Quick, is auto-mdix a mandatory part of the 1000baseT spec? If you’re like me you’re thinking “of course it is; I’ve never seen a 1000baseT port that didn’t auto-flip - that’s why nobody carries crossover cables around in their laptop bag anymore”.

Well, until a couple of weeks ago neither had I…

Turns out it’s an optional part of the spec. One that just about everyone uses. One that Cisco left out of the NPE-G1, a 10-year-old piece of networking hardware that ought to eke out a couple of more years of life from the routers in bro-colo.

Unlike 100baseT, all four pairs are used. So all four pairs must be crossed. In a little deeper detail, since tx and rx are happening on all four pairs at once it’s conceivable that you wouldn’t need crossed cables to run the link, only to actually go through the dance to establish the link. Fun, eh?

https://en.wikipedia.org/wiki/Ethernet_crossover_cable#Crossover_cable_pinouts