Configuring Rate Control

The Switching > Rate Control page provides information and configuration of per-interface flow control.

Both the Rate Control and Flow Control features are controlled on a per port basis.

The bandwidth of ingress frames can be tuned in four modes:

•	Limit All Frames

•	Limit just multicast and flooded unicast frames (including broadcast)

•	Limit just multicast (including broadcast)

•	Limit just broadcast frames

The rate limiting for egress frames can only be enabled or disabled, no mode can be selected.

The ingress rate limit is rounded to the nearest increment, depending on the granularity available for that rate. The granularities are different depending on the range of rates:

•	128kbps ~ 1Mbps – increments of 64kbps

•	1Mbps ~ 100Mbps – increments of 1Mbps

•	100Mbps ~ 1000Mbps – increments of 10Mbps (for gigabit ports)

Back-pressure flow control on half-duplex ports and pause frame-based flow control on full-duplex ports are provided to support zero packet loss under temporary traffic congestion.

Full-duplex flow control requires support from the peer end station. Full-duplex flow control works as follows: when a port’s free buffer space is almost empty, the devices send out a PAUSE frame with the maximum pause time to stop the remote node from sending more frames into the switch. The devices also respond to the pause command. After the PAUSE frame is detected, the port will stop transmission of new data for the amount of time defined in the pause time field of the received PAUSE frame.

Half-duplex flow control is used to throttle the throughput rate of an end station to avoid dropping packets during network congestion.