5 Replies Latest reply on Feb 5, 2007 2:55 PM by gus27

    Network Cards -  Use Auto or Fixed Speed and Duplex?

    taylorb Expert

      My network engineer is trying to force me to set all my servers and ESX hosts to fixed gigabit and full duplex.   I remember reading that it is best practice with ESX (and other servers) to use Auto negotiate for NICs unless you have a specific reason.  I really don't want to change any network settings because we have been running fine for a long time.   Am I correct in my assumption that auto negotiate is best?  Any ammo you could give me that would help in my argument?

        • 1. Re: Network Cards -  Use Auto or Fixed Speed and Duplex?
          biekee Expert

          In your case I would go with your network guy. I have had both situations with no difference in speed or reliability. Not every case is the same though, it depends on the combination of switch and nics. So set it all to fixed speeds and see how it goes.


          If it doesn't work, you have to tell the network guy to set the switch to auto and you will have to set your nics of course to auto too.


          See also:





          • 2. Re: Network Cards -  Use Auto or Fixed Speed and Duplex?
            letoatrads Expert

            Generally, best practices is to force speed and duplex with 10/100 NIC's but leave GIG nics to AUTO. Least that's what the in-house CCIE has been preaching at me for years =)

            1 person found this helpful
            • 3. Re: Network Cards -  Use Auto or Fixed Speed and Duplex?
              esiebert7625 Guru

              With gigabit it is always recommended to go Auto/Auto, with 100MB the recommendation was 100/Full.






              From wkikpedia:




              The standard for 1000BASE-TX requires autonegotiation to be always present and enabled. Other than speed and duplex mode, autonegotiation is used to communicate the port type (single port or multiport) and the master-slave parameters (whether it is manually configured or not, whether the device is master or slave if this is the case, and the master-slave seed bit otherwise).


              Here's the technical mumbo jumbo:


              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.


              If you want to read the whole doc:



              • 5. Re: Network Cards -  Use Auto or Fixed Speed and Duplex?
                gus27 Hot Shot

                Generally, best practices is to force speed and

                duplex with 10/100 NIC's but leave GIG nics to AUTO.


                This is the same as we do it...