Observer nTAPs : Observer nTAPs : FAQ : Choosing crossover or straight-through cables
Choosing crossover or straight-through cables
When choosing whether to use crossover or straight-through cables with a TAP, consider the following:
Crossover or straight-through cables can be used for any TAP having copper ports; either type will operate perfectly. However, straight-through cables are required for the 10/100 Copper TAP only.
Most networking hardware supports Auto-MDIX, which electrically creates a crossover connection where one is needed. For those devices, the proper cable configuration is used automatically regardless of the connection type.
If you encounter any rare issue with cable choice and your TAP, test your TAP with the opposite cable type and then contact VIAVI Support.
 
Typically, when a TAP is installed the existing cable that connects one device to another is used as half of the link. That is, the existing cable connects one device to the TAP and a new cable connects the TAP to the second device. The new, second cable is generally a crossover cable. If the new cable is not a crossover cable, then your endpoint devices may not be able to re-establish a connection if the TAP loses power.
Symptom: The TAP and endpoint devices work fine while the TAP has power, but when power is removed the endpoints do not reconnect as they should.
Cause: It is likely that you are using two straight-through cables. In other words, the existing cable that connected your network devices is a straight-through cable and the new cable you added is also a straight-through cable. This is not a problem so long as the TAP has electricity because the TAP takes care of the switching; however, when power is lost the TAP cannot perform the switching and must rely on the cables themselves to do it. Straight-through cables are not capable of Auto-MDIX, and because they are not the endpoint devices cannot connect through the TAP. Another cause is that Auto-MDIX has been deliberately turned off.
Solution: If Auto-MDIX is turned off in your network, enable it. If Auto-MDIX is enabled, then at least one cable must be a crossover cable; both cannot be straight-through cables. If you are using two straight-through cables, replace one of them with a crossover cable.