Since the dawn of structured cabling, the bane of network administrators has been the crossover cable. Swapping those pairs on one end of an Unshielded Twisted Pair (UTP) cable seems so simple, but it's caused countless hours of headscratching.

The basic problem, as most administrators know, is that you need a "straight-through" cable to connect a regular hub or switch port to an end-node like a PC or a printer. This type of hub port is called MDI for "media dependent interface". But occasionally, administrators will want to connect two hubs or switches together. To accomplish this, you need a MDIX port on one of the devices, which simply switches the "pinouts" by crossing a pair. These ports are generally only found on smaller hubs, and rarely found on switches. If you don't have an actual MDIX port you can simulate one by swapping the same pair in your cable, which we then call a "cross-over" cable. So, the problem is that these crossover cables get tossed in with the regular cables because they often look identical, and when an unsuspecting user grabs one accidentally, it can take a while to troubleshoot them.

Of course, over time, we've mostly learned the hard way to color code our cross-over cables so we don't use them unintentionally, and for quite a while now, they've largely been replaced in the enterprise world by fiber, as administrators have taken to connecting their switches with high-speed Gigabit Ethernet.

But old technologies

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die hard, and twisted pair is making a comeback. This is simple economics; electronics required for Gigabit Ethernet over copper UTP are much cheaper and easier to produce than the optics and lasers required for Gigabit Ethernet over fiber.

But finally, after all these years, hardware manufacturers are implementing something that should have been commonplace on the earliest switches: auto-MDIX. This feature not only lets you configure, in software, whether any given port is MDI or MDIX, but it allows the switch to detect how the port should be configured.

There are a lot of implications to this strategy. One positive, is that you can make all your cables straight-through now, and not worry about someone plugging in the wrong cable.

A caveat, though, is that many network administrators have been using the lack of this feature as a method to control layer-2 bridge loops. That is, years ago, if someone plugged two switches together accidentally, the network would come crashing to a halt (unless Spanning Tree Protocol was properly configured), but only if they accidentally used a cross-over cable, which was rare. With auto-MDIX, which is enabled by default on some switches, and disabled by default on others, a user can far more easily create a problem because either type of cable can be used to connect two switches together. Unfortunately, this sort of problem used to be quite common.

But on a practical level, just about any switch that supports auto-MDIX also supports Spanning Tree Protocol. Don't enable the former without the latter.


Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.


This was first published in December 2004

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