Crossover cable from router to switch? WHY?

It has to do with the "standard" wiring for Data Communication Equipment (DCE) and Data Termination/Terminal Equipment (DTE).  If you get two pieces of equipment that are pinned out the same, you need a crossover.  If you get one DCE and one DTE then you need a straight through cable.  Some newer equipment has the ability to automatically crossover - Media Dependent Interface / Crossover or MDI/X.

I believe a router would be considered DTE and the switch would be DCE - hence, you don't normally need a crossover cable.

See these pages:

http://en.wikipedia.org/wiki/Data_terminal_equipment
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213919,00.html

> I believe a router would be considered DTE and the switch would be DCE - hence, you don't normally need a crossover cable.

I have viewed it as the other way around. Now that MDI/X is commonplace you may as well have all crossovers rather than straights as crossovers are more useful in adhoc situations.

Bleh, misread, ignore plz.

Well, I think the CSU/DSU is DCE, router DTE, switch DCE, and PC/device is DTE.  I could be wrong ... :)

I think SOHO gear is especially accomodating in the MDI/X area - don't know about professional routers though (Cisco) - but I'm seeing it on Cisco switches - 3750's, 4500's, and 6500's.  

Somewhere I saw a little male/female adapter which would convert from 568A to B pinout - converting a straight to a cross or vice versa.  Forget where I saw it.

Oh yea for your first question, If you get it wrong it simply will not work. It will not hurt anything but no data will cross that connection.

Ok but I just have a linksys router and a couple of cheap switches and I have the router than a switch and a couple comp's then another switch off that switch and a couple more computers off that switch and I dont have any cross over cables! All my cables are straight and it works just fine and has for 3 years so is this weird or what?

I thought I answered this ...

Router to switch - usually OK with straight through.
Switch to PC - definitely OK with straight through.
Switch to switch - usually requires a crossover - unless you're usins SOHO (Small Office Home Office) which has crossover circuitry built in so it autodetects what kind of connection is required.

So, no, it's not weird.

The linksys is a device made for the home, and most devices made n the last few years especially for the home do not need crossover cables.

Not weird

In a data cable there are 8 wires.  Only 4 of them are used to complete the circuit.  Out of these 4 two are used to send data and two are used to receive data.  

Nics are normally set up for the following...
Pins one and two are used for transmit.
Pins three and six are used for receive.

If you were to connect two computers to each other then Computer A would send data on pairs 1 and 2 and Computer B would expect to receive that data on 3 and 6.  However, without a crossover cable Computer B would receive this data on pins 1 and 2 (which are its send pins not receive).  A crossover cable switches pins 1/2 and 3/6 in the cable so when Computer A sends data on pins 1 and 2 it is received on pins 3 and 6 by computer B and vice versa.

A switch on the other hand usually automatically crosses over a cable so there is no need for a cross over.  It sounds like you have a router connected to a switch (ok for straight through cable).  A few comps off this switch (ok for straight through cable).  And then another switch hanging off of the first switch.  Most likely you have the cable that runs to the second switch connected to its uplink port which should automatically cross it (so a straight through will be ok).  To test this out try plugging the cable that lights up the second switch into the 4th or 5th port.... does it still work?  If so make a crossover cable and replace the straight through cable... does it still work?  If so then your switch is definitely automatically switching between cable types for you.

Normally, I don't like to whine ... but .... today I'm whiney ... I gave you the exact answer within 6 minutes of your question and no point split - at least!?!?

reason being is you didn't tell me that home networking components usually dont need a crossover cable as the winner of the points did. I knew the answer you gave me but I wanted to know why my network at home was working without any crossover cables and I have switches hooked to switches  "You will almost never need cross over cables for any technology that was designed for home based technology. " was what I needed to know but thanks for your quick answer!!

I thought you were asking, "What is suppose to happen if you hook a switch to a router that doesnt auto cross over and you dont use a crossover cable and same ...  why shouldnt this work and why is mine working for 3 years now?"

Well, my first comment said, "Some newer equipment has the ability to automatically crossover "

My second comment, "I think SOHO gear is especially accomodating in the MDI/X area"  

So, I directly answered "what is supposed to happen" and "why should't this work" along with "why is mine working".

SOHO = Small Office / Home Office.

No big deal.  Thanks.

For an Ethernet connection to work you need an odd number of x's. So for instance if you have a network switch, which is typically an MDIX (medium device Interface crossover) connecting to a pc which is MDI, then you would use a straight through cable. To connect to switches together, you would have two MDIX ports and would need to add a third x, being the cross over cable. A router is typically an MDI since they have been built for years out of standard computers.  
If you have one of the interfaces using auto cross, then a straight through or crossover will work.

I already had a answerto this a year ago when it was asked but thanks for helping