Which of the following Quality of Service (QoS) mechanisms and header fields correctly mat… — Cisco CCNA

Which of the following Quality of Service (QoS) mechanisms and header fields correctly match their corresponding binary/decimal representations or operational behaviors? Match each QoS term on the left with its correct description on the right.

Expedited Forwarding (EF) PHBEncoded as DSCP value $46$ ( $101110_2$ ), providing a low-latency, low-jitter service class typically assigned to strict priority queues.
Assured Forwarding 31 (AF31) PHBEncoded as DSCP value $26$ ( $011010_2$ ), representing Class $3$ traffic with a low drop probability during congestion.
IEEE 802.1Q Class of Service (CoS)Utilizes a $3$ -bit Priority Code Point (PCP) field within the Ethernet frame tag to provide $8$ distinct priority levels at Layer 2.
Traffic Policing MechanismEnforces rate limits by immediately dropping or remarking out-of-profile traffic bursts without using queue buffers.

Answer

Expedited Forwarding (EF) maps to DSCP 46 (

101110_2

) for low-latency traffic. Assured Forwarding 31 (AF31) maps to DSCP 26 (

011010_2

) representing Class 3 with low drop probability. IEEE 802.1Q CoS maps to the 3-bit PCP field in the Layer 2 Ethernet tag. Traffic Policing maps to rate-limiting that drops or remarks traffic without queuing delay.

Each QoS term accurately matches its technical bit value or functional mechanism: Expedited Forwarding (EF) is DSCP 46 (

101110_2

), Assured Forwarding AF31 is DSCP 26 (

011010_2

), IEEE 802.1Q CoS uses the 3-bit PCP field, and Traffic Policing limits bandwidth by dropping or remarking excess traffic without buffering.

Step-by-Step Solution

Analyze Expedited Forwarding (EF) DSCP bit structure

EF uses DSCP binary

101110_2

, which converts to decimal

46

. It is designed for strict low-latency priority queuing.

EF is a single standardized Per-Hop Behavior for real-time traffic.

Calculate Assured Forwarding (AF31) DSCP decimal value

For AFxy, Class

x=3

yields binary

011

, Drop precedence

y=1

yields binary

01

, followed by trailing

0

. This forms

011010_2 = 26

AF class and drop precedence bits directly map to their DiffServ field values.

Identify Layer 2 classification header location and field size

Layer 2 CoS relies on the

3

-bit Priority Code Point (PCP) field within the IEEE 802.1Q VLAN header tag.

Ethernet frames lack IP TOS headers, so priority is encoded inside the 802.1Q tag.

Differentiate Traffic Policing mechanics from Traffic Shaping

Policing compares incoming packet rates against CIR and instantly drops or remarks out-of-profile traffic without buffering.

Shaping buffers excess traffic to smooth bursts, whereas policing acts immediately without introducing latency.

Key Concept

QoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

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