Question 65 of 240 from exam 400-351: CCIE Wireless Written Exam
Question
DSCP values can be expressed in decimal form or by PHB.
Which PHB is the equivalent of DSCP 20?
Answers
Explanations
B.
Differentiated Services Code Point (DSCP) is a field in the IP packet header that defines the priority level of the packet. Per-Hop Behavior (PHB) is a set of rules that define how routers and switches handle traffic based on the DSCP value.
There are 64 possible values for DSCP. The first 6 bits of the 8-bit DSCP field define the codepoint, and the last 2 bits are unused. DSCP values can be represented in decimal form or in PHB notation.
AF (Assured Forwarding) is a PHB that defines four classes of traffic, each with a different priority level. AF class values are defined in the range of 1 to 4, and within each class, there are three drop precedence levels (low, medium, and high), denoted by the values 1 to 3.
The AF PHB corresponding to DSCP 20 can be found by looking at the DSCP to PHB mapping table. DSCP 20 falls into the AF21 class, which has a drop precedence of 1. Therefore, the answer is option B, AF22, which represents the same AF class with a drop precedence of 2.
To summarize, DSCP 20 maps to the AF21 PHB, and the equivalent PHB notation is AF22.
