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As everybody knows Cisco's console ports are RJ45. However, the cable they come bundled with is DB9 to RJ45. We have a KVM that allows managing network devices via a serial port but the port on the KVM is also RJ45, not DB9. What is the name of the cable to allow this connection (serial RJ45 to RJ45). Note: the standard Ethernet patch cable is also RJ45 to RJ45 but does not work.

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2 Answers

up vote 8 down vote accepted

You're correct, standard ethernet cables won't work due to the wiring sequence - cisco uses what's called a "rollover cable" for their serial consoles. An example of the pin-out for rollover can be found here:

http://pinouts.ru/NetworkCables/rj45_rollover_pinout.shtml

It's as simple as pin 8 on one end is swapped to pin 1 on the other end, pin 7 to pin 2, etc. All the way down the cable.

If you have some Cat-5 cable, RJ-45 ends, a crimper, and the know-how to crimp your own cables - you can make your own rollovers fairly easily! If you don't wish to do that, I'd suggest searching for "Cisco RJ-45 Rollover" or similar in order to find vendors from which you can purchase RJ-45 to RJ-45 rollover cables.

P.S: Double-check the documentation of your KVM - some KVMs that allow serial management require their own proprietary adapters for it, even if the port on the KVM is RJ-45. If it does require a proprietary adapter, there's a chance that it will support standard RJ-45.

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Rollover cable is different from Crossover (one used to directly connect two hosts)? –  Alex Dec 12 '11 at 4:33
1  
Yes, crossover has a different pinout as well. It has a different crossing pattern rather than straight "rolling over" - (see pinouts.ru/NetworkCables/Ethernet100BaseT4Crossover.shtml) - so a crossover cable won't work in lieu of a rollover cable. Hope this helps! –  haymaker Dec 12 '11 at 4:56
    
Thanks Haymaker, I think this is also a good page comparing 3 types on the same page with explanations: computercablestore.com/straight-through_crossover_rollover.aspx –  Alex Dec 12 '11 at 6:25
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Haymaker beat me to it...

You don't say what Cisco device or what KVM, but chances are you want what Cisco calls a rollover cable:

Pin 1 on one end connects to Pin 8 on the other
Pin 2                        Pin 7
Pin 3                        Pin 6
Pin 4                        Pin 5
etc.
Pin 8                        Pin 1

This is basically a null-modem cable but with RJ connectors instead of DB. Here's a link to a different Cisco doc that describes console pinouts.

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Thanks! I spend half my day sometimes reading cisco docs yet I didn't even think to link one. :) –  haymaker Dec 12 '11 at 4:58
    
yesterday I made my first rollover cable! Thanks both Ward and Haymaker for spending time to answer! Upvoted both answers. –  Alex Dec 13 '11 at 1:58
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