Why can't I turn Auto-Negotiation off on my Gigabit and 10 Gigabit links?

I have seen that question in various forms over the years and the answer is really simple, you can't because the specification says you can't.

Gigabit and 10 gigabit links use auto-negotiation for more than just fixing the speed and duplex settings. Some types of equipment or drivers might let you fix the speed and the duplex but you are not really turning off auto-negotiation, your are just telling the auto-negotiation module to advertise that it will link only to a peer that will do that speed and duplex. This can make it easier to detect a problem if the peer is not capable of running at that speed since the link never comes up and you are not left trying to determine why throughput is so low. But other types of equipment and drivers will not let you do this, it just depends on the manufacturer.

The following are the relevant and (I think) most understandable portions of the IEEE 802.3 specification. Note that item "a" in both sections 28D.5 (1000Base-T) and 28D.6 (10GBASE-T) indicate that auto-negotiation is mandatory. The rest of the text just gives you so information on why. You can find the full specification at http://standards.ieee.org/about/get/802/802.3.html

From Section 2

28D.5 Extensions required for Clause 40 (1000BASE-T)

Clause 40 (1000BASE-T) makes special use of Auto-Negotiation and requires additional MII registers. This use is summarized below. Details are provided in 40.5.

1. Auto-Negotiation is mandatory for 1000BASE-T (see 40.5.1).
2. 1000BASE-T requires an ordered exchange of Next Page messages (see 40.5.1.2), or optionally an exchange of an Extended Next Page message
3. 1000BASE-T parameters are configured based on information provided by the exchange of Next Page messages.
4. 1000BASE-T uses MASTER and SLAVE to define PHY operations and to facilitate the timing of transmit and receive operations. Auto-Negotiation is used to provide information used to configure MASTER-SLAVE status (see 40.5.2).
5. 1000BASE-T transmits and receives Next Pages for exchange of information related to MASTER-SLAVE operation. The information is specified in MII registers 9 and 10 (see 32.5.2 and 40.5.1.1), which are required in addition to registers 0-8 as defined in 28.2.4.
6. 1000BASE-T adds new message codes to be transmitted during Auto-Negotiation (see 40.5.1.3).
7. 1000BASE-T adds 1000BASE-T full duplex and half duplex capabilities to the priority resolution table (see 28B.3) and MII Extended Status Register (see 22.2.2.4).
8. 1000BASE-T is defined as a valid value for “x” in 28.3.1 (e.g., link_status_1GigT.) 1GigT represents that the 1000BASE-T PMA is the signal source.

28D.6 Extensions required for Clause 55 (10GBASE-T)

Clause 55 (10GBASE-T) makes special use of Auto-Negotiation and requires additional MDIO registers. This use is summarized below. Details are provided in 55.6.

1. Auto-Negotiation is mandatory for 10GBASE-T.
2. Extended Next Page support is mandatory for 10GBASE-T.
3. 10GBASE-T requires an exchange of an Extended Next Page message.
4. 10GBASE-T parameters are configured based on information provided by the exchange of an Extended Next Page message.
5. 10GBASE-T uses MASTER and SLAVE to define PHY operations and to facilitate the timing of transmit and receive operations. Auto-Negotiation is used to provide information used to configure MASTER-SLAVE status.
6. 10GBASE-T transmits and receives an Extended Next Page for exchange of information related to MASTER-SLAVE operation. The information is specified in 45.2.7.
7. 10GBASE-T adds new message codes to be transmitted during Auto-Negotiation.
8. 10GBASE-T adds 10GBASE-T full duplex capabilities to the priority resolution table (see 28B.3).
9. 10GBASE-T is defined as a valid value for “x” in 28.3.1 (e.g., link_status_10GigT.) 10GigT represents that the 10GBASE-T PMA is the signal source.
10. 10GBASE-T supports Asymmetric Pause as defined in Annex 28B.

From Section 3

40.1.3 Operation of 1000BASE-T

A 1000BASE-T PHY can be configured either as a MASTER PHY or as a SLAVE PHY. The MASTER-SLAVE relationship between two stations sharing a link segment is established during Auto-Negotiation (see Clause 28, 40.5, and Annex 28C).

. . . .

40.5 Management interface

40.5.1 Support for Auto-Negotiation

Auto-Negotiation is performed as part of the initial set-up of the link, and allows the PHYs at each end to advertise their capabilities (speed, PHY type, half or full duplex) and to automatically select the operating mode for communication on the link. Auto-negotiation signaling is used for the following two primary purposes for 1000BASE-T:

1. To negotiate that the PHY is capable of supporting 1000BASE-T half duplex or full duplex transmission.
2. To determine the MASTER-SLAVE relationship between the PHYs at each end of the link.
3. To negotiate EEE capabilities as specified in 28C.12.

From section 4

55.1.3 Operation of 10GBASE-T

A 10GBASE-T PHY can be configured either as a MASTER PHY or as a SLAVE PHY. The MASTER-SLAVE relationship between two stations sharing a link segment is established during Auto-Negotiation (see Clause 28, 55.6, Annex 28B, Annex 28C, and Annex 28D).

. . . .

55.6 Management interfaces

55.6.1 Support for Auto-Negotiation

Auto-Negotiation is performed as part of the initial set-up of the link, and allows the PHYs at each end to advertise their capabilities (speed, PHY type, half or full duplex) and to automatically select the operating mode for communication on the link. Auto-Negotiation signaling is used for the following primary purposes for 10GBASE-T:

1. To negotiate that the PHY is capable of supporting 10GBASE-T transmission.
2. To determine the MASTER-SLAVE relationship between the PHYs at each end of the link.
3. To determine whether the local PHY performs PMA training pattern reset.
4. To determine whether the local PHY supports the EEE capability.
5. To determine whether the local PHY supports the fast retrain capability.

This page was last modified on 13-10-06

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