More Information About NIC Autoconfiguration: Gigabit NICs

Some interesting information comes to me from Ty Morton, a network engineer from Network Engines:

I just thought I would share this with you guys, I know there was some confusion on this a while back and Tom just posted on his blog regarding “Beware NIC and Switch Autonegotiation” . Most network engineers are under the impression that you should always force or manually configure links for the designated speed/duplex. Unfortunately with the advent of 1000B-TX that is no longer an option. Although Cisco in their infinite wisdom decided to still make it an option in the IOS, it still does the autoneg because it has to. At least according to the IEEE standard. Below is some decent documentation that I found on a Sun site believe it or not. Let me know if you still have questions.

Here is a link to Cisco that talks about forcing 1000/full.

Cisco Statement:

“If speed=1000 and duplex=full modes are specified for both g0/0 and g0/1 interfaces in copper mode (RJ-45), autonegotiation is still turned on. This is considered to be in forced mode for speed=1000. This occurrence is per the Annex 28D.5 extensions required for clause 40 (1000-BASE-T) IEEE 802.3.”

This is where the information below was obtained from.

New technologies such as 1000BASE-T require autonegotiation. The IEEE 802.3 standard default is to run with autonegotiation enabled. Technology improvements, and better interoperation of autonegotiation make it the preferred  mode of operation, and is required on new technologies such as 1000BASE-T (802.3ab). While the standard on Fast Ethernet allows the ability to disable autonegotiation, it is neither required nor recommended for vendors to implement it.

The IEEE 802.3 standard states that you must support and test autonegotiation enabled to certify a product IEEE 802.3 compliant, and for multivendor  interoperability (for example, testing at the UNH Interoperability Laboratory). There are no requirements in the standard to support locked down or forced configurations using autonegotiation disabled. As a result, there are no requirements for vendors to test multivendor interoperability between products with autonegotiation disabled.

The IEEE 802.3ab specification does not allow for forced mode 1000BASE-T with autonegotiation disabled running at 1000 Mbps. As a result, many switch vendors do not support forced mode. Although the transceiver used in the Sun™ Gigaswift Ethernet UTP adapter 1.0 is configurable for the 1000 Mbps forced mode and the ce driver allows this mode be aware that it does not work under certain circumstances.

Clause 40 (1000BASE-T), subclause 40.5.1 of 802.3 states: All 1000BASE-T PHYs shall provide support for Auto-Negotiation (Clause 28) and shall be capable of operating as MASTER or SLAVE. 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:

a) To negotiate that the PHY is capable of supporting 1000BASE-T half duplex or full duplex transmission.

b) To determine the MASTER-SLAVE relationship between the PHYs at each end of the link. 1000BASE-T MASTER PHY is c from a local source.

The SLAVE PHY uses loop timing where the clock is recovered from the received data stream.

What this means is that although autonegotiation (Clauses 22 and 28) is optional for most variants of Ethernet and manual configuration (forced mode) is allowed, this is not the case for Gigabit copper (1000BASE-T). Per the IEEE 802.3u specification, it not possible to manually configure one link partner for 100 Mbps full duplex and still autonegotiate to full duplex with the other link partner. In all cases, both ends of the link must be set to the same value or the link may not connect or may result in duplex mismatch as shown in following tables.

For CSMA/CD compatible devices that use the eight-pin modular connector of ISO/ IEC 8877: 1992 and that also encompass multiple operational modes, if a signaling method is used to automatically configure the preferred mode of operation, the autonegotiation function SHALL BE USED in compliance with Clause 28.

Ethernet Autonegotiation Best Practices • July 2004

Locked-down port policies (forcing speed, duplex, and link capabilities with autonegotiation disabled) are outdated. Legacy and historical reasons for forced setup with autonegotiation disabled date back many years when the technology was new. Due to the maturity of the technology today, it no longer has the same issues of 5- to-10 years ago when 802.3u Fast Ethernet and 802.3z Gigabit Ethernet were new technology and many vendors had standard compliance issues.

The UNH Interoperability Laboratory is used to ensure vendor compliance. These issues were resolved with NIC-driver patches, switch-firmware, and multiple generations of new product releases over many years. The notion of “autonegotiation is unreliable” can no longer be substantiated.

Not all network devices have the ability to force link capabilities for disabled autonegotiation policies. Some switches and drivers use autonegotiation (enabled) only and its usage is not optional. In the absence of autonegotiation (for example, using forced mode), link syncing between link partners may not occur and the link may not come up.

Even though the standard allows the ability to disable autonegotiation on Fast Ethernet 802.3u and Gigabit Ethernet 802.3z (fiber) technologies, it is neither required nor recommended. Do not disable autonegotiation between switches or NICs unless absolutely required, as physical layer problems may go undetected and result in spanning tree loops. Disabling autonegotiation should only be used as a troubleshooting aid or temporary workaround until the autonegotiation problem is resolved. The alternative to disabling autonegotiation is contacting the vendor for a software or hardware upgrade for IEEE 802.3 compliant Ethernet autonegotiation support.

Note – Forcing devices is problematic and adds administrative overhead. Legacy policies often create more administrative issues and network failures than they were intended to resolve.

When disabling autonegotiation, both ends of the link must match configuration values. This requirement makes it difficult to move physical connections or update devices with different capabilities. The administrative overhead of manually setting both link partners, verifying driver configurations, and checking Ethernet port statistics can be overwhelming on large Ethernet networks.

Enabling forced mode should be reserved only as a troubleshooting aid for link issues or as a temporary workaround until the autonegotiation problem is resolved (or with older pre-standard 100BASE-X switches that have a dated bug or do not comply with the standards).

Upgrading software and hardware of network devices for IEEE 802.3u/z autonegotiation support is highly recommended. Old policies from years past for locked-down forced autonegotiation disabled should be discouraged today.




Thomas W Shinder, M.D.


Email: [email protected]

MVP — Microsoft Firewalls (ISA)

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