Debugging Ethernet Port Duplex and Speed Settings

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Ethernet Port Duplex and Speed Settings are the most common reason customers report speed or bandwidth issues to their location. This also is the most common reason that there are Voice Line Quality of Service Issues, Speed or Performance Issues, or problems when we deliver SIP Trunks.

What Duplex Settings Are

Duplex refers to the way an ethernet port is configured. to send and recieve data

  • Full Duplex, means that the ethernet connection is properly configured to allow simultaneous two way traffic on your ethernet line. This is bidirectional, or two-way communication.
    • In order to get the best speeds you must have your equipment configured for full duplex.
  • Half Duplex, means that the ethernet connection is configured to send or receive traffic, but not at the same time. This is different from simplex, or one-way communication.
    • Half Duplex, will result in impaired and slower performance.

Effects of setting "Half Duplex" communication

  • You are almost guaranteed to have slower speeds. Typically a Half-Duplex setup will decrease speeds by at least 20%, perhaps higher, particularly on faster connections. On Fiber Speed Connections your performance will definitely be degraded, and most high speed Ethernet Service, including ADSL2+ and Ethernet First Mile
  • [Speed Tests] will result in lower speeds.
  • If you are running Voice Over Internet Protocol, SIP Trunks or Hosted PBX, you almost always will have degraded voice. Problems with Voice will occur more often when there is high traffic load.

How to configure your connection for full duplex

There are a lot of incompatibilities related to devices properly negotiating speed and duplex. Usually it is best to manually set the speed and duplex on both ends of the connection, but not always.

If you have a Cisco Router, the Cisco IOS Command Line below is an example of how you would type the commands on the ethernet interface for the speed and duplex.

config t
interface fastethernet0/0
speed 100
duplex full
wr mem
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