Why router connect to switch with a straight-through cable?

Answered Question
Jan 9th, 2008

Everyone said router need a straight-through cable to connect to switch. Why?

I can't see any spec like that on the standard. Is it a habit?

Correct Answer by bvsnarayana03 about 9 years 1 month ago

In networking world, devices are identified as Terminal equipments & communications equipments.


Terminal equipments: includes desktops & routers


communication euipments: switches


Similar devices are connected via cross cable & non-similar devices are connected with straight.

Correct Answer by Danilo Dy about 9 years 1 month ago

Hi,


Its a standard.


DTE to DTE is crossed cable

DCE to DCE is crossed cable

DTE to DCE is straight cable


Same device is crossed, different device is straight.


But now, new devices support Auto-MDIX. Doesn't matter which cable you use. But sometimes, for troubleshooting sanity, use the standard.


http://www.intel.com/support/network/sb/cs-020673.htm


Regards,

Dandy

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Correct Answer
Danilo Dy Wed, 01/09/2008 - 01:07

Hi,


Its a standard.


DTE to DTE is crossed cable

DCE to DCE is crossed cable

DTE to DCE is straight cable


Same device is crossed, different device is straight.


But now, new devices support Auto-MDIX. Doesn't matter which cable you use. But sometimes, for troubleshooting sanity, use the standard.


http://www.intel.com/support/network/sb/cs-020673.htm


Regards,

Dandy

Correct Answer
bvsnarayana03 Wed, 01/09/2008 - 01:08

In networking world, devices are identified as Terminal equipments & communications equipments.


Terminal equipments: includes desktops & routers


communication euipments: switches


Similar devices are connected via cross cable & non-similar devices are connected with straight.

shrikar.dange Wed, 01/09/2008 - 02:14

The new feature call AUTO-MDIX adjusts the signaling of the port according to the type of wire attached and signals it recieves through it.

The standards are as stated by the dandy.But now a days the AUTO-MDIX supports all combinations to work absolutely fine.


Regards,

shri :)

Edison Ortiz Wed, 01/09/2008 - 19:39

Rule of thumb on cabling.


1) non-switch device to switch device = straight-through cable


2) Any other connection = crossover cable


A router is considered a non-switch device.


Until the introduction of Auto-MDIX, you needed a crossover cable for switch-to-switch connectivity.


One caveat on Auto-MDIX, if you configure your speed/duplex manually, you disable Auto-MDIX.


HTH,


___


Edison.


nikhil.engineer Wed, 01/09/2008 - 22:53

To add to above posts auto-mdix supports only to speed above 1000Mbps so if your connecting with ethernet then you have to take care of the devices and cable your going to use.


Cheers,

Nikhil E.

mohammedmahmoud Thu, 01/10/2008 - 03:54

Hi Lin,


If we take this question to the physical level, routers, wireless access point Ethernet ports, and PC NICs all send using pins 1 and 2, whereas hubs and switches send using pins 3 and 6. Straight-through cables are used when connecting devices that use the opposite pairs of pins to transmit data (Tx to Rx and Rx to Tx), if the devices are the same (use the same pins for Tx and Rx) then we must use a cross-over cable to cross the transmission.


BR,

Mohammed Mahmoud.

bvsnarayana03 Thu, 01/10/2008 - 03:59

Your comment was like a dessert after dinner.


In fact it was very valuable point. i never came across this explanation ever. Full points to you.

And last but not the least, when using a patch cable, the easiest way to identify if it is a straight-through or cross-over is, to look at both plugs at the same time with the golden pins facing you, look at the wire's color in the RJ45 plugs and read from left to right


A straight-through cable will shown the next config on both plugs:


pin 1: white-orange

pin 2: orange

pin 3: white-green

pin 4: blue

pin 5: white-blue

pin 6: green

pin 7: white-brown

pin 8: brown


A cross-over cable will shown the next config on one plug:


pin 1: white-orange

pin 2: orange

pin 3: white-green

pin 4: blue

pin 5: white-blue

pin 6: green

pin 7: white-brown

pin 8: brown


and will show the next configuration on the second plug:


pin 1: white-green

pin 2: green

pin 3: white-orange

pin 4: blue

pin 5: white-blue

pin 6: orange

pin 7: white-brown

pin 8: brown


But I strongly recommend you if you are buying a cross-over cable, to buy it of a different color, since not always they are labeled as straight-through or cross-over.


Remember, in ethernet:


pin 1: Tx+

pin 2: Tx-

Pin 3: Rx+

Pin 6: Rx-


And memorize this for the cross-over cable: "1 to 3, 2 to 6".


Hope it helps.

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