350-401 Practice Question: A network team must design a QoS policy for a WAN…

Answer analysis

Option-by-option breakdown

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

• ✗

  Class-based weighted fair queuing (CBWFQ).

  Why it's wrong here

  CBWFQ is a class-based scheduling mechanism that allocates guaranteed bandwidth to each configured class using weighted fair queueing. However, it does not provide a strict-priority queue, so voice packets are treated like any other class and can experience queuing delay or be dropped when the interface is congested. While it ensures minimum bandwidth for voice, it cannot enforce the strict latency and zero-drop guarantee that real-time traffic requires.

• ✓

  Low-latency queuing (LLQ).

  Why this is correct

  LLQ combines class-based weighted fair queueing with a strict priority queue, allowing voice traffic to be placed in a dedicated priority queue that is served before all other queues. This guarantees that voice packets are always served first, providing low latency and eliminating drops for voice traffic even during congestion. To prevent the priority queue from starving other classes, LLQ typically applies a policer to limit the amount of traffic that can use the priority queue.

• ✗

  Weighted random early detection (WRED).

  Why it's wrong here

  Weighted random early detection (WRED) drops packets probabilistically before congestion occurs, which directly violates the requirement that voice traffic must never be dropped. WRED is tempting because it is designed to manage congestion by selectively discarding lower-priority packets, and it would be correct for a data-only class where controlled drops are acceptable to avoid tail-drop. However, for a loss-sensitive voice class, a strict priority queue (LLQ) is required to guarantee zero drops.

• ✗

  First-in, first-out (FIFO) queuing.

  Why it's wrong here

  FIFO is the simplest queuing mechanism, where packets are transmitted in the exact order they arrive with no traffic classification or priority treatment. Under FIFO, voice traffic is not distinguished from any other traffic, so if the queue fills up due to bursty data traffic, voice packets will suffer the same tail-drop as other packets. This makes it completely unsuitable for real-time services that demand low latency and zero packet loss.