Example: Configuring WRED Drop Profiles

Step-by-Step Procedure

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:

• Drop start point—Percentage of average queue fill level when the WRED algorithm starts to drop packets. Before the drop start point, no packets are scheduled to drop.

• Drop end point—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.)

• Maximum drop rate—Drop probability when the average queue fill level reaches the drop end point.

You set the drop start point and the drop end point by specifying two queue fill level percentage values. The first value is the drop start point and the second value is the drop end point.

You set the maximum drop rate by specifying two drop probability percentage values. The first value is always zero (0), which is the minimum drop rate, the probability of dropping a packet at the drop start point. The second value is the maximum drop rate at the drop end point.

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 curve until 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.

Figure 1 shows the graph for a drop profile with a drop start point of 30 percent, a drop end point of 50 percent, and a maximum drop rate of 80 percent.

Figure 1: WRED Drop Profile Packet Drop Example

The graph shows that when the queue fill level is less than 30 percent, the packet drop rate is zero. When the queue fill level reaches 30 percent, packets begin to drop. As the queue fills, a higher percentage of packets drop. When the queue fill level reaches 50 percent, the packet drop rate has climbed to 80 percent. When the queue fill level exceeds 50 percent, all packets drop.

This example describes how to configure the drop profile shown in Figure 1. The drop profile will have:

• The name be-dp1

• 30 percent for the drop start point (first fill-level setting)

• 50 percent for the drop end point (second fill-level setting)

• 0 percent for the minimum drop rate (first drop-probability setting)

• 80 percent for the maximum drop rate (second drop-probability setting)

You apply a drop profile by configuring a drop profile map that maps the drop profile to a packet loss priority, and associate the drop profile and packet loss priority with a scheduler. When you map the scheduler to a forwarding class (queue), the switch applies the drop profile to the packets in the forwarding class that have a matching packet loss priority.

1. Set the drop start point at 30 percent, the drop end point at 50 percent, the minimum drop rate at 0 percent, and the maximum drop rate at 80 percent for the drop profile be-dp1:

Step-by-Step Procedure

Each queue fill level pairs with a drop probability. As the queue fills to different levels, every time it reaches a fill level configured in a drop profile, the queue applies the drop 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 first fill 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.

This example describes how to configure a drop profile with three fill level/drop probability pairs:

• Drop profile name—be-dp1

• Queue fill levels—25 percent, 50 percent, 75 percent

• Drop probabilities—30 percent, 60 percent, 100 percent

Each of the three fill levels pairs with a drop probability to program the interpolated drop profile curve.

You apply a drop profile by configuring a drop profile map that maps the drop profile to a packet loss priority, and associate the drop profile and packet loss priority with a scheduler. When you map the scheduler to a forwarding class (queue), the switch applies the drop profile to the packets in the forwarding class that have a matching packet loss priority.

To configure a drop profile:

1. Set the drop start point at a 25 percent fill level, an intermediate fill level of 50 percent, and a drop end point of 75 percent. Set the paired drop probabilities to 30 percent, 60 percent, and 100 percent, respectively, for drop profile be-dp1: