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I have a cross-over network cable that I used to use to connect two computers using their network cards.

I now want to use that cable to connect a computer to a router. Is this not possible? Why?

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Worst case, you can easily turn a cross-over cable into a normal cable or vice-versa, assuming you have a crimper/wire-stripper/RJ-45 head. Just cut one end of the cable off, swap the 1-2 pair of wires with the 3-6 pair, and recrimp. –  BlueRaja Dec 21 '11 at 18:21

5 Answers 5

up vote 17 down vote accepted

Almost all recent network cards support auto-crossover (Auto-MDIX). With this feature, whether a cable is normal or crossover doesn't matter - you can connect two computers using a normal cable, so although I have not tested it I'm sure it would also allow connecting such a card to a router using a crossover cable.

If this feature is not supported by your network card, you'll have to use a 'normal' cable.

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This is required by the gigabit ethernet standard and therefore will work with any gigabit NIC. Some older 100mb switches supported auto-crossover as well but it is not a standards requirement. –  Lamar B Dec 21 '11 at 7:52
    
We use retractable cat6 crossovers in our conference room. They replaced the cat5 normals when we realized machines with gigabit cards sometimes wouldn't notice they were connected. Likely if both are gigabit capable, odds are very good it will work. –  Melikoth Dec 21 '11 at 12:53

Scenario 1

You have cat5e or Cat 6 cables. (Straight through (ST) or Cross Over (CO) Your switch is a gigabit switch that has the auto sensing Auto MDX standard. Your NIC card is a Gigabit NIC that has the MDX standard as well.

CO or ST cables do not matter in most cases since the MDX feature will auto correct the connection.

Scenario 2 Same as above but your NIC card does not have MDX standard. PC to switch with any cable. Switch to router any cable. Only the Switch can do the correcting. If the Router and the NIC card do not support MDX standard you would need a CO cable to directly connect the two.

Rule of thumb is CO for like devices BUT with the introduction of Auto-MDX in the Gigabit standard that CISCO rule of thumb is becoming an "old" standard that is being phased out with the newer technology.

The new tech is attempting to address all of the issues with ST and CO cabling and requirements in devices. With Auto-MDIX and other features in new equipment the needs of CO and ST technologies are becoming a thing of the past to help ease the issues in installation and upgrading. Down the road there will be just one cabling (Straight through)for cables.

The new rule of thumb is like this. If the equipment is newer than 2010 AND is Gigabit and the cabling is Cat5e or higher then you have a very good chance of having the ability to utilize Auto-MDIX. At least one device will to the auto crossover.

But it is still smart to keep at least one CO cable handy to test connections between older equipment or equipment you are not sure of. It is also good to know the older requirements and rules of thumb since we are still in a "transition phase" and you will find in many situations that there are a mixture of old equipment and new equipment and you may still need a few CO connections made for compatibility.

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I'll have to continue to disagree with everyone here.

My experience and all materials I can find on the Internet except Wikipedia says that barring auto-crossover functionality, a cross-over cable is required to connect a computer to a router directly (i.e. without a switch or hub in between).

I don't know why so many insist otherwise and happily vote up a wrong answer. Maybe I am wrong and everybody else is right, but it doesn't look like the others looked this up or tried it out.

"As is the case with serial cables, Ethernet cables (Cat5 RJ-45) can be straightthrough or crossover. Simply connect the PC to the router via a Cat5 RJ-45 crossover cable. The PC’s and router’s network interface cards should display a green light." www.cs.virginia.edu/~itlab/workshop/Connections.pdf

"Straight-through cable is used to connect:

* Host to switch or hub
* Router to switch or hub

Crossover cable can be used to connect:

* Switch to switch
* Hub to hub
* Host to host
* Hub to switch
* Router direct to host"

http://www.kenmanohar.com/blog/tag/crossover/

"You should use a crossover cable between a router and a server or, if you have a switch, use a straight-through cable from the router to the switch and from the switch to the server."

http://computernetworkingnotes.com/ccna_certifications/back_up_restore.htm

"This type of cable is a “regular” or straight-through cable. You use this type of cable to connect a PC to a hub/switch or a router to a switch. [...] So, you can use a crossover cable to connect a PC to a PC, directly, without any hub or switch. You can also use a crossover cable to connect two switches together."

http://www.petri.co.il/csc_the_basics_of_ethernet_cabling.htm

So here's the deal:

Straight cable: host/router to hub/switch

Crossover cable: host/router to host/router, hub/switch to hub/switch

The router belongs in the host/router class of devices, not into some hub/switch/router class.

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No comment explaining the downvote? Am I wrong? Are all my sources wrong? I am quoting Cisco's own explanation after all. Curious. –  Andrew J. Brehm Dec 23 '11 at 7:58
    
since this is superuser, the expectation is that the OP is wanting to connect to a SOHO router via the included SWITCH ports - thus the situation is SWITCH<->HOST = Straight-through. –  Jeremy W Jun 7 '12 at 13:44
    
That doesn't mean that correct answers should be voted down. –  Andrew J. Brehm Jun 7 '12 at 16:15
    
Not sure why your calling out everyone being wrong for one answer. When I worked on commercial routers crossover cabling was nessicary. The rule of thumb is alike devices get crossover and different devices get straight through. Not sure what makes my answer here wrong, please explain. –  Terrance Jul 31 '12 at 20:28
    
Please read my answer. It explains in some detail why barring autocrossover functionality, a crossover cable is required. Using your rule of thumb, routers and hosts (which are alike devices) get crossover while hosts/routers and switches/hubs (which are different devices) get straight through. What makes your answer wrong is that it contradicts your (correct) rule of thumb. However, my complaint was not about your answer being wrong but about my correct answer being voted down. –  Andrew J. Brehm Jul 31 '12 at 20:36

That would depend on a lot of things. Routers are generally made to use "normal" cables to connect to computers as they are wired to work that way. Most modern devices and network cards can use a normal cable as a cross-over cable or a normal cable. Unless all your devices are pre-1998 you should be able to use a cross-over cable with a router.

Wikipedia entry on Cross Over Cable and Auto-MDIX

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1) computernetworkingnotes.com/ccna_certifications/… begs to differ. As does kenmanohar.com/blog/tag/crossover quoting the CCNA book. It would also surprise me since a xover cable is needed for host to host and a router is a host. 2) If connected via a switch or hub, two straight cables would be used. And yes, there is a via. That's what a switch is, a device via which data travels. –  Andrew J. Brehm Dec 21 '11 at 13:06
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@myotherme No. I was responding to your claim that routers are normally configured to use normal cables to connect to computers. But absent network adapters that automatically adapt to cable type, they are not. I corrected the point and said that connections between routers and hosts are normally xover cables, not straight cables. The Grawity claimed, falsely, that routers are like switches in that they expect straight cable connections to hosts. I corrected the claim again and linked to sources. That's when you suddenly came back and claimed that I was "arguing reality". –  Andrew J. Brehm Dec 22 '11 at 11:07
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@myotherme Pre-1998 routers might not have network adapters that automatically adapt to cable type in which case you would need a xover cable to connect directly to the router. I don't know why the Wikipedia article classes a router like a switch rather than like a host. The article I referred to claims the opposite which has also been m experience: "You should use a crossover cable between a router and a server or, if you have a switch, use a straight-through cable from the router to the switch and from the switch to the server." –  Andrew J. Brehm Dec 22 '11 at 11:12
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Andrew J. Brehm: I have only worked with small routers (baby Cisco and home ADSL stuff), and the ports on them where always just like those on a switch/hub, in that we had to use straight cables to connect with them (from hosts). Perhaps it is on bigger caliber routers that this applies, which would make sense, because there you generally run from the routers to the switches. On most of the small-business/home routers the router IS the switch for the hosts... –  myotherme Dec 22 '11 at 16:49
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Then my answer is still correct as a crossover cable will always work, whether it is a pre-1998 router without built-in switch or a post-1998 router with or without built-in switch. But a normal cable won't work with pre-1998 routers. –  Andrew J. Brehm Dec 23 '11 at 8:49

The other answers explain current solutions but I kinda felt like going into the original problem cause It allows me to reminisce on my time in the army as a cable dog.....sigh.....

The original problem simplified

Think about it like this. For your two pcs (or any alike devices really) your nic has a set of predefined "standard" dedicated directions for transmitting data and receiving data.(See 10baset and Category 5 for a more detailed look) So with that lets say two wire pairs are set to receive traffic and two are set to send.

|PC1 TX|------>X<-----|PC2 TX|
|PC1 RX|()<--- ? --->()|PC2 RX|

Because both sides are sending on the same line neither is getting anything. (Think of two water pipes trying to push water on both ends... )

Enter the router

However with a router the connections are set to the reverse.

|PC1 TX|------------>()|RTR RX|
|PC1 RX|()<----------- |RTR TX|

And with smarter routing devices your router can figure out what kind of cable you are using and act accordingly.

Yes this is a a bit of an oversimplification so forgive me if I left something out.

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Thank you for providing the base info that I made the mistake of taking for granted. –  myotherme Dec 21 '11 at 16:03

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