Detailed Auto-Negotiation Process of Ethernet Interfaces

Latest reply: Aug 9, 2017 14:35:16 2970 4 1 1

There are two types of auto-negotiation mechanisms: auto-negotiation and auto-sensing/speed detection.

Auto-Negotiation: is a standard handshake mechanism for negotiation of multiple interface attributes such as the interface rate. When two interfaces are connected, the interface that has auto-negotiation enabled can be automatically configured with the optimal configurations supported by the two interfaces (for example, the interface can be configured to work at the rate of 100 Mbit/s and in full-duplex mode). Interfaces with auto-negotiation enabled can be connected to interfaces that do not support auto-negotiation.

Auto-Sensing/Speed Detection: is a mechanism only for interface rate negotiation. For example, when two interfaces are connected and one interface works at the rate of 10 Mbit/s or 100 Mbit/s and in duplex mode, the other interface is automatically negotiated to work at the rate of 10 Mbit/s or 100 Mbit/s with its duplex mode unconfigured.

When two 10BASE-T Ethernet interfaces that work only at the rate of 10 Mbit/s and do not support auto-negotiation are connected, they send link test pulses (LTPs) to check whether the link can go Up before it is Up. When two 100BASE-T Ethernet interfaces that work only at the rate of 100 Mbit/s and do not support auto-negotiation are connected, they send Fast Ethernet IDLE flows to check whether the link can go Up before it is Up.

Ethernet interfaces that support auto-negotiation send fast link pulses (FLPs) before the link is Up. The FLPs combine a group of LTPs and data pluses, indicating certain interface attributes, for example, the interface can work at the rate of 100 Mbit/s and in full-duplex mode. Interfaces that do not support auto-negotiation such as 10BASE-T interfaces still identify FLPs as LTPs. A device that supports auto-negotiation can identify interface attributes carried in FTPs and configure the link to be optimal by exchanging the FLPs with the remote device. If a device that supports auto-negotiation receives LTPs rather than FTPs, the device configures the local interface to work at the rate of 10 Mbit/s and in half-duplex mode. If a device that supports auto-negotiation receives Fast Ethernet IDLE flows, the device configures the local interface to work at the rate of 100 Mbit/s and in half-duplex mode. The preceding auto-negotiation mechanism only applies to 10M/100M copper wire interfaces, 1000M optical interfaces, and 1000M electrical interfaces. 100BASE-FX interfaces do not support auto-negotiation.

The following describes auto-negotiation between interfaces working at the rates of 10 Mbit/s, 100 Mbit/s, or 10 Mbit/s and 100 Mbit/s respectively.

In Figure 1-1, two devices that support auto-negotiation are connected.

Figure 1-1 Connection between two devices that support auto-negotiation

http://support.huawei.com/enterprise/product/images/4296b8874b534335bfdf87530edf6ed0

 

Device A and Device B send FTPs. Each device sets the confirmation bit in its own FLPs after receiving FLPs from the remote device, sets its rate and duplex mode to the optimal configurations supported by the two devices, and then starts to send Fast Ethernet IDLE flows. The link then goes Up.

In Figure 1-2, an interface in auto-negotiation mode is connected to a 10BASE-T interface.

Figure 1-2 Connection between an interface in auto-negotiation mode and a 10BASE-T interface

20170719170828817002.png

 

Device A sends FLPs and Device B sends LTPs. Device A configures the local interface to work at the rate of 10 Mbit/s and in half-duplex mode after receiving LTPs from Device B. This brings a risk. When Device B works at the rate of 10 Mbit/s and in full-duplex mode, Device A is still negotiated to work at the rate of 10 Mbit/s and in half-duplex mode.

In Figure 1-3, an interface in auto-negotiation mode is connected to a 100BASE-T interface.

Figure 1-3 Connection between an interface in auto-negotiation mode and a 100BASE-T interface

20170719170828567003.png

 

Device A sends FLPs and Device B sends Fast Ethernet IDLE flows. After receiving the flows from Device B, Device A configures the local interface to work at the rate of 100 Mbit/s and in half-duplex mode and starts to send Fast Ethernet IDLE flows. This brings a risk. When Device B works at the rate of 100 Mbit/s and in full-duplex mode, Device A is still negotiated to work at the rate of 100 Mbit/s and in half-duplex mode.

In Figure 1-4, auto-negotiation causes a duplex mode mismatch.

Figure 1-4 Duplex mode mismatch caused by auto-negotiation

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Device A works in auto-negotiation mode. Device B is configured to work at the rate of 100 Mbit/s and in full-duplex mode and has auto-negotiation disabled though it supports auto-negotiation. Therefore, Device A sends FLPs and Device B sends Fast Ethernet IDLE flows. After receiving Fast Ethernet IDLE flows from Device B, Device A configures the local interface to work at the rate of 100 Mbit/s and in half-duplex mode. Device B is forced to work at the rate of 100 Mbit/s and in full-duplex mode, causing the following problem. When Device A and Device B send a frame simultaneously, Device A considers that a conflict occurs. It then destroys its own frame, discards the frame from Device B, and attempts to resend a frame. Device B does not resend a frame and considers the frames from Device A as "corrupted frames". Therefore, Device A records many delayed collision frames and device B records many CRC error frames.

In Figure 1-5, the link is not Up because of incorrect configurations.

Figure 1-5 Link not being Up because of incorrect configurations

http://support.huawei.com/enterprise/product/images/a435650453904cf3893ed34053420f29

 

Device A is configured to work at the rate of 100 Mbit/s and in full-duplex mode and has auto-negotiation enabled, and Device B is configured to work at the rate of 100 Mbit/s and in half-duplex mode (or only in half-duplex mode) and has auto-negotiation enabled. Each device receives FLPs from the remote device, but cannot change their duplex modes because their duplex modes are inconsistent. Therefore, the link does not go Up.

Figure 1-6 shows operations between a device in auto-negotiation mode and a device in auto-sensing mode.

Figure 1-6 Operations between a device in auto-negotiation mode and a device in auto-sensing mode

http://support.huawei.com/enterprise/product/images/96f370b388584024bd036c1c7942f5ad

 

Devices in auto-sensing mode do not send FLPs and identify FLPs as LTPs. They originally send Fast Ethernet IDLE flows and continue sending the flows after receiving Fast Ethernet IDLE flows from remote devices, but start to send LTPs after receiving LTPs from remote devices. For example, Device A works in auto-negotiation mode and Device B works in auto-sensing mode. Originally, Device A sends FLPs and Device B sends Fast Ethernet IDLE flows. After receiving the flows from Device B, Device A configures the local interface to work at the rate of 100 Mbit/s and in half-duplex mode and starts to send Fast Ethernet IDLE flows. Device B first receives FLPs from Device A and identifies FLPs as LTPs. Device B then configures the local interface to work at the rate of 10 Mbit/s and in half-duplex mode and starts to send LTPs. The link is Up. (Device B regards the Fast Ethernet IDLE flows from Device A as junk data, and Device A may not respond to LTPs from Device B.) Therefore, one interface works at the rate of 10 Mbit/s and the other interface works at the rate of 100 Mbit/s.

In Figure 1-7, two devices that support auto-negotiation are connected.

Figure 1-7 Connection between two devices that support auto-negotiation

http://support.huawei.com/enterprise/product/images/f2e6472e0d4b491184b1b94c7107fde6

 

Originally, Device A sends FLPs and Device B sends Fast Ethernet IDLE flows. Device B first receives FLPs from Device A and identifies FLPs as LTPs. Device B then configures the local interface to work at the rate of 10 Mbit/s and in half-duplex mode and starts to send LTPs. After receiving LTPs from Device B, Device A configures the local interface to work at the rate of 10 Mbit/s and in half-duplex mode. The link then goes Up.

The following describes auto-negotiation between interfaces working at the rate of 1000 Mbit/s.

The auto-negotiation mechanisms for interfaces working at the rate of 1000 Mbit/s and interfaces at the rates of 10 Mbit/s and 100 Mbit/s are the same. 1000BASE-LX and 1000BASE-SX interfaces use the auto-negotiation mechanism to negotiate their duplex modes and flow control. Auto-Negotiation of 1000BASE-T interfaces also includes negotiation of other interfaces attributes (such as negotiation of the master and slave interfaces). Interfaces that work at the rate of 1000 Mbit/s do not work in half-duplex mode (through they can be negotiated to work in such mode). Therefore, there is no problem about their duplex modes.

 

In Figure 1-8, 1000BASE-LX and 1000BASE-SX interfaces in auto-negotiation and non-auto-negotiation modes are connected.

Figure 1-8 Connection between 1000BASE-LX and 1000BASE-SX interfaces in auto-negotiation and non-auto-negotiation modes

http://support.huawei.com/enterprise/product/images/9183670e7358408380e5d6b2b31fd698

 

Device A has auto-negotiation enabled and Device B has auto-negotiation disabled. Therefore, one interface is Up and the other interface is Down, or neither interface is Up.

In Figure 1-9, 1000BASE-T interfaces in auto-negotiation and non-auto-negotiation modes are connected.

Figure 1-9 Connection between 1000BASE-T interfaces in auto-negotiation and non-auto-negotiation modes

http://support.huawei.com/enterprise/product/images/0be23d9953bb4f4293064a951c0ba8cb

 

Device A has auto-negotiation enabled and Device B has auto-negotiation disabled. Therefore, one interface is Up and the other interface is Down, or neither interface is Up. However, when interfaces are configured according to Figure 1-9, the interfaces can go Up.

note

When two electrical interfaces are connected and one interface works in forcible mode and the other works in auto-negotiation mode, the two interfaces can go Up. However, it is strongly recommended that you do not use such configuration. Otherwise, packets may be exchanged abnormally.

 

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gululu
Admin Created Jul 19, 2017 09:45:51 Helpful(0) Helpful(0)

good
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Come on!
walidnawar
Created Jul 19, 2017 12:21:43 Helpful(0) Helpful(0)

Appreciated , Very good participation , keep on Detailed Auto-Negotiation Process of Ethernet Interfaces-2326011-1
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Try to Add Value anywhere anytime , Tomorrow never waits
m7g
Created Aug 9, 2017 07:37:23 Helpful(0) Helpful(0)

Good!
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WBR, Evgeny K.
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Suhail
Created Aug 9, 2017 14:35:16 Helpful(0) Helpful(0)

Super thanks for this sharing valuable info
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Senior Eng Telecom/Datacom

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