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?
Correct because on ASR routers, the priority command includes an implicit policer to prevent starvation of other queues; exceeding this policer causes drops.
Why this answer
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.
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.
How to eliminate wrong answers
Option B is wrong because setting the interface bandwidth too low would affect the overall shaping and scheduling, but the priority queue overflow is specifically caused by a policer, not by under-provisioning of the queue itself. Option C is wrong because if the class map were not matching voice traffic, the traffic would not be placed into the priority queue at all, and the 'priority queue overflow' log would not occur. Option D is wrong because WRED (Weighted Random Early Detection) is not applied to priority queues on Cisco ASR routers; priority queues use tail drop or a policer, and WRED is typically used on non-priority queues to manage congestion.