I’ll admit it, I’ve become a lazy, fat Cat.

Once I got away from Cisco 5000’s, I stopped caring if I used a crossover cable or a straight through cable. A little thing called Auto-MDIX caused me to be that way. In the old days, the crossover cable rules went like this:

If you are connecting like devices, you must use a crossover cable. So, switch to switch was crossover.

If you are connecting two different devices, you must use a straight through cable. This is also know as a standard Cat 5 cable. So, switch to computer or switch to router required a straight through.

Along the way, Cisco decided to make life easier by using Auto Sensing to figure out of the cable in use was a straight through or a crossover. And so, something meant to make life easier, has in a sense, made things more complicated.

Now, you have things like 4 Port HWIC’s which add more ethernet ports to a router and guess what, they have Auto-MDIX, so you could connect a router to a router via the 4 port HWIC, without using a crossover cable. UGH!

Does your head hurt yet?

Mine does….

I was helping a buddy move an AT&T TLS circuit. You might know it better as a 100Mb MetroEthernet connection. On our side, we were moving from a 2821 Router to a 4507r switch. The other side was an unknown device at the time. The far end of our connection was a Router, but AT&T had a device inbetween.

The 4507r does Auto-MDIX. Well, sort of. It only does Auto Sensing if the port is configured for Auto/Auto. If the port is forced to 100/Full, it will not use Auto-MDIX.

We connect up the 4507r as a Forced 100/Full connection just like the current Router port. No link light. Huh? It matches the settings on the Router. Why isn’t this working. Remember, I said it only Auto Senses the cable in Auto/Auto mode. So, I switch to Auto/Auto and the link light comes up and I show the connection has Auto-Negotiated at 100/Half. Uh, Oh! Now we have a duplex mismatch, which tells me the other end is 100/Full.

Now, we have a problem. If I can’t do Auto/Auto or 100/Full, then what can I do. We call AT&T. Keep in mind, this is after over 12 hours in a loud, cold data center. We’re tired and cranky. It’s at this point we learn the switch in the middle is a Cisco 3550 switch.

I didn’t look up the 3550 switch until later, but it does not do Auto-MDIX. This is the reason why 100/Full on both ends does not provide a link, but Auto/Auto does a duplex mismatch. It is also the reason the AT&T tech can see the connection is sending and receiving, but we are not receiving any data.

3550 Switch# show interfaces transceiver properties
Name : Fa0/1
Administrative Speed: auto 10 100
Administrative Duplex: auto
Administrative Auto-MDIX: N/A
Administrative Power Inline: enable
Operational Speed: 100
Operational Duplex: full
Operational Auto-MDIX: N/A

The real issue is the partial use of Auto-MDIX made troubleshooting this very tricky. Even worse, the fact that I hadn’t though crossover cable in some 3+ years made me even forget the rules.

And if you are wondering if this worked, I’ll let you know down the road. We had to leave everything in place until a few weeks later, so we can get the cable changed out at the datacenter.