ip precedence and dscp mapping

Unanswered Question
Dec 31st, 2008

Hi every body!

Let say a router configured to support dscp,receives a ip packet with ip precedence=1, the router will use the ip precedence - dscp map to find the corresponding dscp value.But which dscp value would be chosen as there are multiple dscp entries for each ip precedence value. For example

ip precedence dscp

1 AF11 (001 010)

AF12 (001 100)

AF13 (001 110)

So which dscp value would be used?

Thanks a lot and happy new year!

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Jon Marshall Fri, 01/02/2009 - 14:05


IP Precedence 1 = CS1. Output from 3550 switch

SW2#sh mls qos maps ip-prec-dscp

IpPrecedence-dscp map:

ipprec: 0 1 2 3 4 5 6 7


dscp: 0 8 16 24 32 40 48 56

Because there is no space in the IP Precedence field for a drop preference eg. AF11, AF12 , AF14 then IP Prec 1 is CS1.

Is there somewhere you have seen this output from ?

By the way, any reason you are posting these questions here ? - if you want to avoid the Switching/Routing forums and the people there just say and i'll let others answer :-)

Happy New Year to you.


sarahr202 Sat, 01/03/2009 - 10:30

My apology .

I did not notice I posted the questions in wrong forum. Thanks Jon for correcting me. I will keep it in mind next time. The last thing thing I want to do is to avoid switching and routing forum where net pros like you helped me understand the complex topics.

Now back to my question :-)

You mentioned ip prec 1= CS1, what is cs1, ?

Happy new year to as well

Jon Marshall Sat, 01/03/2009 - 12:40

Before DSCP came along only the first 3 bits in the TOS byte in the IP header were used to mark packets. These first 3 bits were called the IP Precedence (IPP) values.

When DSCP came along a further 3 bits from the TOS byte were added to mark packets so now there were six bits in total with which to mark packets. To provide backwards compatability DSCP defines the CS classes which map directly to the old IP Precedence value.

Put simply the first 3 bits of the ToS byte are common to both IPP and DSCP. So CS1 which is a DSCP class simply leaves the extra 3 bits as 0 so you get

IP Precedence 1 = binary 001

DSCP CS1 = binary 001 000

So a device that doesn't understand DSCP can still interpret correctly a packet marked with CS1.


Jon Marshall Sun, 01/04/2009 - 12:07


One additional point to this. As you pointed out

AF11 (001 010)

AF12 (001 100)

AF13 (001 110)

a router that did not understand DSCP but only IP Precedence would also be able to interpret the above and it would interpret them all as IP Precedence 1.



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