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What Is A Duplex Mismatch?


A duplex mismatch is a common cause of end-to-end performance problems. It is often manifested by a connection that will work at low speed, but drops packets when high-speed flows are attempted (usually with some traffic flowing in the opposite direction). It is due to a failure in the protocol used to negotiate whether an Ethernet connection can have traffic flowing only in one direction at a time (half-duplex) or in both directions at the same time (full-duplex). It is apparent that a mismatch occurs when one machine is set at full-duplex and another at half-duplex. What is not so obvious is that:

  • When auto-negotiate meets auto-negotiate, the transmission rate is full-duplex;

  • When half-duplex meets auto-negotiate, the rate is half-duplex.


  • In both cases, no problems occur. However, when auto-negotiate meets full-duplex, auto-negotiate converts to half-duplex and not, as you would expect, to full-duplex -– the auto-negotiation capability is programmed to assume half-duplex unless it meets another machine set at auto-negotiate.

    If, for any reason, one side believes it is full-duplex, and the other half-duplex, the side that is full-duplex may send packets when the other side is not expecting them, and those packets are dropped. To avoid these problems, a common campus network recommendation is to always force both sides to auto-negotiate (because most new devices come preset to auto-negotiate).



    Back to Case Study

    t is a cakebox? It is a small, inexpensive PC running Linux. It is configured such that, when you plug it into a DHCP-enabled Ethernet port and give it power, it registers its presence with an LDAP server where it logs its current IP address so you can “find” it. You can then connect to it remotely and run a series of network utilities (like Iperf, traceroute, and pchar). Cakeboxes were developed by Internet2 to test H.323 video conferencing capabilities and have been used for a variety of other end-to-end performance tests. Instructions on “building” a cakebox are available to member institutions upon request.
    DHCP-enabled Ethernet port and give it power, it registers its presence with an LDAP server where it logs its current IP address so you can “find” it. You can then connect to it remotely and run a series of network utilities (like Iperf, traceroute, and pchar). Cakeboxes were developed by Internet2 to test H.323 video conferencing capabilities and have been used for a variety of other end-to-end performance tests. Instructions on “building” a cakebox are available to member institutions upon request.


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