119
Local Management: Overview, Setup, and Navigation
Rate Limiting Screen
The Rate Limiting Screen, Figure 41, is used to control the rate of traffic entering
the network on a per port/priority basis. One of two actions to control traffic must
be programmed with each priority entry on a port. The allowable range for the
rate limit is 0.5 Mbps to 1 Gbps. The sampling time for FDDI or 10/100 Mbps
operation is 100 milliseconds; for 1 Gbps, the sampling time is 10 milliseconds.
The rate limit is configured for a given port and list of priorities. The list of
priorities can include one, some, or all of the eight 802.1p priority levels. The
combined rate of all traffic entering the port with the priorities configured to that
port is not allowed to exceed the programmed limit. If the rate exceeds the
programmed limit, frames are dropped until the rate falls below the limit.
Some protocols - such as TCP - with “built-in” intelligence mechanisms that
optimize a “loss free” data transfer rate are particularly sensitive to frame loss. In
this case, the use of the Rate Limiting (RL) feature dropping frames that exceed
the rate threshold may actually undermine the intelligence of these protocols. For
example, an administrator configures a user port to allow 5 Mbps. When a TCP
session is established, the 5 Mbps rate limit is exceeded, causing the RL feature to
discard frames. As a result, TCP will see long acknowledgment times due to
frames being dropped and consequently lower its transmit rate (to 1 Mbps, for
example).
To prevent this scenario, the SmartSwitch device has a secondary mode of
operation that allows it assert flow control to the attached device(s) before the RL
threshold is reached. When an individual SmartSwitch device port reaches 95% of
its set RL maximum rate value, it sends IEEE 802.3x pause frames (in full duplex
mode operation) or a jam signal (in half duplex mode) to its attached device. By
asserting flow control, end stations will lower the data transfer rate to match the
RL setting.
To access the Rate Limiting Configuration Screen, use the arrow keys to highlight
the RATE LIMITING menu item in the 802.1 Configuration Menu Screen and
press ENTER.
