Foundry Networks FESX User Manual

Foundry Configuration Guide for the FESX, FSX, and FWSX
14 - 2 © Foundry Networks, Inc. December 2005
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 either sends or receives, in one second intervals. The
direction that the software monitors depends on the direction you specify when you configure the rate limit on the
port. If the number of bytes exceeds the maximum number you specify when you configure the rate, the port
drops all further packets for the rate-limited direction, 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 14.1 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 14.1 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.
Configuration Notes
The following FastIron Ironware releases support fixed rate limiting for inbound traffic on individual ports (port-
based rate limiting):
• FESX releases 01.1.00 and later
• All FWSX software releases
• All FSX software releases
One-second
interval
Zero bps
Beginning of
one-second
interval
One-second
interval
One-second
interval
One-second
interval
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.
500000 bps (62500 bytes)

Foundry Networks FESX Specifications

General

Brand	Foundry Networks
Model	FESX
Category	Switch
Language	English