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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Weighted tail drop (WTD) is a queue-management mechanism that gives traffic classifications different drop thresholds within a queue. As the queue fills, traffic assigned a lower threshold is dropped sooner; traffic with a higher threshold can continue to enter. WTD controls which arriving frames are admitted to the queue—not which queued frames are transmitted first.
How weighted tail drop works
In Cisco’s documented Catalyst 9500 implementation, a frame’s QoS label selects a threshold for its destination egress queue. The switch checks whether adding the frame would exceed that threshold. If it would, the frame is dropped, even if traffic assigned a higher threshold can still use space in the same queue. Cisco describes WTD as a way to manage queue lengths and provide drop precedence for different traffic classifications (Cisco IOS XE Everest 16.8.x Catalyst 9500 QoS Configuration Guide).
A worked example
Cisco illustrates the mechanism with a queue that can hold 1,000 frames and thresholds at 40%, 60%, and 100%—400, 600, and 1,000 frames in that example. If 600 frames are already queued, a new frame assigned the 60% threshold is dropped if enqueueing it would exceed 600. A frame assigned the 100% threshold can still be admitted, provided the queue has room below its maximum.
These percentages explain how thresholds differ; they are not universal recommended settings. The same guide documents default threshold values of 80%, 90%, and 400% when fewer than three queue-limit percentages are configured. Those defaults apply to the documented Catalyst 9500 and IOS XE Everest 16.8.x context, not to WTD as a general standard.
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What WTD does—and does not—control
WTD changes drop precedence: classifications sharing a queue can have different cutoffs for admission as the queue grows. It does not require one separate queue per traffic class. Nor does it determine which queued class is served first or how bandwidth is divided. Queue scheduling and bandwidth allocation are separate QoS functions; Cisco’s guide treats these separately in its queue and scheduling configuration material (Cisco WTD documentation; Cisco queueing documentation).
WTD, tail drop, and AQM compared
| Approach | When packets are dropped or marked | How treatment is selected | Primary distinction |
|---|---|---|---|
| Ordinary tail drop | New arrivals are accepted until the queue reaches its configured maximum; subsequent arrivals are dropped until transmissions reduce the queue. | Queue occupancy and its maximum length. | A single full-queue cutoff can contribute to tail-drop lock-out, where a few flows monopolize queue space, and to synchronization effects. The IETF discusses these concerns in RFC 7567, section 1.3. |
| Weighted tail drop (WTD) | An arriving frame is dropped when adding it would exceed the threshold assigned to its classification, even if the queue has room below a higher threshold. | Traffic classification or QoS label mapped to a threshold. | Provides differentiated drop precedence within the documented shared-queue implementation; it is not itself a scheduling policy. |
| Active queue management (AQM), such as RED/WRED | Can drop or mark packets before the queue is full, depending on the AQM method and configuration. | Method-specific; behavior depends on the chosen algorithm and implementation. | Uses early congestion signals or actions rather than relying solely on a full-queue cutoff. Platform support and exact behavior vary. |
WTD should not be treated as another name for WRED. WTD applies configured occupancy thresholds to classifications; AQM methods can act before a queue fills. RFC 7567 recommends AQM procedures for operational deployments and notes that AQM and scheduling can complement one another. That does not make any one method a universal winner: the right choice depends on congestion conditions, desired delay and fairness behavior, traffic, and platform capabilities (RFC 7567).
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When to consider WTD and what to verify
WTD is relevant when a supported platform needs different classes of traffic to have different drop precedence while sharing queue capacity. It is not, by itself, a solution to bufferbloat, flow unfairness, or synchronization; those outcomes depend on the wider queue-management and scheduling design.
- Confirm the exact switch model and software release: queue architecture and threshold controls are platform-specific.
- Check how QoS labels or classifications map to egress queues and thresholds; the intended precedence depends on those mappings.
- Review queue scheduling and bandwidth allocation separately, since WTD does not set transmission order or shares.
- Decide whether the goal is differentiated drop precedence, delay control, or another congestion objective. Compare supported WTD and AQM capabilities against that goal.
- Do not carry Cisco’s Catalyst 9500 IOS XE Everest 16.8.x example values or defaults over to another platform without checking its documentation.
RFC 7567’s guidance on tail drop and AQM is available from the RFC Editor; the Cisco release-specific WTD details are in its Catalyst 9500 QoS guide.
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