Link-level flow control is a congestion management technique that pauses data transmission until the congestion in the system is resolved. When a receiving device becomes congested, it communicates with the transmitter by sending a PAUSE frame. When the transmitting device receives a Pause frame it stops the transmission of any further data frames for a short period of time. The link-level flow control feature applies to all the traffic on the link. The transmit and receive directions are separately configurable. By default, link-level flow control is disabled for both directions.

• show commands with the internal keyword are not supported.

• Beginning with Cisco NX-OS Release 7.0(3)I7(8), syslog messages are generated at 5 minute intervals for accounting all the incoming global and link level pause frames.

• Changing or configuring LLFC on FEX HIF or FEX HIF PO interfaces is not supported.

• Link-level flow control (LLFC) is supported on Cisco Nexus 9500 platform switches with Network Forwarding Engine (NFE) (and Cisco Nexus 3164Q switch with NFE).

• Link-level flow control (LLFC) is supported on Cisco Nexus 9200 platform switches (7.0(3)I3(1) and later releases.

• Beginning with Cisco NX-OS Release 7.0(3)I5(1) and later releases, Link-level flow control (LLFC) is supported on the Cisco Nexus 9300 and 9300-EX platform switches.

• The 100G Cisco Nexus 9408PC-CFP2 line card does not support link-level flow control (LLFC).

• Ethernet interfaces do not auto-detect the link-level flow control capability. You must configure the capability explicitly.

• Enabling link level flow control requires a part of the buffer to be reserved. This reduces the available shared buffer space.

• Data Center Bridging Exchange Protocol (DCBX) is not supported.

• Configuration time quanta of the pause frames is not supported.

• On each Ethernet interface, the switch can enable either PFC or LLFC, but not both.

  Note	When both PFC and LLFC are enabled, LLFC is selected.

• Only pure CoS-based classification of traffic classes is supported.

• Setting of pause threshold values is restricted.

• Configuring Link Level Flow Control on the interfaces will flap the interfaces which results in a momentary traffic loss.

• When a no-drop QoS group is configured, you must ensure that packets received on ports that do not have flow control send-on configured are not classified to a no-drop QoS group.

• Only a no-drop QoS group is capable of generating link level pause frames.

• Weighted Random Early Detection (WRED) should not be enabled on a no-drop class because it can cause egress queue drops.

• It is recommended to use default buffer sizes for no-drop classes because if the buffer size is specified through CLI, it will allocate the same buffer size for all ports irrespective of the link speed, and MTU size.

• It is recommended to change the LLFC configuration when there is no traffic, otherwise packets already in the MMU of the system may not get the expected treatment.

• Beginning with NX-OS 7.0(3)I1(2), link level flow control and PFC are supported on Cisco Nexus 9300 platform switches and line cards that contain the ALE (Application Leaf Engine).

• 3232C does not support a combination of cut-through and LLFC enabled ports. Cut-through and LLFC are mutually exclusive and will work without the presence of the other feature. Post 9.3(8) release, on a cut-through enabled switch, if LLFC is enabled on a port, that port will operate in store and forward mode.

• First generation Cisco Nexus 9000 switches (NFE-based) can properly operate in cut-through switching mode only when there is no back pressure, i.e. LLFC pause frames should not be coming in. If RX LLFC is enabled alongside the default switching mode of cut-through, in some circumstances this can lead to output errors on the port configured with RX LLFC. To mitigate that, the user needs to either configure store and forward switching mode via the CLI "switching-mode store-forward" or disable RX LLFC on the given port. This limitation applies to the following PIDs:

  • N9K-C93120TX, N9K-C93128TX, N9K-C9332PQ, N9K-C9372PX, N9K-C9372PX-E, N9K-C9372TX, N9K-C9372TX-E, N9K-C9396PX, N9K-C9396TX, N9K-X9408PC-CFP2, N9K-X9432PQ, N9K-X9464PX, N9K-X9464TX, N9K-X9464TX2, N9K-X9536PQ, N9K-X9564PX, N9K-X9564TX, and N9K-X9636PQ.

When link level flow control is configured the system changes the interface state to Down if the specified interface is in UP state and then applies the flow control configuration. After the configuration is successfully applied to the interface, the system restores the interface to the UP state.

During a port shutdown event, the flow-control settings on an interface are retained, however no traffic is received or transmitted on the link. During a port startup event the flow-control settings are reinstated on to the hardware.

The transmit and receive directions can be configured separately, and each device on the network can have a different Link Level Flow Control (LLFC) configuration. The following table describes how devices with mis-matched configurations interact.