What is Weighted Fair Queue?

Understanding Weighted Fair Queue

Weighted Fair Queue (WFQ) is a packet-scheduling algorithm that divides a link's bandwidth fairly among active traffic flows while letting some flows receive more service based on assigned weights. It prevents a few high-volume flows from starving smaller, latency-sensitive ones, supporting Quality of Service (QoS) on congested links.

WFQ classifies packets into flows using attributes such as source/destination address, port, and protocol, then places each flow in its own queue. A scheduler computes a finish time for each packet using its size and the flow's weight, and dispatches packets in order of those finish times. On Cisco IOS, weight is derived from the IP Precedence or DSCP value, so higher-priority markings get proportionally more bandwidth. Low-bandwidth interactive flows are serviced ahead of large bulk transfers, reducing their delay.

For security and operations, WFQ matters because it protects availability - one pillar of the CIA triad - during congestion. Without fair queuing, a single heavy flow (a large backup, or traffic from a misbehaving or compromised host) can monopolize a link and effectively deny service to critical applications like VoIP signaling or management sessions. Combined with policing and marking, weighted queuing helps ensure that security telemetry, control-plane traffic, and real-time communications survive periods of saturation.

For example, a branch router has a saturated WAN uplink carrying a bulk file sync alongside VoIP calls. With WFQ, voice packets - marked with higher precedence and thus higher weight - are scheduled with minimal delay, keeping calls clear, while the file transfer consumes the remaining capacity. If the bulk flow surges, WFQ still guarantees the voice flows their fair share rather than letting the transfer crowd them out and drop calls.