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
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)
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.