Configuring Auto-Negotiation

Devices with different transmission capabilities exist on the network. Two devices must negotiate a proper data transmission capability to communicate. The auto-negotiation function provides an information exchange method for connected devices. After auto-negotiation is enabled, devices at both ends of a physical link can exchange information and automatically choose the same working parameters. In this way, the two devices can work at the maximum rate supported by both of them.

The duplex mode and rate of interfaces at both ends of a link are negotiated. If the negotiation succeeds, the two interfaces use the same duplex mode and rate. The auto-negotiation function takes effect only when both the connected devices support it. If the remote device does not support auto-negotiation or uses a different auto-negotiation mode, you can configure the connected interfaces to work in non-auto-negotiation mode, and forcibly set the same rate and duplex mode on the interfaces.

When the working mode of an interface is changed from auto-negotiation to non-auto-negotiation, the interface works at the maximum rate and uses the default duplex mode. For example, when a GE electrical interface works in auto-negotiation mode, the rate negotiated with the remote interface is 100 Mbit/s and the duplex mode is half-duplex. When the GE electrical interface is configured to work in non-auto-negotiation mode, the interface rate is restored to 1000 Mbit/s and the duplex mode is full-duplex.

• For details about Ethernet interfaces supporting the auto-negotiation function, see Licensing Requirements and Limitations for Ethernet Interfaces.
• By default, auto-negotiation is enabled on GE optical interfaces and rate auto-negotiation is disabled. You can run the speed auto-negotiation command to enable rate auto-negotiation.
• When a GE optical module is installed on an XGE optical interface, the interface works in auto-negotiation mode by default. To configure the interface to work in non-auto-negotiation mode, run the undo negotiation auto command. If you replace the GE optical module with an XGE optical module, the interface does not support the auto-negotiation mode and works in non-auto-negotiation mode by default. If you replace the XGE optical module with a GE optical module, the undo negotiation auto command configuration becomes ineffective. If you need the interface to still work in non-auto-negotiation mode, reconfigure this command.
• After configuring the auto-negotiation function on an interface, if you remove and install a single optical fiber on the interface, the interface may be Up and the remote interface may be Down. You can run the shutdown and undo shutdown commands on the remote interface to make the remote interface go Up.