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Configuring CoS Asymmetric Ethernet PAUSE Flow Control

Ethernet PAUSE flow control is a congestion relief feature that works by providing link-level flow control for all traffic on a full-duplex Ethernet link, including Ethernet links that belong to link aggregated (LAG) interfaces. Ethernet PAUSE works in both directions on the link. In one direction, an interface generates and sends PAUSE messages to stop the connected peer from sending more traffic. In the other direction, the interface responds to PAUSE messages it receives from the connected peer to stop sending traffic.

Asymmetric flow control allows you to configure the PAUSE functionality in each direction independently on an interface. The configuration for generating PAUSE messages and for responding to PAUSE messages does not have to be the same. It can be enabled in both directions, disabled in both directions, or enabled in one direction and disabled in the other direction.

Symmetric flow control means that the interface has the same configuration in both directions. The PAUSE generation and PAUSE response functions are both configured as enabled or they are both disabled. If you do not want to PAUSE all of the traffic on a link, you can use priority-based flow control (PFC) to selectively pause traffic based on its IEEE 802.1p code point.

Asymmetric flow control provides the ability to configure the receive buffer and transmit buffer Ethernet PAUSE actions independently on an interface. The buffers perform the following actions:

  • The receive buffers generate and send PAUSE messages to the connected peer to ask the peer to stop sending traffic for a time period specified in the PAUSE frame. The peer interface’s buffers may store outgoing frames until the PAUSE period elapses and the interface can resume sending traffic.

  • The transmit buffers respond to PAUSE messages received from the connected peer to stop sending traffic to the peer. The transmit buffer may store outgoing frames until the PAUSE period elapses and the interface can resume sending traffic.

Asymmetric flow control enables you to specify independently whether or not the interface receive buffer generates and sends PAUSE messages to stop the connected peer from transmitting traffic, and whether or not the interface transmit buffer responds to PAUSE messages it receives from the connected peer and stops transmitting traffic. The receive buffer configuration determines if the interface transmits PAUSE messages, and the transmit buffer configuration determines if the interface receives and responds to PAUSE messages:

  • Receive buffers on—Enable PAUSE transmission (generate and send PAUSE frames)

  • Transmit buffers on—Enable PAUSE reception (respond to received PAUSE frames)

You must explicitly set both the receive buffer and the transmit buffer to configure asymmetric flow control.

  • To configure asymmetric flow control on an interface:

    For example, to configure interface xe-0/0/24 to generate and send PAUSE messages but not to respond to received PAUSE messages:

    For example, to configure interface xe-0/0/30 to respond to received PAUSE messages but not to generate and send PAUSE messages:

Note:

If you configure both buffers to be on, that is equivalent to symmetric flow control. If you configure both buffers to be off, there is no flow control (flow control is disabled).

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