Correct answer & explanation
✓Tail-drop: Drops all arriving packets when the queue is full
Tail-drop drops all packets when queue is full; RED starts dropping packets probabilistically before queue full; WRED uses IP precedence or DSCP to vary drop probability per class.
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Same concept, more angles
2 more ways this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Drag and drop each congestion avoidance mechanism on the left to its method on the right.
medium- ✓ P1.Tail-drop: Drops all packets when queue is full
- ✓ P2.RED: Random early detection based on average queue depth
- ✓ P3.WRED: Weighted random early detection using IP precedence or DSCP
- ✓ P4.Tail-drop (variant): No selective drop before congestion
- ✓ P5.WRED (variant): Drops packets with lower priority more aggressively
Why P1: Tail-drop drops all packets when queue is full. RED randomly drops packets before congestion. WRED uses IP precedence or DSCP to vary drop probability.
Variation 2. Which QoS mechanism is used to prevent head-of-line blocking by ensuring that a single queue does not consume all available buffer space?
medium- A.Policing
- B.Shaping
- C.Weighted Random Early Detection (WRED)
- ✓ D.Priority Queuing
Why D: Priority Queuing is the correct answer because it prevents head-of-line blocking by using multiple priority levels, ensuring that a single queue does not consume all available buffer space. Head-of-line blocking occurs when the first packet in a FIFO queue blocks subsequent packets due to congestion. By separating traffic into multiple priority queues, Priority Queuing allows high-priority traffic to be served first, reducing the chance of a single queue blocking others. However, it can cause starvation for lower-priority traffic if not configured properly.
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