Use of straight through and cross over cable

[vishalpatil86]

hi,

according to icnd1, we need to use crossover cable to connect similar devices, lets say router to router or switch to swicth..

does this really matter? I mean, what if i use straight through cable to connect one switch to another?

thanks

[michael o'nan]

For the test, yes. For real life device now use auto MDIX and can detect the cable and correct the pinout. 

[Rowell Dionicio]

Most newer devices will use auto MDIX. Here's some configuration examples:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960x/software/15-0_2_EX/int_hw_components/configuration_guide/b_int_152ex_2960-x_cg/b_int_152ex_2960-x_cg_chapter_011.html

[vishalpatil86]

Hi,

I want to extend this discussion a little bit. If only two pairs are used in receiving and transmitting.

What are other two pairs for?

[rajeshroshan02]

Hi Vishal,

The cable Pinouts depends on type of cable and ethernet speed.

In Cat5 cables supports 10BaseT, 100BaseT, we use on two pairs for Rx and TX.

But CAT5 E and CAT6 supports 1000 baseT, we use all 4 pairs for bi derection transmission.

Pin	Name	Description
1	BI_DA+	Bi-directional pair A +
2	BI_DA-	Bi-directional pair A -
3	BI_DB+	Bi-directional pair B +
4	BI_DC+	Bi-directional pair C +
5	BI_DC-	Bi-directional pair C -
6	BI_DB-	Bi-directional pair B -
7	BI_DD+	Bi-directional pair D +
8	BI_DD-	Bi-directional pair D -

[Joseph W. Doherty]

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Posting

Hmm, I don't know, does it really matter whether the link between devices can be used?  (Laugh)

As the other posters have noted, many (newer) devices now support auto MDI/MDIX, but some don't.  (Also, on devices that do, auto negotiation might be required to be enabled.)  Some older devices sometimes have "special" uplink ports or manual method to switch the port's MDI/MDIX setting, and for those, too, you can use a strait through cable.

If none of the above, however, you'll need a cross over cable for "similar" device types, i.e. those whose tx/rx pins, on the port, are the same.

You need a cross over cable if both sides are MDI or MDIX (i.e. the same), straight cable if both sides are different.

[Sajith Joseph · Accepted solution]

Hi Vishal,

A switch is an MDI-X device. That means the pin arrangement is as follows:

Pin#  Function

1       Receiver+

2       Receiver-

3       Transmitter+

6       Transmitter-

So if you want to connect another switch to this switch, you have to make sure the Transmitter pins are connected to Receiver pins and vice versa. If you use a straight thru cable, Receiver pins will be connected to Receiver pins on the other end and same for transmitter pins and this will not work for sending or receiving data. Therefore you need to use a crossover cable. Remember in crossover: 1->3 ; 2 -> 6 ; 3 -> 1 ; 6 -> 2

Hope that helps

Sajith

[gmicek · Accepted solution]

Also, note that switches and hubs are both MDI-X devices while routers and PCs are MDI devices. You have to use a crossover cable when using MDI together or MDI-X together, but you would use a straight-through cable when connecting MDI to MDI-X and vice versa.

Thus, a router and a router would use a crossover cable to connect, a router and a switch would use a straight-through cable, but a router and a PC would use a crossover cable.

Again, in practical use most devices nowadays use auto-MDI, so it usually doesn't matter.