20.2.1 Flow Control
The flow control functionality sends a pause packet to the remote device to instruct it to temporarily stop sending frames so that received frames are not discarded when the reception buffer on the switch is full. Conversely, when the switch receives a pause packet, it regulates sending to the remote device. Note that flow control is available only in full-duplex mode.
- <Structure of this section>
(1) Flow Control Settings and Operation
To prevent the Switch from discarding received frames when the reception buffer has become full, the Switch needs to send a pause packet to the remote device to request regulated sending. The remote device must be able to receive pause packets and regulate sending in response to a received pause packet.
Whether the Switch regulates sending when it receives a pause packet from the remote device depends on the settings.
For the 48-port model, communication between ports 1 to 24 and 49 to 50 and ports 25 to 48 and 51 to 54 does not send pause packets from the receiving port due to congestion on the sending port.
Flow control configurations can be selected for transmission and reception in modes that determine operation by enable, disable, or negotiation result, respectively. Match the settings of this device with those of the other device in transmission and reception.
The following table describes the flow control operation when the pause packet send setting of the Switch is combined with the peer device setting.
|
Of the equipment Send pause packet (send parameter) |
On the other device Receive pause packet |
Flow control operation |
|---|---|---|
|
on |
Enabled |
Sending on the remote device is regulated. |
|
off |
Disabled |
Sending on the remote device is not regulated. |
|
desired |
Desired |
Sending on the remote device is regulated. |
Legend: Desired: How negotiations determine what happens
The following table describes the flow control operation when the pause packet reception settings of the Switch are combined with the settings of the remote device.
|
Of the equipment Receive pause packet (receive parameter) |
On the other device Send pause packet |
Flow control operation |
|---|---|---|
|
on |
Enabled |
Sending on the Switch is regulated. |
|
off |
Disabled |
Sending on the Switch is not regulated. |
|
desired |
Desired |
Sending on the Switch is regulated. |
Legend: Desired: How negotiations determine what happens
During auto-negotiation, if the Switch is set to off and the remote device is Desired, and if the Switch is set to desired, the flow-control operation follows the negotiation outcome.
(2) Flow Control Operation When Auto-Negotiation is Used
The Switch can negotiate whether to send and receive pause packets to and from the remote device when auto-negotiation is used on an interface that supports auto-negotiation.
The following table describes the flow control operation when auto-negotiation is used.
|
Switch (Parameter) |
Remote device |
Result of auto-negotiation on the Switch |
Flow control operation |
||||
|---|---|---|---|---|---|---|---|
|
Send pause packet |
Receive pause packet |
Send pause packet |
Receive pause packet |
Send pause packet |
Receive pause packet |
Is sending regulated on the Switch? |
Is sending regulated on the remote device? |
|
on |
desired |
Enabled |
Enabled |
on |
on |
Yes |
Yes |
|
Disabled |
on |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Disabled |
Enabled |
on |
on |
No |
Yes |
||
|
Disabled |
on |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Desired |
Enabled |
on |
on |
Yes |
Yes |
||
|
Disabled |
on |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
off |
Enabled |
Enabled |
on |
on |
Yes |
Yes |
|
|
Disabled |
off |
on |
Yes |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Disabled |
Enabled |
on |
on |
No |
Yes |
||
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Desired |
Enabled |
on |
on |
Yes |
Yes |
||
|
Disabled |
off |
on |
Yes |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
desired |
on |
Enabled |
Enabled |
on |
on |
Yes |
Yes |
|
Disabled |
off |
on |
Yes |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Disabled |
Enabled |
on |
on |
No |
Yes |
||
|
Disabled |
off |
on |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Desired |
Enabled |
on |
on |
Yes |
Yes |
||
|
Disabled |
off |
on |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
off |
Enabled |
Enabled |
off |
off |
No |
No |
|
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
off |
off |
No |
No |
|||
|
Disabled |
Enabled |
on |
off |
No |
Yes |
||
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
on |
off |
No |
Yes |
|||
|
Desired |
Enabled |
off |
off |
No |
No |
||
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
off |
off |
No |
No |
|||
|
desired |
Enabled |
Enabled |
on |
on |
Yes |
Yes |
|
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Disabled |
Enabled |
on |
on |
No |
Yes |
||
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
|
Desired |
Enabled |
on |
on |
Yes |
Yes |
||
|
Disabled |
off |
off |
No |
No |
|||
|
Desired |
on |
on |
Yes |
Yes |
|||
Legend: Desired: How negotiations determine what happens
When auto-negotiation is used on the following ports, flow-control is available only when the Pause packet reception setting is desired, set to on. The following table describes the ports for which flow control is operating differently.
|
Model |
Port number |
|---|---|
|
AX2340S-16T4X |
Port 17~20 |
|
AX2340S-24T4X |
Port 25~30 |
|
AX2340S-24TH4X |
Port 25~30 |
|
AX2340S-48T4X |
Port 53~54 |
|
AX2340S-24P4X |
Port 25~30 |
|
AX2340S-24PH4X |
Port 25~30 |
|
AX2340S-48P4X |
Port 53~54 |
|
AX2340S-16P8MP2X |
Port 25~26 |
(3) Loose mode
Precise flow control is necessary to minimize packet loss when connecting to the server. However, precise flow control may create a momentary loop configuration that triggers status in which both parties regulate each other (shown in the figure below). Loose flow control mode is suited for this type of network.
|
|
![[Figure Data]](./GRAPHICS/ZU111030.GIF)
Because the pause packet sending interval is equal to or shorter than the transmission restriction time in default mode, transmission from the receiving side of pause packets will be completely stopped. The following figure describes the default behavior sequence:
|
|
![[Figure Data]](./GRAPHICS/ZU111040.GIF)
Because the pause packet transmission interval is longer than the transmission restriction time in loose mode, transmission will not be completely stopped when Switches communicate with each other. The following figure describes the sequence of operations in loose mode:
|
|
![[Figure Data]](./GRAPHICS/ZU111050.GIF)