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Defining Packet Drop or ECN Behaviors by Configuring RED Drop Profiles

You enable random early detection (RED) by applying a drop profile to a scheduler. When RED is operational on an interface, the queue no longer drops all excess packets at the tail of the queue. Rather, a controlled fraction of packets are dropped, or marked with ECN (if enabled). Some output-buffered routers perform RED drops of oldest packets at the head of the queue. Other routers perform RED as packets enter a queue. When a queue becomes full, tail-drops (100%) supersede random dropping.

To configure a drop profile, include either the interpolate statement and its options, or the fill-level and drop-probability percentage values. These two alternatives enable you to configure either each drop probability at up to 64 fill-level/drop-probability paired values, or a profile represented as a series of line segments, as discussed in Managing Congestion Using RED Drop Profiles and Packet Loss Priorities.

For example, the following shows a discrete configuration and an interpolated configuration that correspond to the graphs in Figure 1. The values defined in the configurations are matched to represent the data points in the graph lines.

Figure 1: Discrete and Interpolated Drop ProfilesDiscrete and Interpolated Drop Profiles

Creating a Discrete Configuration

To create the discrete profile graph as shown in Figure 1 on the left, the software begins at the bottom-left corner, representing a 0-percent fill level and a 0-percent drop probability. This configuration creates a line horizontally to the right on the fullness level (l) until it reaches the first defined fill level, 50-percent for this configuration, which is designated to have a drop probability (p) of 20-percent. The software then continues the line horizontally along the fill level until the next drop probability is reached at the designated data point of 75-percent fill level, which has a designated drop-probability of 40-percent. The line is then continued horizontally to the next fill level of 85-percent and the designated drop probability of 75-percent. The line continues horizontally to the next designated fill level of 90-percent, which has a designated drop probability of 90-percent, and a line is created to data point 90-percent (l), 90-percent (p) (l90 p90). From the l90 p90 point, the line continues horizontally to the 100-percent fill level, which has a drop probability of 100 percent, at which the line rises to the end-point of 100-100, which is 100 percent fill level with a 100 percent drop probability.

A smoother graph line can be created by configuring the profile with the interpolate statement. This enables the software to automatically generate 64 data points on the graph beginning at (0, 0) and ending at (100, 100). Along the way, the graph line intersects specific defined data points, which you define as follows:

Creating an Interpolated Configuration

To configure a drop profile:

  1. Create the drop profile by specifying a name for it.
  2. (Optional) Specify the fill-level and drop-probability values for the drop profile.

    Repeat this step for each fill-level and drop-probability.

  3. (Optional) Specify values for interpolating the relationship between queue fill level and drop probability.
  4. Verify your configuration.
  5. Save your configuration.

After you configure a drop profile, you must assign the drop profile to a drop-profile map, and assign the drop-profile map to a scheduler, as discussed in Determining Packet Drop Behavior by Configuring Drop Profile Maps for Schedulers.