Tail-drop is a queue management algorithm used for congestion avoidance. When an egress port's queue buffer becomes full (congested), the FortiSwitch will discard any new incoming packets destined for that queue. This process of dropping all subsequent packets continues until packets are transmitted from the queue, freeing up buffer space. This behavior directly matches the scenario described in the question, where incoming packets are dropped until congestion on the egress port is relieved.

B. Weighted round robin mode: This is a packet scheduling algorithm that allocates egress port bandwidth among different traffic queues based on configured weights, not a policy for dropping packets during congestion.

C. Random early detection mode: This is a proactive congestion avoidance mechanism that starts dropping packets probabilistically before a queue becomes completely full, aiming to prevent total congestion. It does not wait for the queue to be full.

D. Strict mode: This is a priority-based packet scheduling algorithm that services the highest-priority queue exclusively until it is empty before servicing any lower-priority queues. It is not a drop policy.

1. Fortinet FortiSwitchOS 7.4.0 Administration Guide, Page 138, "Configuring QoS".

The guide describes the two available drop policy modes. For Tail-drop mode, it states: "When the queue is full, the incoming packet is dropped. This is the default mode." This confirms the behavior described in the question.

2. Fortinet FortiSwitchOS 7.4.0 CLI Reference, Page 378, config switch qos drop-policy.

This section details the CLI commands for QoS drop policies. It lists the two available modes for set mode: taildrop and red. The description for taildrop implicitly defines its function as dropping packets when the queue limit is reached.

3. Fortinet FortiSwitchOS 7.4.0 Administration Guide, Page 138, "Configuring QoS".

The guide distinguishes between drop policies and queue (scheduling) policies. It describes "Strict mode" and "Weighted round-robin mode" under the config switch qos queue-policy section, confirming they are for scheduling egress traffic, not for dropping packets due to congestion.