Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 421
53-1002269-02
Chapter
16
Configuring Rate Limiting and Rate Shaping on the
PowerConnect B-Series TI24X
Rate limiting overview
This chapter describes how to configure rate limiting and rate shaping on PowerConnect B-Series
TI24X devices.
Rate limiting applies to inbound ports and rate shaping applies to outbound ports.
Port-based fixed rate limiting is supported on inbound ports. This feature allows you to specify the
maximum number of kilobits a given port on a PowerConnect device can receive. The port drops
bytes or kilobits that exceed the limit you specify. You can configure a Fixed Rate Limiting policy on
a port inbound direction only. Fixed rate limiting applies to all traffic on the rate limited port.
Fixed rate limiting is at line rate and occurs in hardware. Refer to “Rate limiting in hardware” on
page 421.
When you specify the maximum number of kilobits, you specify it in bits per second (bps). The
Fixed Rate Limiting policy applies to one-second intervals and allows the port to receive the number
of kilobits you specify in the policy, but drops additional bytes or kilobits. Unused bandwidth is not
carried over from one interval to the next.
NOTE
Dell recommends that you do not use Fixed Rate Limiting on ports that receive route control traffic
or Spanning Tree Protocol (STP) control traffic. If the port drops control packets due to the Fixed Rate
Limiting policy, routing or STP can be disrupted.
Rate limiting in hardware
Each device supports line-rate rate limiting in hardware. The device creates entries in Content
Addressable Memory (CAM) for the rate limiting policies. The CAM entries enable the device to
perform the rate limiting in hardware instead of sending the traffic to the CPU. The device sends
the first packet in a given traffic flow to the CPU, which creates a CAM entry for the traffic flow. A
CAM entry consists of the source and destination addresses of the traffic. The device uses the
CAM entry for rate limiting all the traffic within the same flow. A rate limiting CAM entry remains in
the CAM for two minutes before aging out.
How Fixed Rate Limiting works
Fixed Rate Limiting counts the number of kilobits (PowerConnect devices) that a port receives, in
one second intervals. If the number exceeds the maximum number you specify when you configure
the rate, the port drops all further inbound packets for the duration of the one-second interval.
Once the one-second interval is complete, the port clears the counter and re-enables traffic.