Question 376 of 1,958
hardMultiple ChoiceObjective-mapped
Priority Queue Default Policer: Voice Traffic Drops Explained
An engineer is troubleshooting QoS on a Cisco ASR 1002 router. The router is configured with a policy map that includes a class for voice with a priority command. During congestion, the engineer notices that voice traffic is being dropped even though the priority queue is not congested. The router logs show 'QoS: priority queue overflow'. What is the most likely cause?
Quick Answer
The answer is that the priority queue has a default policer that drops voice traffic exceeding a certain rate. This is because on Cisco ASR 1000 series routers, the priority command in a QoS policy map automatically enables a hardware-based policer that limits the bandwidth for priority traffic, typically to a default value like 75% of the interface bandwidth. Even when the priority queue itself is not congested, if the voice traffic rate surpasses this policer, the router drops the excess packets and logs the "priority queue overflow" message. On the ENCOR 350-401 exam, this tests your understanding that priority queuing is not simply a strict priority queue; it includes an implicit policer to prevent starvation of other queues. A common trap is assuming drops only happen when the queue is full, but the policer acts independently. Remember the memory tip: "Priority is policed, not just queued"—the default policer is the silent drop culprit.
⚠ Common exam trap
Cisco often tests the misconception that a priority queue never drops traffic unless the queue is full, but the trap here is that the 'priority' command on ASR routers includes a default policer that drops traffic exceeding a rate limit, even when the queue has available buffer space.
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
✓
The priority queue has a default policer that drops traffic exceeding a certain rate
The 'priority queue overflow' log message indicates that the priority queue itself is dropping packets due to exceeding its configured or default policer rate. On Cisco ASR 1000 series routers, a priority class without an explicit 'police' command still has a default policer that limits the priority queue to a percentage of the interface bandwidth (often 33% by default). When voice traffic exceeds this implicit rate, packets are dropped even if the queue depth is not full, because the policer enforces a strict rate limit.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The priority queue has a default policer that drops traffic exceeding a certain rate
Why this is correct
Correct because on ASR routers, the priority command includes an implicit policer to prevent starvation of other queues; exceeding this policer causes drops.
- ✗
The interface bandwidth is set too low, causing the priority queue to be under-provisioned
Why it's wrong here
Incorrect because the priority queue is not bandwidth-constrained; the policer is independent of interface bandwidth.
- ✗
The class map is not matching the voice traffic correctly
Why it's wrong here
Incorrect because if the class map did not match, the traffic would not enter the priority queue at all.
- ✗
The router is using WRED on the priority queue
Why it's wrong here
Incorrect because WRED is not applied to priority queues; it is used for non-priority queues to manage congestion.
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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 engineer is troubleshooting QoS on a Cisco Nexus 9000 switch. The switch is configured with a policy map that uses a class-default with a bandwidth remaining percent of 100. However, during congestion, traffic in a priority queue (class-map for EF) is experiencing drops even though the priority queue is not fully utilized. What is the most likely cause?
hard- ✓ A.The priority queue is implicitly policed to a default rate on Nexus switches
- B.The class-default bandwidth remaining percent should be set to 0
- C.The priority queue is not configured with a queue-limit
- D.The switch is using strict priority queuing without any shaping
Why A: On Cisco Nexus 9000 switches, a priority queue (class-map for EF) is implicitly policed to a default rate of 1 Gbps (or the interface speed, whichever is lower) when no explicit policer is configured. This implicit policing can cause drops in the priority queue even if the queue itself is not fully utilized, because the policer rate limits the traffic before it enters the queue. The class-default bandwidth remaining percent of 100 is unrelated to this issue, as it only affects non-priority queues during congestion.
Last reviewed: Jul 4, 2026
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