Configuration Guide User guide

1948 FastIron Configuration Guide
53-1002494-02
Rate limiting in hardware
When you specify the maximum number of bytes, you specify it in kilobits per second (kbps). The
Fixed rate limiting policy applies to one-second intervals and allows the port to receive the number
of bytes you specify in the policy, but drops additional bytes. Unused bandwidth is not carried over
from one interval to the next.
NOTE
Brocade 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 Brocade 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 bytes 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.
Figure 218 shows an example of how Fixed rate limiting works. In this example, a Fixed rate
limiting policy is applied to a port to limit the inbound traffic to 500000 bits (62500 bytes) a
second. During the first two one-second intervals, the port receives less than 500000 bits in each
interval. However, the port receives more than 500000 bits during the third and fourth one-second
intervals, and consequently drops the excess traffic.
FIGURE 218 Fixed rate limiting