Medium-dependent interface

Auto–MDI-X ports on newer network interfaces detect if the connection would require a crossover and automatically choose the MDI or MDI-X configuration to complement the other end of the link.

The terminology generally refers to variants of the Ethernet over twisted pair technology that use a female 8P8C port connection on a computer, or other network device.

The general convention is for network hubs, bridges and switches to use the MDI-X configuration, while all other nodes such as personal computers, workstations, servers and routers use an MDI interface.

The confusion of needing two different kinds of cables for anything but hierarchical star network topologies prompted a more automatic solution.

[2] A pseudo-random–number generator determines whether a network port will start in MDI or MDI-X configuration to begin auto-negotiating the link.

[3][4] When two auto–MDI-X ports are connected together, which is normal for modern products, the algorithm resolution time is typically < 500 ms.

However, a ~1.4 second asynchronous timer is used to resolve the extremely rare case (with a probability of less than 1 in 5×1021) of a loop in which each end keeps switching.

[6] This may or may not be implemented on a given device, so occasionally a crossover cable may still be necessary when connecting an auto-MDI-X to an MDI-X (hub or switch) port, especially when autonegotiation is deactivated.