Configuring Bidirectional Flow Control
Context
Flow control prevents packet loss caused by network congestion. If network congestion occurs on the local device after flow control is configured, the local device sends a message to the remote device, requesting the remote device to temporarily stop sending packets. After receiving the message, the remote device temporarily stops sending packets to the local device regardless of the interface working rate. In this case, the device can receive and send pause frames.
- GE0/0/1 of SwitchA and GE0/0/2 of SwitchB are connected, and their negotiation rate is 1000 Mbit/s. The two interfaces send data packets to each other at the rate of 1000 Mbit/s.
- The outbound interface Eth0/0/3 of SwitchB provides a maximum transmission rate of only 100 Mbit/s. When congestion occurs during packet forwarding, SwitchB caches the received packets. As the cached packets accumulate, Eth0/0/3 cannot forward all the packets due to forwarding capability limitations, and packet loss will occur.
- After flow control is configured on GE0/0/1 of SwitchA and GE0/0/2 of SwitchB, GE0/0/2 sends a Pause frame to GE0/0/1, requesting GE0/0/1 to temporarily stop sending packets. After receiving the Pause frame, GE0/0/1 stops sending packets to GE0/0/2 within the period specified in the Pause frame. If the congestion persists, GE0/0/2 continues to send Pause frames to GE0/0/1.
- Eth0/0/3 continues to send cached data packets until the congestion is eliminated.
- After the congestion is eliminated, SwitchB stops sending Pause frames to SwitchA. SwitchA then continues to send data packets to SwitchB at the rate of 1000 Mbit/s.
If the remote device does not support auto-negotiation, you can configure flow control on the local and remote devices. If both the local and remote devices support auto-negotiation, you can configure flow control auto-negotiation on the two devices. The local device then negotiates with the remote device to determine whether to enable flow control based on the network congestion status.
For details about Ethernet interfaces supporting the flow control function, see Licensing Requirements and Limitations for Ethernet Interfaces.
Flow control and flow control auto-negotiation cannot be concurrently configured on Ethernet interfaces.
Flow control auto-negotiation takes effect only when configured on interfaces at both ends of a link.
When an interface works in half-duplex mode, flow control does not take effect on the interface.
A GE optical interface supports flow control and flow control auto-negotiation after it has a GE copper module or a GE optical module installed.
An XGE optical interface supports flow control and flow control auto-negotiation after it has a GE copper module installed.
A 25GE optical interface supports flow control and flow control auto-negotiation after it has a GE copper module installed. Before installing a GE copper module on a 25GE optical interface, you need to run the port mode ge command to configure the interface to work at the rate of 1 Gbit/s.
On the S2720-52TP-EI, S5720-52P-LI-AC, if interfaces 0 to 23 work as inbound interfaces (or outbound interfaces) and interfaces 24 to 47 work as outbound interfaces (or inbound interfaces), flow control auto-negotiation does not take effect on these interfaces.
For the S5735-L, S5735S-L, S5735S-L-M, S5735-S, S5735-S-I, and S5735S-S, the multicast functions (both Layer 2 and Layer 3 multicast functions) conflict with the flow control function.