CCNP Infrastructure Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  CoS 5, DSCP AF41

  Why it's wrong here

  AF41 (Assured Forwarding 4.1) is an admission-controlled video marking, not a voice marking. Voice traffic requires the strict-priority forwarding behavior of EF (Expedited Forwarding) to guarantee low latency, low jitter, and minimal loss; AF41, by contrast, is a lower-priority class that can be dropped during congestion. Using CoS 5 with DSCP AF41 creates a mismatch where Layer 2 says high priority but Layer 3 treats the traffic as ordinary video, leading to suboptimal queuing and potential voice quality degradation.

• ✗

  CoS 5, DSCP CS3

  Why it's wrong here

  CS3 (Class Selector 3) is a DiffServ marking used for lower-priority traffic such as signaling or multimedia conferencing, not for voice bearer traffic. In the standard QoS model, voice must be marked as EF (DSCP 46) to receive strict-priority treatment; CS3, with a lower numerical DSCP value, gets placed in a lower queue and is subject to congestion-related delays and drops. Combining CoS 5 with DSCP CS3 contradicts the intended high-priority voice classification and causes inconsistent behavior on devices that trust only one marking.

• ✓

  CoS 5, DSCP EF

  Why this is correct

  CoS 5 combined with DSCP EF (Expedited Forwarding, DSCP 46) is the industry-standard marking for voice bearer traffic in a campus network. This dual marking ensures that voice frames are placed in the strict-priority queue at both Layer 2 and Layer 3, providing the low latency, low jitter, and minimal packet loss that real-time audio requires. The EF PHB (Per-Hop Behavior) is designed to guarantee a configured bandwidth and queue service, while CoS 5 aligns with the Cisco-recommended voice VLAN and switch port trust settings, making this the only correct answer.

• ✗

  CoS 4, DSCP EF

  Why it's wrong here

  CoS 4 is conventionally assigned to video conferencing traffic on Cisco switches, placing it in a lower-priority queue to prevent it from starving voice. Although DSCP EF is correct for voice at Layer 3, pairing it with CoS 4 creates a marking conflict: a switch that trusts only CoS will classify this traffic as video, not as strict-priority voice. This mismatch can lead to voice frames being queued with video and experiencing delay or jitter, so the proper pairing for voice is CoS 5 with DSCP EF, not CoS 4.