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Crossover Cables

Ethernet crossover cables are required when connecting like devices—such as PC to PC or router to router—because MDI interfaces share the same transmit and receive pin assignments and need the cable to swap them. Straight-through cables work for MDI-to-MDIX connections, such as a computer or router plugging into a switch.

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About this video

Ethernet interfaces are classified as either MDI or MDIX based on how their transmit and receive pins are assigned. MDI devices—including computers and routers—transmit on pins one and two and receive on pins three and six, while MDIX devices—typically switches—reverse that arrangement. This complementary design means that connecting an MDI device to an MDIX device with a standard straight-through cable automatically aligns each side's transmit pins with the other's receive pins, allowing communication without any modification to the cable itself. Problems arise when two MDI devices are connected directly to each other. With a straight-through cable in place, both devices transmit on the same pins and listen on the same pins, so neither side receives a signal. A crossover cable corrects this by physically swapping pins one and two with pins three and six inside the cable, ensuring that one device's transmit path feeds the other device's receive path. This is the scenario that requires a crossover: PC to PC, PC to router, router to router, or switch to switch when the switch ports involved are both MDI or both MDIX. In a typical network layout, end devices and routers connect into switches using straight-through cables, since the MDI-to-MDIX pairing handles the pin alignment automatically. Crossover cables become necessary only when connecting like device types directly. Some switches also include dedicated uplink ports that change the pin assignment to accommodate inter-switch connections with a straight-through cable. Modern networking hardware commonly supports Auto MDI-MDIX, a feature that allows individual ports to detect the connected device type and automatically adjust their pin configuration, effectively eliminating the need to select the correct cable type in most situations—though legacy or non-auto-sensing equipment may still require a crossover cable in certain scenarios.

What you'll learn

What's covered

Crossover vs. Straight-Through Cables

Aligned to

Cisco CCNA
1.3 Compare physical interface and cabling types
1.4 Identify interface and cable issues
CompTIA Network+
1.5 Compare and contrast transmission media and transceivers
Cisco CCST Networking
3.1 Identify cables and connectors commonly used in local area networks
CompTIA A+ Core 1
3.1 Explain basic cable types and their connectors, features, and purposes

Key terms

MDI
Medium Dependent Interface; a standard Ethernet interface type found on end devices such as PCs and routers where pins 1 and 2 transmit and pins 3 and 6 receive.
Medium Dependent Interface Crossover
MDIX
A feature in modern network interfaces that automatically detects and electrically corrects straight-through versus crossover cable requirements, eliminating the need to use specific cable types when connecting similar devices. Auto-MDIX is standard on most contemporary switches and NICs.
Straight-Through Cable
An Ethernet cable terminated with the same wiring standard (T568A or T568B) on both ends, used to connect dissimilar devices such as a computer to a switch.
Crossover Cable
An Ethernet cable terminated with T568A on one end and T568B on the other, crossing transmit and receive pairs to connect two similar devices such as computer to computer.
Auto MDI-MDIX
A feature on modern network equipment that automatically detects and adjusts a port's MDI or MDIX configuration to match the connected device, eliminating the need for manual cable selection.

Transcript

I often find that there's confusion on crossover cables and when to use them, so let's go over the different types of equipment there are and when to use a regular cable.

MDI and MDIX

When it comes to Ethernet, there are two different types of equipment out there: MDI and MDIX. MDI stands for medium dependent interface, and the X at the end just means that it's crossed over. What I have here are two pieces of equipment: a computer, which functions as an MDI, and a switch, which has MDIX interfaces on it. The MDI interfaces have pins one and two as the transmit and pins three and six as the receive, versus the MDIX interfaces, which have pins one and two as the receive and pins three and six as the transmit.

So when you're connecting these two devices with a straight-through cable, everything lines up correctly. You don't have any problems, and so most of your equipment that's meant to connect together, that is switches connecting into computers, in those cases a straight-through cable is fine. The problem is when we start connecting MDI to MDI equipment, because now if we were to connect MDI together with a straight-through, then the transmits are both trying to transmit on the same line but no one's listening, and they're trying to receive on the same line but no one's talking. So this is a problem.

What a crossover cable does

That's why we use a crossover cable. A crossover cable will take pins one and two and bring them to three and six, and pins three and six to pins one and two. Now this computer that's talking on pins one and two is receiving on pins three and six, and vice versa. Typically with crossover cables, there are a couple of different types, but with our T568A and T568B crossover cables, these are all going to be straight-through lines from there. So this is when we're connecting MDI to MDI equipment.

A typical network

Let's take what a network typically looks like. We have all of these devices on the outside that plug into a switch. All of these devices are MDI type connectors, where the switch tends to be an MDIX type connector. Then we usually plug the router into a switch, so this again is typically an MDI. Whenever we're plugging an MDI into an MDI, we want a crossover cable. But if it's an MDI to MDIX, then we just want a straight-through. So all of these cables here, and this cable here, are fine as straight-through cables. But if we were to connect these two devices together, or these two devices together, then these would require a crossover cable to connect from the MDI to MDI equipment.

Here's a table that might describe it in a little different way. Here we have MDI equipment, which would be the PCs and the routers, and MDIX interfaces on the switches. If I were to follow this through with this table, a switch plugged into a PC is a straight-through, and a switch plugged into a router is a straight-through. Same thing: switch to PC is a straight-through, switch to a router is a straight-through. So there we see all of the straight-through connectors. The rest of these connections, though, are like equipment. This is MDI to MDI. So when we're connecting like equipment, we want crossover cables. A PC to PC is a crossover. PC to router, a crossover. So that's where we see these crossover cables. And also a switch to switch is a crossover.

Uplink ports and auto MDI/MDIX

Now, we can see switches that have both MDI and MDIX on them, and there are times when we run through where it's a switch to switch and you would still use a straight-through to connect that, if those are the MDI connectors or the uplink connectors. There is also such a thing as an auto MDI/MDIX. Let's say this switch is an auto MDI/MDIX. When we connect whatever this device is into this switch on whatever port, it would auto sense and change that port to be either MDI or MDIX to adjust to whatever equipment is on the other side. So a lot of our equipment now can actually auto adjust to this, making it even less likely that we have those crossover cables. Although every once in a while I'll run into a case where I still have to use a crossover cable to connect two different pieces of equipment that are the same type.

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