How to tell if crossover cable is needed or straight-throgh?

gwhuang5398 · ‎02-02-2006

I know a cross over cable is needed to connect router directly to router, but how about switch to switch? switch to router? etc.

I had to use a cross over cable to connect a 2849 switch to a 6509 with Sup720/MSFC3. Is it because 6509 is a Layer 3 switch while 2948 is a Layer 2 device?

Thanks

Gary

Bobby Thekkekandam · ‎02-02-2006

Hi Gary,

Any time you connect homogenous devices together (router to router, switch to switch, PC to PC), you need to use a crossover cable.

Ethernet ports on Routers and switches have pinouts such that the tx and rx wires are reversed, so a straight through cable will match up the tx on one end to the rx on the other, and vice versa.

Routers and PCs have the same pinout, so if you connected a Router to a PC, you would also need a crossover cable.

Some switches have a capability to detect which pin-out is necessary. This feature is called auto-mdix.

HTH,

Bobby

thisisshanky · ‎02-02-2006

Networking equipments can be generally classified into DTE and DCE equipments. Based on these simple rules, we can identify when to use an Xover or straight through.

a. DTE to DTE devices use Xover

b. DCE to DCE devices use Xover

c. DTE to DCE needs straight through.

Router, PC, laptop, server etc are DTE devices

Switch, Hub etc are DCE devices.

You do the math!

Now there is a feature called MDIX thats available in new routers, switches etc. This feature if enabled on a per port basis will allow the use of a straight or cross cable and doesnt care if the other end is a DTE or DCE type device.

HTH

PS: please remember to rate helpful posts!

Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus

saravanababu22 · ‎02-02-2006

like devices - cross cables

unlike devices - straight cables

bhedlund · ‎02-02-2006

Router - Router = xover

Router - Switch = straight

Switch - Switch = xover

PC - PC = xover

PC - switch = straight

PC - router = xover

For the purposes of this discussion, a 6500, Layer3 or not, is considered a switch.

HTH. Please rate this post.

-Brad

felipe_alfaro · ‎02-03-2006

Generally speaking, you will need to use a cross-over cable when connecting two similar equipments, like switch-to-switch or router-to-router.

Basically, we classifiy equipment into DTE (Data Terminal Equipment) and DCE (Data Circuit Equipment). End-user stations are DTE (computers, laptops, Ethernet printers, Wireless access points), while switching and routing devices are DCE (switches, hubs, routers).

Cross-over cables are used to connect DCE-to-DCE, or DTE-to-DTE. However, some modern equipment is able to detect whether its peer is a DCE or DTE and whether the connection is using a cross-over cable or a direct-through cable and adjust properly. For example, my Apple PowerBook, although it is a DTE, has the ability to connect to my switch using either a cross-over cable or a direct-through cable. My low-end Surecomm switch has also this ability.