The Detailed Process of Auto-negotiation on Ethernet

There are two types of auto-negotiation mechanisms: Autonegotiation and

Autosensing/Speed Detection.

Autonegotiation is a handshake mechanism of the standard negotiation speed. It can be

automatically configured to the most possible connection (such as 100M full-duplex) and

be connected to old devices or the devices that do not support auto-negotiation.

Autosensing/Speed Detection can be automatically configured to only 10M or 100M and

does not support the setting of duplex.

The 10BASE-T Ethernet interface that supports 10M rather than auto-negotiation sends

LTPs before the link is Up. The LTP is used to check whether the link is Up. The

100BASE-T Ethernet interface that supports 100M rather than auto-negotiation sends

FAST ETHERNET IDLE flows to check whether the link is Up.

The Ethernet interface that supports Autonegotiation sends FLPs before the link is Up.

The FLP is actually a group of LTPs and data pulses, indicating a certain meaning. For

example, the 100M full-duplex is supported. The old device such as the 10BASE-T

device still regards such FLPs as LTPs. The auto-negotiation device, however, can

identify the meaning of the FLPs and configure the optimal link by exchanging such

handshake messages. The auto-negotiation device sets the local end to 10M half-duplex

if common LTPs (not FLPs of specific meanings) enter the device. The auto-negotiation

device set the local end to 100M half-duplex if FAST ETHERNET IDLE flows enter the

device. The preceding auto-negotiation mechanisms are applicable to only 10/100M

copper wire interfaces or 1000M optical/electrical interfaces. The 100BASE-FX device

does not support auto-negotiation.

The following takes an example to describe the process of 10/100M auto-negotiation: As

shown in Figure 1, the two devices that support auto-negotiation are connected.

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

Both device A and device B send FLPs. Each device confirms the BIT position in its own

FLPs after receiving the FLPs from the peer device. Then each device sets its rate and

duplex to the best modes supported by both devices and starts sending FAST 1

ETHERNET IDLE flows. Thus, the link is Up.

As shown in Figure 2, an auto-negotiation device is connected to a 10BASE-T device.

Device A sends FLPs but device B sends common LTPs. Device A sets the local end to

10M half-duplex after detecting the LTPs of the peer end in a parallel way. This may bring

risks. That is, when device B is set to 10M full-duplex, the negotiation result of device A is

also 10M halfduplex.

As shown in Figure 3, an auto-negotiation device is connected to a 100BASE-T device.

Figure 3 Connection between an auto-negotiation device and a 100BASE-T device

Device A sends FLPs but device B sends FAST ETHERNET IDLE flows. Device A sets

the

local end to 100M half-duplex and starts sending FAST ETHERNET IDLE flows after

detecting the IDLE flows of the peer end in a parallel way. This may bring risks. That is,

when device B is set to 100M full-duplex, the negotiation result of device A is also 100M

half-duplex.

As shown in Figure 4, the duplexes caused by two auto-negotiation devices are

inconsistent.

2

Device A is in auto-negotiation state. Device B is set to 100M full-duplex and auto-

negotiation is disabled although it supports auto-negotiation. As a result, device A sends

FLPs but device B sends FAST ETHERNET IDLE flows. Device A sets the local end to

100M half-duplex after receiving the IDLE flows from device B. Device B, however, is set

to 100M full-duplex forcibly, causing the following problem: If device A and device B send

a frame at the same time, device A regards that the sending conflicts. It destroys the

frame sent by itself, discards the frame of device B, and tries to send the frame of its own

again. Device B regards the frame of device A as a corrupted frame without sending the

frame again. As a result, many late collisions are recorded on device A and many CRC

error frames are recorded on device B.

As shown in Figure 5, the link is not Up because of incorrect configurations.

FLPs and device B sends FAST ETHERNET IDLE flows. Device A sets the local end to

100M half-duplex and starts sending FAST ETHERNET IDLE flows after receiving the

IDLE flows; device B identifies the FLPs received from device A as LTPs, sets the local

end to 10M halfduplex, and then sends LTPs. As a result, the link is Up. The FAST

ETHERNET IDLE flows sent by device A after the link is Up are regarded as trash data

by device B. Or device A may not process the LTPs from device B. Therefore, one end is

set to 10M and the other is set to 100M.

3

Or device A sends FLPs and device B sends FAST ETHERNET IDLE flows during

initialization. Device B identifies the FLPs received from device A as LTPs, sets the local

end to 10M halfduplex, and sends LTPs; device A sets the local end to 10M half-duplex

after receiving the LTPs from device B. Thus, the link is Up.

The process of 1000M auto-negotiation is as follows:

The same mechanism is adopted for 1000M auto-negotiation and 10/100M auto-

negotiation.

The 1000BASE-LX and 1000BASE-SX devices use the auto-negotiation mechanism to

negotiate duplexes and flow control. The auto-negotiation on the 1000BASE-T device

also

includes the negotiation of the additional region (master or slave region). Half-duplex

rarely exists (although can be negotiated) on 1000M devices. Thus, many duplex

problems do not exist.

As shown in Figure 8, the 1000BASE-LX and 1000BASE-SX devices in auto-negotiation

and non-auto-negotiation are connected.

and the other Down, or neither end is Up.

As shown in Figure 9, the 1000BASE-T devices in auto-negotiation and non-auto-

negotiation are connected.4

Auto-negotiation is enabled on device A and disabled on device B. As a result, one end is

Up and the other Down, or neither end is Up. In this case, the ends can be Up with

Huawei devices.

More related:

Huawei PING command

Summary of switch configurations

Huawei S5700 basic configuration command

Huawei switch restores factory settings

More Huawei products and Reviews you can visit: http://www.huanetwork.com/blog

Huanetwork.com is a world leading Huawei networking products distributor, we wholesale

original new Huawei networking equipments, including Huawei switches, Huawei routers,

Huaweisymantec security products, Huawei IAD, Huawei SFP and other Huawei networking

products. Our customers include telecom operators, Huawei resellers, ISP and system integrators.

Right now most of our sales are contributed by regular customers

Our website: http://www. hu anetwork.com

Telephone: +852-30501940

Email:  sales@huanetwork.com

Address: 23/F Lucky Plaza, 315-321 Lockhart Road, Wanchai, Hongkong

5