Weighted random early detection
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.
