 000 100 (ECN). how can this value be even valid  ? it doesnt have a class selector value and only drop precedence.

This should not mean ECN, as the ECN flags are outside the 6-bit DCSP field. However, I am also somewhat puzzled by the DSCP values used in your example.

DSCP values 4 and 6 do not correspond to any valid DSCP codepoint. Instead, if we reinterpreted the DSCP field including ECN bits as the former ToS field, setting the DSCP value 4 would toggle the "minimize delay" bit, and the DSCP value 6 would toggle the "minimize delay" and "maximize throughput" bits in the ToS field. The value 10 corresponds to AF11 (in general, the decimal value of AFxy can be computed as 8*x+2*y).

Personally, I believe that the example is just to show how the DSCP values are set, but does not actually use sensible, meaningful DSCP codepoints.

Best regards,

Peter

Alexander Maroukian · ‎11-26-2011

Hi,

I would add to what Peter said that if you are looking at the historical TOS octet there you will see the differentiated services and ECN fields. But they have exact bits allocated and they are not overlapped. Bits 0-5 are used for DS and bits 6,7 are for ECN. If you look at figure.2 in RFC 3168 section 5 you will see how the fields are located.

Link to RFC 3168:

http://tools.ietf.org/html/rfc3168#section-5

I hope this will help you with the study. Please rate helpful posts.

Best regards,

Alex

EDIT: Here is good link on this topic too:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml#back