IEEE 802.3X Pause Frame Transmit

Table 1. IEEE 802.3X Pause Frame Transmit product support

Feature

Product

Release introduced

IEEE 802.3X Pause frame transmit

5320 Series

Fabric Engine 8.6

5420 Series

VOSS 8.4

5520 Series

VOSS 8.2.5

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

Note

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

Table 2. Advertised abilities

Interface configuration

Pause

ASM

Capability advertised

Flow control enabled

1

0

Symmetric pause

Flow control disabled

1

1

Both Symmetric pause and asymmetric pause

The following tables identifies the pause resolution.

Table 3. Pause resolution

Local device pause

Local device ASM

Peer device pause

Peer device ASM

Local device resolution

Peer device resolution

0

0

Do not care

Do not care

Disable pause transmit and receive.

Disable pause transmit and receive.

0

1

0

Do not care

Disable pause transmit and receive.

Disable pause transmit and receive.

0

1

1

0

Disable pause transmit and receive.

Disable pause transmit and receive.

0

1

1

1

Enable pause transmit. Disable pause receive.

Disable pause transmit. Enable pause receive.

1

0

0

Do not care

Disable pause transmit and receive.

Disable pause transmit and receive.

1

Do not care

1

Do not care

Enable pause transmit and receive.

Enable pause transmit and receive.

1

1

0

0

Disable pause transmit and receive.

Disable pause transmit and receive.

1

1

0

1

Disable pause transmit. Enable pause receive.

Enable pause transmit. Disable pause receive.

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