Question 200 of 1,420
LLQ for Voice Traffic: QoS Queuing Mechanism | Cisco ENCOR 350-401
A network administrator is deploying a new QoS policy to prioritize voice traffic across a WAN link. The policy must ensure that voice packets are not dropped even during congestion, and that bandwidth is guaranteed for voice. Which queuing mechanism should be used for the voice class?
Quick Answer
The correct answer is Low Latency Queuing (LLQ). LLQ is the right QoS queuing mechanism for voice traffic because it combines a strict priority queue with Class-Based Weighted Fair Queuing (CBWFQ), ensuring that voice packets are serviced before all other traffic to guarantee low latency and jitter, while also policing the priority queue to a configured bandwidth limit so that voice is never starved or dropped during congestion. On the ENCOR 350-401 exam, this question tests your understanding of how to meet both latency and bandwidth guarantees for real-time traffic; a common trap is choosing plain Priority Queuing (PQ), which lacks bandwidth policing and can starve other classes. Remember the mnemonic: “Voice needs Low Latency, so LLQ is the only way—PQ without the police will cause a traffic cease.”
⚠ Common exam trap
Cisco often tests the distinction between CBWFQ and LLQ, where candidates mistakenly choose CBWFQ because it offers bandwidth guarantees, but fail to recognize that only LLQ provides the strict priority queuing required for real-time voice traffic to avoid drops and delay.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Low Latency Queuing (LLQ)
LLQ is the correct choice because it combines strict priority queuing with CBWFQ, ensuring that voice traffic is placed into a strict priority queue that is serviced before any other queues. This guarantees low latency and prevents voice packet drops during congestion by allowing the priority queue to be policed to a configured bandwidth limit, while still providing bandwidth guarantees for the voice class.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Weighted Random Early Detection (WRED)
Why it's wrong here
WRED is a drop policy, not a queuing mechanism.
- ✓
Low Latency Queuing (LLQ)
Why this is correct
LLQ provides strict priority queuing for real-time traffic like voice.
- ✗
Class-Based Weighted Fair Queuing (CBWFQ)
Why it's wrong here
CBWFQ does not offer strict priority for voice.
- ✗
First-In, First-Out (FIFO) queuing
Why it's wrong here
FIFO does not prioritize any traffic.
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Same concept, more angles
1 more way this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network administrator is troubleshooting a performance issue in a large enterprise campus network. The network consists of Cisco Catalyst 9300 switches acting as access switches and Cisco Catalyst 9500 switches as distribution. Users on VLAN 10 report intermittent slow file transfers to a server on VLAN 20. The administrator has verified that there are no errors on the links, CPU utilization is normal, and STP topology is stable. The administrator suspects a possible QoS issue. Upon checking the QoS configuration on the access switch, the administrator finds that the default QoS configuration is in place, which trusts the CoS value at the port level. The connected devices are IP phones and PCs; the IP phones mark voice traffic with CoS 5. The server on VLAN 20 is connected to a distribution switch. Which action should the administrator take to most likely resolve the issue?
easy- A.Apply a policy map that polices voice traffic to 128 kbps to free bandwidth for data.
- B.Disable QoS entirely on all switches to eliminate any potential QoS-related drops.
- ✓ C.Configure auto QoS for VoIP on the access ports to ensure proper classification and queuing.
- D.Configure trust DSCP on the access ports to prioritize all traffic based on DSCP values.
Why C: Auto QoS for VoIP automatically configures the necessary class maps, policy maps, and trust settings to properly classify and queue voice traffic (CoS 5) while ensuring data traffic is not starved. The default QoS configuration trusts CoS at the port level, but without proper queuing and scheduling, voice and data may compete for buffers, causing intermittent slow file transfers. Auto QoS sets up strict priority queuing for voice and allocates bandwidth for data, resolving the performance issue without manual misconfiguration.
Last reviewed: Jun 11, 2026
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