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Configuring CoS WRED Drop Profiles

You can configure an interpolated weighted random early detection (WRED) profile to control traffic congestion by controlling packet drop characteristics for different packet loss priorities.

Drop profiles specify two values, which work as pairs:

  • Fill level—The queue fullness value, which represents a percentage of the memory used to store packets in relation to the total amount of memory allocated to the queue.

  • Drop probability—The percentage value that corresponds to the likelihood that an individual packet is dropped.

Note:

Do not enable WRED on lossless traffic flows (forwarding classes configured with the no-loss packet drop attribute). Use priority-based flow control (PFC) to prevent packet loss on lossless forwarding classes.

Except on QFX10000, you cannot enable WRED on multidestination (multicast) queues on. You can enable WRED only on unicast queues.

OCX Series switches do not support lossless flows or PFC.

Note:

On ECN-enabled queues, the drop profile sets the threshold for when the queue should mark a packet as experiencing congestion (see Understanding CoS Explicit Congestion Notification). On ECN-enabled queues, the switch does not use the drop profile to control dropping packets that are not ECN-capable packets during periods of congestion. Instead, the switch uses the tail-drop algorithm to drop non-ECN-capable packets during periods of congestion. When a queue fills to its maximum level of fullness, tail-drop simply drops all subsequently arriving packets until there is space in the queue to buffer more packets. All non-ECN-capable packets are treated the same way.

Drop Profiles on Switches Except QFX10000

Interpolated means that the switch creates a smooth drop curve from a drop start point to a drop end point, with a maximum drop rate that is reached at the drop end point.

The dropstart point is the average queue fill level when the WRED algorithm starts to drop packets. Before the drop start point, no packets are scheduled to drop. Specify the drop start point using the first of two fill-level statements.

The drop end point is the average queue fill level at which all subsequently arriving packets are dropped. When the queue fill levels falls below the drop end point, packets begin to be forwarded again. (At the drop end point, the packet drop probability becomes 100 percent.) Specify the drop end point using the second of two fill-level statements.

The minimum drop rate is always 0. Specify the minimum drop rate using the first of two drop-probability statements. The maximum drop rate is the drop probability when the average queue fill level reaches the drop end point. Specify the maximum drop rate using the second of two drop-probability statements.

The drop rate is zero until the queue fill level reaches the drop start point. As the queue continues to fill, packets drop in smooth linear curveuntil the queue reaches the drop end point, when packets drop at the maximum drop rate. If the queue fills beyond the drop end point, all packets that match the drop profile are dropped.

To configure a WRED profile using the CLI on switches except QFX10000:

  1. Name the drop profile and set the drop start point, drop end point, minimum drop rate, and maximum drop rate for the drop profile:

Drop Profiles on QFX 10000 Switches

Each queue fill level pairs with a drop probability. As the queue fills to different levels, every time it reaches afill level configured in a drop profile, the queue applies thedrop probability paired with that fill level to the traffic in the queue that exceeds the fill level. You can configure up to 32 pairs of fill levels and drop probabilities to create a customized packet drop probability curve with up to 32 points of differentiation.

Packets are not dropped until they reach the first configured queue fill level. When the queue reaches the firstfill level, packets begin to drop at the configured drop probability rate paired with the first fill level. When the queue reaches the second fill level, packets begin to drop at the configured drop probability rate paired with the second fill level. This process continues for the number of fill level/drop probability pairs that you configure in the drop profile.

Drop profiles are interpolated. An interpolated drop profile gradually increases the drop probability along a curve between each configured fill level. When the queue reaches the next fill level, the drop probability reaches the drop probability paired with that fill level.

To configure a WRED profile using the CLI on QFX10000 switches:

  1. Name the drop profile and set the fill levels and their associated drop probabilities as percentages. For every fill level, there must be a paired drop probability (you must configure the same number of fill levels and drop probabilities).

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