Flow Control

1. Function Overview

A switching hub initially stores received frames in memory and then performs relay processing.
When many frames are sent at the same time and relay processing cannot keep up (a congested state), exceeding the available memory capacity for storage, the frames to be relayed are discarded.
This product includes the following function to help mitigate such congestion.

  • When ports are operating at full duplex: IEEE 802.3x flow control can be enabled.

2. Definition of Terms Used

Bit Time

On a 100BASE network, the speed is 100 Mbps, so 1 bit time is 10 nsec.
In the same way, the bit time on 1000BASE is 1 nsec.

3. Function Details

3.1. IEEE 802.3x flow control

For full duplex communication, the MAC control protocol with IEEE802.3x option can be used.
The MAC control frame in the diagram below is used for flow control.

  • MAC control frame
    image

The following flow control operations are performed, based on the restriction start threshold and the restriction cancel threshold.

  • Flow control: processing flow
    image

This product can be used for either transmitting or receiving MAC control frames. The operations for each are shown below.

  • MAC control frame transmission processing

    • Frames are stored in the received buffer. When the number of the frames exceeds the restriction start threshold, a PAUSE frame with a pause time of 65535 is sent.

    • When the overflow in the received buffer is resolved, and the number of the frames falls below the restriction cancel threshold, a PAUSE frame with a pause time of 0 is sent.

  • MAC control frame reception processing

    • When a PAUSE frame with a pause time of 1–65535 is received, the transmission processing will be stopped if the corresponding bit time has elapsed, or if the a PAUSE frame with a pause time of 0 has been received.

Use the flowcontrol command to enable or disable the flow control (when transmitting/receiving MAC control frames).
This setting can be made for the system and for each transmitting/receiving LAN/SFP port, and is set to “disable” by factory default.
In order to enable flow control for an individual port, flow control must be enabled for the system.
If the Qos function is enabled, flow control cannot be enabled.
When the stack function is enabled, only Pause frames can be received.

4. Related Commands

Related commands are indicated below.
For details, refer to the Command Reference.

Operations Operating commands

Set (system) flow control (IEEE 802.3x PAUSE send/receive)

flowcontrol

Set (interface) flow control (IEEE 802.3x PAUSE send/receive)

flowcontrol

Show flow control operating status

show flowcontrol

5. Examples of Command Execution

  • Enable flow control on LAN port #1.
    After the function is enabled, check the flow control operating status.

    Yamaha(config)#flowcontrol enable
Yamaha(config)#interface port1.1
Yamaha(config-if)#flowcontrol both
Yamaha(config-if)#end
Yamaha#show flowcontrol port1.1
Port        FlowControl       RxPause TxPause
---------   -----------       ------- -------
port1.1     Both                    0      64

6. Points of Caution

None

7. Related Documentation

None