IEEE 802.3x Flow Control

With autonegotiation enabled, Summit family switches advertise the ability to support pause frames.

This includes receiving, reacting to (stopping transmission), and transmitting pause frames. However, the switch does not actually transmit pause frames unless it is configured to do so, as described below.

IEEE 802.3x flow control provides the ability to configure different modes in the default behaviors. Ports can be configured to transmit pause frames when congestion is detected, and the behavior of reacting to received pause frames can be disabled.

TX

You can configure ports to transmit link-layer pause frames upon detecting congestion. The goal of IEEE 802.3x is to backpressure the ultimate traffic source to eliminate or significantly reduce the amount of traffic loss through the network. This is also called lossless switching mode.

The following limitations apply to the TX flow control feature:

Flow control is applied on an ingress port basis, which means that a single stream ingressing a port and destined to a congested port can stop the transmission of other data streams ingressing the same port that are destined to other ports.
High volume packets destined to the CPU can cause flow control to trigger. This includes protocol packets such as, EDP (Extreme Discovery Protocol), EAPS (Extreme Automatic Protection Switching), VRRP (Virtual Router Redundancy Protocol), and OSPF (Open Shortest Path First).
When flow control is applied to the fabric ports, there can be a performance limitation. For example, a single 1G port being congested could backpressure a high-speed fabric port and reduce its effective throughput significantly.

To configure a port to allow the transmission of IEEE 802.3x pause frames, use the following command:

enable flow-control tx-pause ports port_list|all

Note

To enable TX flow-control, RX flow-control must first be enabled. If you attempt to enable TX flow-control with RX flow-control disabled, an error message is displayed.

To configure a port to return to the default behavior of not transmitting pause frames, use the following command:

disable flow-control tx-pause ports

RX

You can configure the switch to disable the default behavior of responding to received pause frames. Disabling rx-pause processing avoids dropping packets in the switch and allows for better overall network performance in some scenarios where protocols such as TCP handle the retransmission of dropped packets by the remote partner.

To configure a port to disable the processing of IEEE 802.3x pause frames, use the following command:

disable flow-control rx-pause ports port-list | all

Note

To disable RX flow-control, TX flow-control must first be disabled. If you attempt to disable RX flow-control with TX flow-control enabled, an error message is displayed.

To configure a port to return to the default behavior of enabling the processing of pause frames, use the following command: