When to Use a Crossover Cable

For technicians that are beginning their network careers today, there is often a struggle to learn things that have just always automagically worked for them. One example of this is the crossover cable. Most modern network devices have a function called auto-mdix which will allow the incorrect use of straight through and crossover cables. As a result no one really has to learn the correct use. The exception is, of course, the CCNA exam.

For the CCNA exams, it is important to understand which cable is appropriate, a crossover or straight through cable. So how does one know which cable to use? The answer is quite simple, but is often confused and made complicated. The confusion is a result of people trying to memorize all of the scenarios for which a crossover is used. That is the wrong approach.

To easily understand where to use a crossover cable, we need to examine a couple of key concepts. The first concept is that there are Ethernet NICs that are wired like a hub or switch and there are NICs that are wired like a PC. So we basically have these two categories. Any time that we are connecting a device from one category to a device in the other category, we use a standard straight through cable. If we ever need to connect two devices that are in the same category, we should be thinking about a crossover cable.

Key Concept–Two devices in the same category use a crossover cable

Looking at our two categories, our first category is hubs and switches. Is there anything else? Well I guess someone could argue that there are routers with ESW (ethernet switch modules or cards), but their name pretty much tells us that they are a switch that is inserted into the router. As such, these specific cards act like a switch. So the simple and correct answer is that this first category only includes switches (and hubs if you have any of those left).

What is in the second category? This is really easy–everything else. To be complete, this includes the following: routers (except ESW ports), print servers, PCs, access-points, servers, and any other Ethernet connected device. If it isn’t a switch or a hub it goes into category two.

Now we can go back to the previous concept and apply it to these categories. This concept stated that when connecting two devices in the same category, a crossover cable was appropriate. So let’s test our knowledge.

For each of the connections, what is the appropriate cable type?

Hub<—>Router (answer: straight through–hub in category 1, router in category 2)

Hub<—>Hub (answer: crossover–both devices in category 1)

PC<—>Print Server (answer: crossover–both devices in category 2)

PC<—>Router (answer: crossover–both devices in category 2)

Switch<—>Hub (answer: crossover–both devices in category 1)

Switch<—>Switch (answer: crossover–both devices in category 1)

For those who want to take a look at the pinout for a crossover cable there is this and many other examples on the internet.

Understanding the use case for a crossover cable is not that difficult. What is difficult is trying to memorize all of the use cases. I hate memorizing, so I always recommend simply understanding that we have two categories of Ethernet devices. A crossover cable is only needed when we are interconnecting two devices in the same category. It is also worth mentioning that in practice auto-mdix has been available for many years and makes this discussion a non issue in most real world cases.

About Paul Stewart, CCIE 26009 (Security)

Paul is a Network and Security Engineer, Trainer and Blogger who enjoys understanding how things really work. With over 15 years of experience in the technology industry, Paul has helped many organizations build, maintain and secure their networks and systems.
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25 Responses to When to Use a Crossover Cable

  1. Timo says:

    Hi,
    Here’s a great blog post that explains the possible problems with auto-mdix and incorrect cabling, http://cciepursuit.wordpress.com/2007/07/08/switch-cabling-and-auto-mdix/

    Timo

  2. Will says:

    I took a new postion recently and had to get my Network+ certification(currently 13 year network engineer). I was told it was very easy and just go take it the next day. So I did (and passed).
    One of the questions was what type of cable do you use to connect a PC to a router
    A)Rollover
    B)Straight Through
    C)Cross Over
    I was looking for D) all of the above. No clue what they were looking for. MDIX would let me use either to connect to the NIC and rolloever would let me connect to the console port. I chose cross over anyway.

    • pen says:

      And you were wrong here.

      • I think the challenge with a question like that is that it is subject to the material. It could be A (typical cable for bootstrapping a Cisco), B (are we talking about a router with an integrated switch–aka most every homo and SOHO router, or does it do Auto-MDIX) or C (the correct CCNA answer).

        I get the frustration, answer to a question should be technically the same regardless of whether it is being asked by Comptia, Cisco or Jack and Jill’s pre-hire technology testing.

        Will says he chose C which would have also been my choice (without being able to read the mind of the test author). If there are specifics around Comptia Network+ in regard to this, feel free to share the logic and why you think that is incorrect.

  3. Pingback: Steps On How To Make A Ethernet Crossover Cable-The How To Man

  4. Pradeep says:

    Thanks alot Paul, Blog is very useful. I appreciate your effort.

    • Thanks for the feedback. It is funny that this still comes up time to time. Most equipment actually supports an automatic crossover process. However, there are some exceptions and there can always be questions (interview and test) about this. In those cases, they are assuming there is no such thing as auto crossover.

  5. anil kumar says:

    sir i have some doughts
    my friend was asking these questions
    1.why to use cross cable(T568a) in between the system and cisco 300mbps access point?
    2.all devises must having the ip address and mac address?

  6. user says:

    Thanks, this made it sensible for me.

  7. Frank DeAngelo says:

    Sometime ago I remember a new guy telling me we did not need a cross over cable when connecting two HP server because they had either a 100 Mbs or 1000 Mbs NIC. I forget which speed. He correctly stated that the NIC in question will use auto sensing. Do you recall which speed? Perhaps auto sensing is part of the specification for the speed in question. I do agree that it is always a good idea to have a cross over cable handy just in case. Plus I always have an RJ45 coupler handy in case I have to combine two cables.

  8. Jamie Young says:

    Thanks for the explanation Paul.

    I would suggest visualising it like this to make it easier to remember. ( I hope the format comes out okay when this posts. There’s no “preview button”! )

    Group 1 Group 2
    __________ _________
    | | | |
    | Switch | | PC |
    | | | Router |
    | Hub | | Servers |
    | | | APs |
    |_________| |_________|

    Now… If you can draw a STRAIGHT Line, between devices you are connecting, you should use a STRAIGHT through cable. (eg: Switch [straight line] to Router …. Hub [Straight line] to Server.)

    If you connect a HUB to a SWITCH, your line will move out of the group 1 box, and bend back into the group 1 box, to join up with “SWITCH”. Therefore, you CAN’T USE a STRAIGHT line to connect them… so you should NOT use a straight through cable.

    • Jamie Young says:

      Oh no. WordPress ate all my whitespace!

      I’ll explain the diagram instead.

      Draw two boxes. In the left box, write “Hub” and “Switch”. In the right box, write, “PC”, “Router”, “Servers”, “APs”. Or if you like, simply put “Everything Else”.

      Then follow the other instructions in my comment.

  9. john stephen says:

    what about router to router?

    • Routers have the same type of Interface as a PC. In any case, any connection between two like interfaces traditionally require a crossover cable. I say traditionally because a lot of current routers will automagically work through what is known as auto mdix.

      Router <> Router (crossover)
      Router <> Switch (straight through)

  10. please answer this question:
    we can connect router and switch by crossover cable?

    • ANTONIO SANTOS says:

      @Alisina. It depends. “If” you are referring to a traditional Layer-2 Switch and a Layer-3 Router, the answer is NO. For anything else, we’d need more specifics on each device.

    • You shouldn’t. A router connected to a switch should use a straight through cable. Modern switches can use auto-mdix to automagically make it work if you use a straight through cable. There are also some routers that use integrated switches. So if you are connecting an integrated switch to another switch, a crossover cable would be appreciate.

      • Mark says:

        Paul This was a wonderful explanation for someone like me who really has no CCNA experience.
        Just as how you came up with an excellent explanation to understand it, I have found an excellent explanation to remember it. Here goes;

        I use Man and Woman. Man is in First category and Woman is in the Second Category.
        When a Man connects with a Woman he uses his boner with is somewhat straight or just straight, which represents the straight line.

        However if a Man connects to another Man, he would have to cross the line to do it. So this results in a Cross Over situation. Hence if you connect two devices of the same type, you use a cross over cable.

        Hope this helps those who still had issues understanding.

  11. Richard Sliski says:

    Haha!

  12. oldandnew says:

    Ok. so I have” cable Aires modem with 4 port router and wifi built in.” going to “netgear router”( for dhcp )going tool older linksys 24 port 10/100 …… now where should i use the crossover?

  13. richard farmer says:

    This made a lot more since to me. since I’m honorific .lol well played

  14. kdalts says:

    Folks

    I’m no expert but I’ve built a fairly complex home network to connect multiple devices all around the house. It also allows me to switch use a proxy where for media that must have a UK IP (I live in Italy).

    I have my incoming router (Italy IP) attached to 2 hubs (UK IP – 1 upstairs and 1 downstairs). The upper floor hub then connects to an 8 port managed switch that feeds cable connected devices (Pc, printer, Amazon Fire TV, PS3, Sky decoder and TV, NAS) so they each have a UK IP.

    On the lower floor the UK IP hub then connects to a 3rd Hub configured to Italy IP. This is for Netflix because it knows if you are behind a proxy so I have a choice of UK or Italy all around the house depending on the device. On the lower floor I have TV, another Sky decoder, Sonos and another Amazon Fire TV box. IE: Sky needs a UK IP, Netflix does not like a proxy and my NAS needs to be on the same network as my Sonos system.

    all sounds incredibly complex but it actually works.

    My point, I did not know about cross-over cables and so everything works using straight through cables. Maybe my Netgear D6200 router is smarter enough to figure it out?

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