Weighted random early detection
Weighted random early detection (WRED) is a traffic control feature that uses IP precedence to determine how it treats or drops traffic.
On the device, queues are provided to buffer traffic levels that exceed the bandwidth of individual ports. For each output port, a set of eight priority queues is allocated. When traffic exceeds the bandwidth of a port, packets are dropped randomly as long as the congestion persists. Under these conditions, traffic of greater priority can be dropped instead of traffic with a lesser priority.
Instead of being subject to random selection,
you can configure a device to monitor traffic congestion and drop packets according
to a WRED algorithm. This algorithm enables the system to detect the onset of
congestion and take corrective action. In practice, WRED causes a device to start
dropping packets as traffic in the device starts to back up. WRED provides various
control points that can be configured to change a system's reaction to congestion.
The following variables are used when calculating whether to drop or forward
packets:
- Queue-Size—The user-configurable queue size.
- Current-Q-Size—The current size of the queue as calculated on the device.
- Min-Q-Threshold—The queue threshold in percentage below which all packets
are accepted. This variable is user configurable.
- Max-Q-Threshold—The queue threshold in percentage above which all packets
are dropped. This variable is user configurable.
- Drop-Probability—The maximum drop probability when the Queue-Size is at
Max-Q-Threshold. This variable is user configurable.
