How can two interfaces at both ends of a link determine the rate and duplex mode through auto-negotiation


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. For example, a GE interface connects to an FE interface and their rate is negotiated to 100 Mbit/s, which is the maximum rate supported by both of them.

Other related questions:
Can two interface communicate with each other if one interface works in auto-negotiation mode and the other works in forcible mode
This configuration is not recommended. When this configuration is used, the interface working in auto-negotiation mode changes to half-duplex state according to the auto-negotiation requirements. This reduces the network utilization efficiency and may cause other problems. It is strongly recommended that the local and remote interfaces work in the same mode. That is, they both work in auto-negotiation mode or forcible mode.

Can two electrical interfaces communicate normally when one interface is forcibly configured to work in full duplex mode and the other work in auto-negotiation mode
According to IEEE 802.3, when electrical interface A in auto-negotiation mode connects to electrical interface B that is forcibly configured to work in full duplex mode, the two interfaces can go Up. However, interface A starts to work in half-duplex mode, and interface B still works in full duplex mode. In this case, the interface working in half-duplex mode encounters error packets and packet loss due to collision of the physical link. Therefore, it is strongly recommended that you configure the interconnected electrical interfaces to work in the same mode. That is, enable auto-negotiation mode on both interfaces or configure the interfaces to work at the same speed and duplex mode by using the following commands: system-view [HUAWEI] interface gigabitethernet 0/0/1 [HUAWEI-GigabitEthernet0/0/1] undo negotiation auto [HUAWEI-GigabitEthernet0/0/1] speed 100 [HUAWEI-GigabitEthernet0/0/1] duplex full

How can packet loss be avoided when two interfaces at both ends of a link work in auto-negotiation mode and use the half-duplex mode
You can configure the local and remote devices to work in non-auto-negotiation mode, and set their duplex mode to full-duplex to avoid packet loss. The configuration is as follows: system-view [HUAWEI] interface gigabitethernet 0/0/1 [HUAWEI-GigabitEthernet0/0/1] undo negotiation auto [HUAWEI-GigabitEthernet0/0/1] duplex full

Can I configure parameters such as rate, auto-negotiation mode, and flow control on interfaces of CE series switches
You can use Interface Query Tool to obtain interface attributes such as the default auto-negotiation status, rate, and support for the auto-negotiation and flow control auto-negotiation configuration based on the interface type and connection medium. To use the interface query tool for CE series switches, visit

Rates on all interfaces working in auto-negotiation mode are different on a WLAN device
When all interfaces of a WLAN device work in auto-negotiation mode, the negotiated rate is the maximum rate supported by the interfaces. However, an interface may negotiate a rate of 100 Mbit/s with a peer interface, while other interfaces negotiate a rate of 1000 Mbit/s with peer interfaces. The possible cause is that the Ethernet cable connecting the GE interfaces on the local and peer devices cannot support a high working rate due to deterioration or RJ-45 connector faults. Therefore, the negotiated rate can only be 100 Mbit/s or 10 Mbit/s. In this case, replace the Ethernet cable.

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