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This chapter contains the following sections:
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.
Ethernet interfaces do not auto-detect the link-level flow control capability. You must configure the capability explicitly.
Only link-level flow control is supported. Priority flow control (PFC) is not supported.
Enabling link level flow control requires a part of the buffer to be reserved. This reduces the available shared buffer space.
Flow control is not supported on 40G ports.
Data Center Bridging Exchange Protocol (DCBX) is not supported.
Configuration time quanta of the pause frames is not supported.
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.
When configuring a no-drop class for QoS you must use QoS-Group 1 and map the QoS Group 1 to the no-drop class.
Information About Link Level Flow Control
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.
Switch A | Switch B | Description |
---|---|---|
LLFC configured to receive and transmit PAUSE frames. |
LLFC configured to receive PAUSE frames. |
Switch A can transmit 802.3x PAUSE frames and honor 802.3x PAUSE frames. Switch B can only receive 802.3x PAUSE frames. |
LLFC configured to receive and transmit PAUSE frames. |
LLFC configured to transmit PAUSE frames. |
Switch A can transmit 802.3x PAUSE frames and honor 802.3x PAUSE frames. Switch B can transmit 802.3x PAUSE frames but will drop all received PAUSE frames. |
How to Configure Link Level Flow Control
To configure link-level flow control transmit on an interface, you enable flow control on the interface, configure a network-qos type QoS policy to enable a no-drop QoS group, and apply a qos type QoS policy to classify the traffic that requires no-drop behavior to the no-drop class.
You must ensure that bandwidth is allocated for the No-Drop QoS class using a queuing policy when you define a no-drop class. For more information, see the "Configuring Type Queuing Policies" section.
Note | 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. This is required as any ingress port that does not have flow-control send-on configured, can not generate a link level pause frame and there is no way to request the transmitting device to stop the transmission. Therefore, if flow-control send-on is not configured on all the interfaces you should not use a system policy to classify the packets to the no-drop QoS group. Instead, you should apply an interface QoS policy to the interfaces that having flow-control send-on enabled. |
Configuration Examples for Link Level Flow Control
The following example shows how to configure Link Level Flow Control receive on the device:
Device# configure terminal Device(config)# interface ethernet 1/1 Device(config-if)# flowcontrol receive on Device(config-if)# exit