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Routing & Switching · Foundational

Tail Drop

The default congestion behavior where packets arriving at a full queue are discarded.

Definition

Tail drop treats all traffic equally and can cause TCP global synchronization, where many flows back off and ramp up simultaneously, underutilizing the link. RED and WRED mitigate this by dropping early and selectively.

Example

An output queue at 100 percent depth drops all new arrivals.

Exam tip

Tail drop leads to TCP global synchronization; WRED is the answer to prevent it.

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