Crossover Cables

When You Need a Crossover Cable

When you're working with Packet Tracer and you're networking different machines, most of the time you're going to want to connect with a straight-through cable, but as I mentioned, occasionally you may need to connect with a crossover cable.

Let's just look at an example of where we might use a crossover cable. What I'm going to do is go to the end devices here and bring out a couple of PCs, and we're going to interconnect these using our standard straight-through copper cable. I'm going to select that and we're going to go from fast ethernet to fast ethernet, and you can see it immediately says that these are down — it has the down arrow and it's red. So that is not going to work for us. But if I were to use a copper cable to connect both of those, now we see that it is up. So this is an example where we would use a crossover cable.

Why That's the Case

Here's a typical local area network. We would have a router on the outside with a switch, and all of our end devices connecting into the switch. There's a cable that interconnects these different devices, and we call it ethernet.

Ethernet has eight different conductors to it, however we only use four of those, and they're coupled into pairs. Here we see a transmit positive and negative pair, and a receive positive and negative pair, and it uses pins 1, 2, 3 and 6 — those are the ones that ethernet uses. So we see the transmit and the receive.

The problem is that if we were to just connect this into another computer, as you can see, this transmit is connecting to that transmit and this receive is connecting to that receive. It's kind of like an ear listening to an ear and a mouth talking to a mouth. That's what is happening here, and what we want is an ear that's listening to a mouth and a mouth that's talking to an ear. That's what we want across here.

A switch is set up differently. A switch is set up to communicate to computers. What computers have is what we call an MDI type connection, versus a switch, which is set up with an MDI-X. We've got the mouthpiece of the computer talking to the ear, the receive end, of the switch, and then the talking piece of the switch is connected to the receive, the ear, of the computer. Now one of them is talking and the other one's listening on one line, and the other one's listening and the other one's talking on the other line, so it all works out. Now we have communication happening, because we have this MDI versus MDI-X setup on each of the sides.

So this is where you'd use a straight-through cable: one where MDI is talking to an MDI-X and communication is going to happen.

What the Crossover Does

The cases where we need a crossover are when there's a device on the network that has this MDI type connection talking to another device that also has an MDI connection, as in this case right here. Or it could be an MDI-X talking to an MDI-X. The point being, both devices can't be listening on the same connection, and they can't be talking on the same connection.

What this crossover cable does is exactly what it says it does: it crosses over. These transmit pairs go to the receive pairs, and the transmit over here goes to these receive pairs, and so now that communication is happening successfully.

When to Use Them

So then the question is when do we use these crossover cables. We're going to fill out this little chart of what's connecting to what and when we would use that. During this process, just remember that switches tend to be MDI-X, because they're used to connecting to all of the end devices, which are all MDI. But we also have this router here, and it usually connects into a switch, so in that case this router is an MDI as well.

So what we have is that when a switch connects to a PC, as in a case like this, or it connects to a router, then we just use a straight-through cable. This means that if a PC connects to a PC, or a switch connects to a switch, or a router connects to a router, then that's when we're going to use a crossover cable.

You may find that this is not always true, that you can connect a switch to a switch. Perhaps that's because auto-MDI-X is set up. Auto-MDI-X is when the switch is set up to auto-sense what's on the other side and adjust accordingly. So in this case right here, if you have two switches that are auto-MDI-X and they're connecting to these other devices, it doesn't matter which one it is — you can use a crossover, you can use a straight-through.

Why Not Just Use Crossovers Everywhere

Looking at this chart, it might give the impression that if your switches are using auto-MDI-X, then maybe you should just use all crossover cables. That is not advisable. In fact, if you're using auto-MDI-X, you actually want to use even more straight-through cables, because switch-to-switch connections can use these straight-through cables as well.

So why would we want to use all straight-through? The reason is that the majority of your connections really are going to be straight-through cables. You're really not going to use crossover cables very much at all; it's going to be pretty rare that you would use those.

Number two, it makes troubleshooting a little more complicated, because when you get too many crossover cables — let's say you're using patch panels and stuff, and you end up with two crossover cables connected together, that makes another straight-through cable. So that can cause problems. It really is that most of the time you want to use those straight-through cables.