general QOS question

[Amit Shah]

Hi

Qos is confusing the heck out of me. From the attached word doc can someone explain why we should mark packets as CS1 and af11. From what i understand they are one and the same at layer 3. This whole config is from the QOS SRND 3.3

[Jon Marshall · Accepted solution]

Amit

They are not quite the same.

CS1 in binary = 001000

AF11 in binary = 001010

If a device that can only interpret the first 3 bits ie. the IP Precedence looks at this marking then yes it will be the same. But a device that can interpret the full DSCP value will understand that they are different with AF11 having a "Low Drop Probability" specified in the last 3 bits.

Jon

[francisco_1 · Accepted solution]

Jon,

I was trying to understand the DSCP/IP Precedence values the other day.My understanding without confusing anyone is in the 6bits on TOS, the first 3 bits used by ip precedence is the major class 0-7. The higher the value the better the traffic is treated. For example C5 is Expidite fowarding given higest priority. DSCP uses all 5 bits 0-63 (sixth is not used, always 0!!). starting with AF11,, the lower values are given better priority instead for example AF11 is better that AF13 even though AF11,AF12,AF13 = CS1. AF11 has better priority than AF12/13 and will not be dropped. Because IP Precednece/DSCP are backward compatable, AF11 and CS1 is treated the same by Qos.

DSCP to IP Precedence Markings

af11 Match packets with AF11 dscp (001010) - IP Precedence 1 (CS1)

af12 Match packets with AF12 dscp (001100)

af13 Match packets with AF13 dscp (001110)

af21 Match packets with AF21 dscp (010010) - IP Precedence 2 (CS2)

af22 Match packets with AF22 dscp (010100)

af23 Match packets with AF23 dscp (010110)

af31 Match packets with AF31 dscp (011010) - IP Precedence 3 (CS3)

af32 Match packets with AF32 dscp (011100)

af33 Match packets with AF33 dscp (011110)

af41 Match packets with AF41 dscp (100010) - IP Precedence 4 (CS4)

af42 Match packets with AF42 dscp (100100)

af43 Match packets with AF43 dscp (100110)

EF = - IP Precedence 5 (CS5)

[Jon Marshall]

Franco

I think you have explained it better than i did. Yes DSCP markings are meant to be backwards compatible with IP Precedence markings.

So AF1x is backwards compatible with CS1 but what i was trying to explain is that if the device interpreting the markings is capable of using full DSCP markings you have the additional option of being able to express the drop probability by either marking it AF11, AF12 or AF13. You can't do this with packets marked as CS1 because there is no additional field. So at layer 3 they are not exactly the same - AF11 packets are viewed in relation to AF12 and AF13 packets as well as all the other classes whereas CS1 packets are viewed in relation to the other classes only.

Does this make sense ?

Jon

[francisco_1]

yes it does.

Thanks Jon.

Francisco

[Amit Shah]

thank you guys

[Joseph W. Doherty]

Jon's posts provide a very nice summary, but to fully understand the difference between CS1 and AF11, it helps to read the source RFCs that defined them. The DSCP Class Selector (CSx) is, I believe, first defined in RFC 2474. The DSCP Assured Forwarding (AFxy) Group is, I believe, first defined in RFC 2597.

Recommendation for DSCP traffic marking, can be found in RFC 4594.