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Difficulty: MediumQoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

Match each Quality of Service (QoS) mechanism, field, or per-hop behavior on the left to its corresponding operational description or header specification on the right.

  • Assured Forwarding (AF31)Uses a 6-bit DSCP value specifying Class 3 traffic with a low drop probability (binary 011010).
  • Class of Service (CoS)Utilizes 3 Priority Code Point (PCP) bits located within the IEEE 802.1Q trunking header.
  • Traffic ShapingBuffers excess egress packets in memory queues to smooth out traffic bursts and match a defined rate.
  • Strict Priority Queuing (PQ)Services delay-sensitive traffic ahead of all other queues, potentially starving non-priority traffic if unpoliced.

Answer

Assured Forwarding (AF31) corresponds to the 6-bit DSCP value specifying Class 3 traffic with low drop probability (011010); Class of Service (CoS) corresponds to the 3-bit PCP field in the IEEE 802.1Q header; Traffic Shaping corresponds to buffering excess egress packets in memory queues to smooth traffic bursts; Strict Priority Queuing corresponds to servicing delay-sensitive traffic ahead of all other queues.
Each term correctly aligns with its exact QoS definition: AF31 represents DSCP value 26 (011010), CoS utilizes the 3-bit 802.1Q header field, Traffic Shaping smooths bursts via packet buffering, and Strict Priority Queuing services high-priority packets ahead of all other queues.

Step-by-Step Solution

1
Identify Layer 3 marking specifications for Per-Hop Behaviors (PHBs).
Assured Forwarding (AF) uses 6 bits in the DSCP field. AF31 breaks down into Class 3 (3×8=243 \times 8 = 24, or binary 011000) with low drop probability (add 2, resulting in decimal 26 or binary 011010).
DSCP AF values use the formula 8x+2y8x + 2y where xx is the class (1-4) and yy is the drop precedence (1-3).
2
Identify Layer 2 marking specifications.
Class of Service (CoS) resides exclusively in the 3-bit Priority Code Point (PCP) field of the 802.1Q VLAN tag header.
CoS is a Layer 2 QoS marking mechanism and does not persist across non-trunked Ethernet hops or Layer 3 boundaries.
3
Differentiate between traffic conditioning mechanisms (shaping vs policing).
Traffic Shaping uses software queues to buffer packets that exceed the configured rate, smoothing egress flow.
Unlike policing (which drops or remarks excess packets), shaping delays out-of-profile traffic by placing it into memory queues.
4
Evaluate queuing algorithms and packet scheduling.
Strict Priority Queuing always services its designated queue first until empty before serving lower-priority queues.
Strict priority scheduling provides minimal latency for real-time traffic (such as voice), but requires policing to prevent queue starvation of other traffic classes.

Key Concept

QoS Classification, Marking, Queuing, and Traffic Conditioning Mechanics
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Topics

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

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