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

Match each Differentiated Services Code Point (DSCP) Per-Hop Behavior (PHB) standard on the left to its corresponding bit/decimal value and network application profile on the right.

  • Expedited Forwarding (EF)DSCP 46 (binary 101110); assigned to strict priority queuing for real-time voice traffic requiring minimal delay and jitter.
  • Assured Forwarding 41 (AF41)DSCP 34 (binary 100010); assigned to high-priority enterprise data applications with low packet drop precedence.
  • Class Selector 6 (CS6)DSCP 48 (binary 110000); reserved for core IP routing protocols and network infrastructure control traffic.
  • Default Forwarding (DF)DSCP 0 (binary 000000); represents unclassified best-effort traffic managed with standard FIFO queuing.

Answer

Expedited Forwarding (EF) matches DSCP 46 (voice/low-latency queue); Assured Forwarding 41 (AF41) matches DSCP 34 (high-priority data with low drop precedence); Class Selector 6 (CS6) matches DSCP 48 (network control traffic); Default Forwarding (DF) matches DSCP 0 (best-effort standard FIFO).
Each DiffServ PHB maps directly to a standardized 6-bit DSCP value: EF maps to DSCP 46 for priority low-latency queues, AF41 maps to DSCP 34 for high-priority low-drop data, CS6 maps to DSCP 48 for network control traffic, and DF maps to DSCP 0 for best-effort traffic.

Step-by-Step Solution

1
Identify the DSCP encoding and purpose of Expedited Forwarding (EF).
EF uses binary 101110101110 (DSCP 46) to provide a guaranteed low-delay, low-jitter expedited path for voice media.
RFC 2598 defines EF for real-time applications using a priority queue.
2
Determine the DSCP binary and decimal values for Assured Forwarding AF41.
For AF class xx and drop precedence yy (AF xyxy), the 6-bit DSCP structure is xxxyy0xxxyy0. For AF41, x=4x=4 (1002100_2) and y=1y=1 (01201_2), giving binary 100010100010 (decimal 34).
AF41 allocates guaranteed bandwidth while maintaining low drop probability under congestion.
3
Map Class Selector 6 (CS6) to its corresponding 6-bit DSCP value.
Class Selector values set the 3 MSBs to match IP Precedence while zeroing the 3 LSBs, yielding binary 110000110000 (DSCP 48).
CS6 is reserved by network equipment for critical control traffic like OSPF and BGP routing updates.
4
Identify the characteristics of Default Forwarding (DF).
DF uses binary 000000000000 (DSCP 0).
DF describes standard best-effort traffic subject to FIFO queuing and tail drop when queues overflow.

Key Concept

DiffServ Per-Hop Behaviors (PHB) and DSCP Marking Standards
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Topics

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

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