Question

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

Match each Quality of Service (QoS) mechanism or Per-Hop Behavior (PHB) on the left to its corresponding operational characteristic or header specification on the right.

  • Expedited Forwarding (EF)Uses DSCP decimal 46 (binary 010110) to designate low-loss, low-latency, low-jitter traffic typically placed in a Strict Priority Queue.
  • Assured Forwarding 41 (AF41)Uses DSCP decimal 34 (binary 100010) to designate Class 4 traffic with low drop probability.
  • Traffic PolicingMonitors traffic rate against a configured limit and immediately drops or remarks excess traffic without queuing it.
  • Traffic ShapingSmooths outbound traffic bursts by buffering excess packets in memory and releasing them at a steady rate.

Answer

Expedited Forwarding matches DSCP decimal 46 / Strict Priority Queue service; Assured Forwarding 41 matches DSCP decimal 34 / Class 4 low drop probability; Traffic Policing matches immediately dropping or remarking excess traffic without queuing; Traffic Shaping matches buffering excess traffic to smooth bursty egress rates.
Expedited Forwarding (EF) is standard DSCP 46 (010110) intended for real-time traffic requiring low latency/jitter via priority queuing. Assured Forwarding 41 (AF41) corresponds to DSCP 34 (100010) offering Class 4 high-priority bandwidth with a low drop probability. Traffic Policing measures rate limits and drops or remarks exceeding traffic immediately. Traffic Shaping regulates egress rate spikes by holding overflow packets in memory buffers.

Step-by-Step Solution

1
Identify Differentiated Services Code Point (DSCP) values and binary encodings for Per-Hop Behaviors (PHBs).
EF uses DSCP 46 (010110). AF41 uses class 4 (001) with low drop (01) plus trailing zero (100010 = 34).
DiffServ DSCP uses 6 bits in the IPv4 ToS field. EF guarantees strict priority service while AFxy defines class x (1-4) and drop preference y (1-3).
2
Differentiate traffic enforcement mechanisms based on memory buffer utilization.
Policing discards/remarks non-conforming packets without delay; Shaping delays non-conforming packets in egress buffers.
Policers do not support queuing/buffering and operate inbound or outbound, whereas shapers require queue buffers and operate outbound only.

Key Concept

QoS Per-Hop Behaviors (EF/AF) and Rate Enforcement (Policing vs Shaping)
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