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644 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Rate limiting in hardware
21
Rate limiting in hardware
Each Dell PowerConnect 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 113 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 113 Fixed rate limiting
NOTE
The software counts the bytes by polling statistics counters for the port every 100 milliseconds,
which provides 10 readings each second. Due to the polling interval, the Fixed Rate Limiting policy
has an accuracy of within 10% of the port's line rate. It is therefore possible for the policy to
sometimes allow more traffic than the limit you specify, but the extra traffic is never more than 10%
of the port's line rate.
Zero bps
Beginning of
one-second
interval
500000 bps (62500 bytes)
The Fixed Rate Limiting policy
allows up to 500000 bits
(62500 bytes) of inbound traffic
during each one-second interval.
Once the maximum rate is reached,
all additional traffic within the
one-second interval is dropped.
One-second
interval
One-second
interval
One-second
interval
One-second
interval