CCNP Enterprise Core (350-401) Cisco Services

DSCP Per-Hop Behaviour: Assured Forwarding

Assured Forwarding will ensure that certain amount of bandwidth is reserved to an assured forwarding class, and will allow access to extra bandwidth if it available on the link.

Packets that will utilise assured forwarding will have a field format of XXXYY0. XXX represents the binary value of the assured forwarding class, which is between 5 and 7. YY represents the drop probability.

AF ClassAF IP PrecedenceAF IP Precence
Binary values of the AF class
Drop Probability DecDrop Probability Bin
1 (Low)01
2 (Medium)10
3 (High)11
Binary values of drop probity

Assured Forwarding has four standard classes from AF1 to AF4. It is important to note that the class number does not determine priority. Each class is treated independently and placed into different queues.

Each AF class has its own IP Precedence with three drop probabilities, low, medium, and high.

The class and drop probability is combined into an AF Name, classed as AF12 – this would be IP Precedence 1 with Drop Probability 2.

It can be converted into a Differentiated Services Code Point value with the formula 8x + 2y. x being the IP Precedence and y being the Drop Probability.

AF Class NameIP Precedence DecimalIP Precedence BinaryDrop ProbabilityDrop Probability BinaryDrop Probability DecimalAF NameDSCP Value BinaryDSCP Value Decimal
Decimal / Binary Table for AF Names

Assured Forwarding must detect and respond to long-term congestion within each class, dropping packets using a congestion-avoidance algorithm such as weighted random early detection.

Weighted random early detection uses the AF Drop Probability value in a class to determine which packets should be dropped first when congestion is experienced. It should be able to handle short term congestion resulting from bursts if each class is placed into separate queues using a dedicated queuing algorithm such as class-based weighted fair queuing (CBWFG).

Assured Forwarding does not specify the use of a particular algorithm but it does specify the requirements of these algorithms.

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