Why data rate must be "auto" while using Auto-MDIX?

Question

I've just learned that ethernet port does not link up if you keep auto-MDIX working even if both speed and duplex is fixed (100 Mbps with full duplex, in my case) at both end.

Wikipedia states as follows:

For auto-MDIX to operate correctly, the data rate on the interface and duplex setting must be set to "auto".

And here is cisco's page.

When you enable auto-MDIX, you must also set the speed on the port to auto so that for the feature to operate correctly.

Both page says that you must use auto for for speed and duplex with auto-MDIX but doesn't say why.

Is there any technical limitation for that? Or, is it just US Patent 7,366,771?

Answer 1

The patent says:

However, there are particular devices that do not follow the IEEE 802.3 Auto-Negotiation Protocol, and are instead forced into either 10 Mbps or 100 Mbps operation. Customers who use the Auto-MDI/MDIX algorithm have grown used to not requiring special crossover cables with products that implement the algorithm. When they force speed/duplex for some limited applications, the algorithm breaks down and link is not established. The root of the problem lies in the Auto-Negotiation state machine which turns off an auto-negotiating (AN) transmitter if it detects non-FLP (non Fast Link Pulse) energy on the wire (as occurring during forced mode). When that AN transmitter is turned off, it will cause the Auto-MDI/MDIX node to incorrectly determine its link state and cycle endlessly. The logical elements described below will allow operation of Auto-MDI/MDIX with nodes that are Auto-Negotiating even when the node in question is operated in forced mode.

So if you set the speed/duplex manually, that turns off autonegotiation, and changes the signal on the wire. That causes the other end to turn off autonegotiation as well. The Auto-MDI/MDIX algorithm can no longer work out whether it has a link or not and cannot function.

(The patent basically covers "try swapping the connection pairs, randomly, every few miliseconds, until a link is established")