Question

A network engineer is reviewing the QoS policy on a branch router WAN interface. Voice traffic is marked EF, and the following policy-map is applied outbound:

policy-map WAN-EDGE
 class VOICE
  priority percent 20
 class VIDEO
  bandwidth percent 30
 class class-default
  fair-queue

During periods of severe WAN congestion, users report that voice quality is acceptable but a new bulk backup application (which falls into class-default) is being starved of throughput at times, while at other times video streams experience delay. What is the correct interpretation of how this policy handles traffic under congestion?

Accepted Answer

The VOICE class uses a low-latency (priority) queue that is policed to 20% of link bandwidth during congestion, guaranteeing low delay for EF traffic while capping its maximum usage.

Answer

The VOICE class is guaranteed a minimum of 20% bandwidth but can burst without limit, which is why class-default is being starved.

Answer

The VIDEO class receives strict priority over VOICE because bandwidth statements are serviced before priority statements.

Answer

All three classes share bandwidth equally through fair-queue, so the priority and bandwidth percentages are ignored under congestion.