IEEE 802.3X Pause Frame Transmit

The switch uses MAC pause frames to provide congestion relief on full-duplex interfaces.

Overview

When congestion occurs on a port, the system can send or receive pause frames, also known as flow control, to temporarily pause the packet flow. The system uses flow control if the rate at which one or more ports receives or sends packets is greater than the rate the switch can process or accept the packets.

The switch can generate pause frames to tell the sending device to stop sending additional packets for a specified time period. After the time period expires, the sending device can resume sending packets. During the specified time period, if the switch determines the congestion is reduced, it can send pause frames to the sending device to instruct it to begin sending packets immediately.

Flow control mode and pause frames

If you enable flow control mode, the switch drops packets on ingress when congestion occurs. If the switch is not in flow control mode, it drops packets at egress when congestion occurs.

Configure an interface to send pause frames when congestion occurs to alleviate packet drops due to flow control mode.

Auto-Negotiation

Interfaces that support auto-negotiation advertise and exchange their flow control capability to agree on a pause frame configuration. IEEE 802.3 annex 28b defines the auto-negotiation ability fields and the pause resolution. The switch advertises only two capabilities. The following table shows the software bit settings based on the flow control configuration.

Note

Not all interfaces support Auto-Negotiation. For more information, see your hardware documentation.

The following tables identifies the pause resolution.

The following list identifies the type of interfaces that support auto-negotiated flow control:

• 10 Mbps/100 Mbps/1 Gbps copper

• 100 Mbps/1 Gbps/10 Gbps copper

• 1 Gbps fiber (in both SFP and SFP+ ports)