How Auto Negotiation Works

How auto-negotiation works

 A pulse signal called FLP (Fast Link Pulse) burst is transmitted when interfaces with the auto-negotiation function enabled are physically connected to each other.

 In the case of 10BASE-T, a pulse signal called NLP (Normal Link Pulse) is used for similar signals. FLP is a signal that extends this NLP and is compatible with NLP.

●100BASE-TX operation

 By adding the communication mode information that you can support to the FLP and sending it to each other, the mode with the highest priority among the communication modes supported by each other is set.

Communication mode selection priority is shown in the table below.

Prioritycommunication mode
1100BASE-T2 full duplex
2100BASE-TX full duplex
3100BASE-T2 half duplex
Four100BASE-T4 half duplex
Five100BASE-TX half duplex
610BASE-T full duplex
710BASE-T half duplex

Case of mode mismatch in 100BASE-TX (10 Mbps full duplex on the other side)

Let's take the configuration shown below as an example.

 The auto-negotiation function of the client PC is enabled, the setting of the port on the switch side of the connection destination is fixed, the communication speed is 10 Mbps, and the communication mode is full duplex.

1. The client PC side sends FLP because it is auto-negotiation. However, the client PC does not receive the FLP because the other switch is in fixed mode and does not send the FLP.

2. The switch in fixed mode is 10Mbps, so it sends out NLP signals to make sure the link is working properly.

3. The client PC that receives the NLP judges that the communication speed of the switch on the other side is 10 Mbps from the shape of the signal peculiar to 10Base.

4. From the shape of the signal, the PC side can detect that the speed is 10 Mbps, but the other switch cannot determine whether it is full duplex or half duplex.

5. If the communication mode cannot be determined, devices set to auto-negotiation are set to half-duplex. In this case, the client PC will be set to half duplex.

6. Ultimately, the communication mode of each other will be full 2 ​​on one side and half duplex on the other side, and since the communication modes will be different, appropriate communication will not be possible.

To prevent this reduction, both should have the same baud rate and baud mode settings.

Case of mode mismatch in 100BASE-TX (100Mbps full duplex on the other side)

Let's take the configuration shown below as an example.

 The auto-negotiation function of the client PC is enabled, the setting of the port on the switch side of the connection destination is fixed, the communication speed is 100 Mbps, and the communication mode is full-duplex.

1. The client PC side sends FLP because it is auto-negotiation. However, the client PC does not receive the FLP because the other switch is in fixed mode and does not send the FLP.

2. A switch set to static mode sends an idle signal at 100Mbps to ensure the link is working properly.

3. The client PC that receives the idle signal judges that the communication speed of the switch on the other side is 100 Mbps from the shape of the signal peculiar to 100BASE-TX.

4. From the shape of the signal, the PC side can detect that the speed is 100 Mbps, but the other switch cannot determine whether it is full duplex or half duplex.

5. If the communication mode cannot be determined, devices set to auto-negotiation are set to half-duplex. In this case, the client PC will be set to half duplex.

6. Ultimately, the communication mode of each other will be full 2 ​​on one side and half duplex on the other side, and since the communication modes will be different, appropriate communication will not be possible.

To prevent this reduction, both should have the same baud rate and baud mode settings.

For 1000BASE-T

1000BASE-T mandates auto-negotiation.

 By adding communication mode information that you can support to FLP (Fast Link Pulse) at the time of connection and sending it to each other, you can set the mode with the highest priority among the communication modes supported by each other.

The order of priority is shown in the table below.

Prioritycommunication mode
11000BASE-T full duplex
21000BASE-T half duplex
3100BASE-T2 full duplex
Four100BASE-TX full duplex
Five100BASE-T2 half duplex
6100BASE-T4 half duplex
7100BASE-TX half duplex
810BASE-T full duplex
910BASE-T half duplex

 If you are 1000BASE-T and the other party is 10BASE-T or 100BASE-TX, backward compatibility is achieved by transmitting NLP and idle signals.

Notes on auto-negotiation

 As described so far, auto-negotiation may cause problems depending on the combination of settings. In order to avoid these problems, it is necessary to keep the following settings in mind.

  • If one side is in auto-negotiation mode, the other side is also in auto-negotiation mode.
  • If one side is set to fixed mode, the other side is also set to fixed mode with the same line speed and communication mode.

 However, what has been explained so far does not always hold true. Depending on the vendor's implementation, even if the combination of settings that causes a problem with the other party's settings, the optimum mode may be used for communication.

However, combinations that cause problems can cause problems, so basically keep the above settings in mind.