This article applies to:

• We have RMA’d a number of taps since the introduction of the TAR product. Subsequent to their return, each of these taps was tested and none have proven to be faulty. After consulting with the tap manufacturer, a likely cause of perceived tap failure is the Auto MDI/MDI-X feature on network switches.

Symptoms:

• MDI (Medium Dependent Interface port)

  Also called an "uplink port," MDI is a port on a network hub or switch used to connect to other hubs or switches without requiring a crossover cable. The MDI port does not cross the transmit and receive lines as executed by the regular MDI-X ports that connect to end stations (the X stands for "crossed"). The MDI port instead connects to the MDI-X port on the other device. There are typically one or two ports on a device that can be toggled between MDI (not crossed) and MDI-X (crossed).

  Automatic MDI/MDI-X

  Some time ago, automatic line sensing was developed within Ethernet, detecting whether the crossover is needed or not. Referred to as "auto MDI/MDI-X," "universal cable recognition," and "auto sensing," this feature eliminates having separate MDI and MDI-X ports, selector switches, and special crossover cables on the equipment.

  Based on the above definitions, it appears all should be well on the network, however, if the Auto MDI/MDI-X feature is turned on, the switch can randomly reverse the cable connection (due to a cable change, switch reset, line down/line up sequence, etc.). A network tap cannot cope with this connection reversal and thus is rendered inoperable.

Resolution:

The user should turn off the Auto MDI/MDI-X feature for the ports connected to a tap. If the user’s switch is not capable of disabling the Auto MDI/MDI-X feature, the switch will need to be replaced, not the tap.