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