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Saturday, January 28, 2012

AF Drop Level

RFC 2597 leavingcisco.com defines the assured forwarding (AF) PHB and describes it as a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. The Assured Forwarding PHB guarantees a certain amount of bandwidth to an AF class and allows access to extra bandwidth, if available. There are four AF classes, AF1x through AF4x. Within each class, there are three drop probabilities. Depending on a given network's policy, packets can be selected for a PHB based on required throughput, delay, jitter, loss or according to priority of access to network services.
Classes 1 to 4 are referred to as AF classes. The following table illustrates the DSCP coding for specifying the AF class with the probability. Bits DS5, DS4 and DS3 define the class; bits DS2 and DS1 specify the drop probability; bit DS0 is always zero.
Drop Class 1 Class 2 Class 3 Class 4
Low 001010
AF11
DSCP 10
010010
AF21
DSCP 18
011010
AF31
DSCP 26
100010
AF41
DSCP 34
Medium 001100
AF12
DSCP 12
010100
AF 22
DSCP 20
011100
AF32
DSCP 28
100100
AF42
DSCP 36
High 001110
AF13
DSCP 14
010110
AF23
DSCP 22
011110
AF33
DSCP 30
100110
AF43
DSCP 38 

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