Rollover vs. Straight through

JohnnyBiggles · June 2011

Quick dumb question: What is the real purpose of using a rollover cable vs. using a straight through? I know the rollover is for connection to a console port, but what is the significance in the difference of wiring? In other words, is it for security purposes (making the console port less accessible by means of a less common cable)? Why did Cisco design it this way?

hiddenknight821 · June 2011

JohnnyBiggles wrote: »

Quick dumb question: What is the real purpose of using a rollover cable vs. using a straight through? I know the rollover is for connection to a console port, but what is the significance in the difference of wiring? In other words, is it for security purposes (making the console port less accessible by means of a less common cable)? Why did Cisco design it this way?

First of all, this is not a proprietary design. Cisco has nothing to do with it. It's how the computer transmits the signal on the pins. The rollover cable is based on a completely different standard unlike the TIA/EIA. It's based on Yost Serial Device Wiring standard and R-232 standard (for serial port pinouts). Cisco could have come up with a different proprietary cable to use to access the console, but a rollover cable seems to be a more reasonable and managable choice of cabling since it has the same connector as the RJ-45 connector, but is labeled differently though. You need to understand the wiring concept if you want to make a rollover cable with RJ connectors at both ends.

EDIT: I forgot to mention this is not for security purpose. If it was then the wiring would be very unique, and only certain people can acquire it. You should really ask an electrical engineer this question

instant000 · June 2011

JohnnyBiggles wrote: »

Quick dumb question: What is the real purpose of using a rollover cable vs. using a straight through? I know the rollover is for connection to a console port, but what is the significance in the difference of wiring? In other words, is it for security purposes (making the console port less accessible by means of a less common cable)? Why did Cisco design it this way?

The cables have different purposes:

straight - connect unlike devices
crossover - connect like devices
rollover - program a device

If you want to go into figuring out why someone did it a certain way, you have to ask more than Cisco, as tons of companies out there use serial connections, and the pinouts can be different, for each different connection you look at.

If I was a programmer, I might give you the technical reasons behind it, but right now, the best I can tell you is that how the devices are designed to work, and that is the cable that works for it.

Read this one for more information:

Zonker's Minor Scroll of Console Knowledge

Also, read these two:
Cabling Guide for Console and AUX Ports - Cisco Systems

Cisco Cable Product Numbers, Part Numbers, and Descriptions - Cisco Systems

^^^ This just shows there's tons of different cables to use.

Truth be told, Cisco has acquired a lot of companies, so that explains it, a little bit.

smashedpumpkins · June 2011

JohnnyBiggles wrote: »

Quick dumb question: What is the real purpose of using a rollover cable vs. using a straight through? I know the rollover is for connection to a console port, but what is the significance in the difference of wiring? In other words, is it for security purposes (making the console port less accessible by means of a less common cable)? Why did Cisco design it this way?

The others have already answered your question but I thought I'd mention the pins used.

Rollover Cable - 1 to 8, 2 to 7, 3 to 6, 4 to 5 etc... Here's a great example.

Straight through cable example.

Crossover cable example.

Straight through and crossovers are the normally used wires. Straight through means just that. 1 to 1, 2 to 2, 3 to 3 etc... Crossover is 1 to 3, and 2 to 6.

Lets say you're connecting two switches together. If you connect a straight through cable between the two you're going to have 1 switch sending data over a pin that arrives at another switches sending pin. Using a crossover makes it so the data is sent from a sending pin and arrives at another switches receiving pin. Obviously you'll want the results of the crossover cable. As said by another user, alike devices send and receive from the same pins. You'll want a crossover for that. Different devices like a router and a switch, send and receive on different pins. In this case the crossing is already done because their different and a straight through is required.

JohnnyBiggles · June 2011

I appreciate the refresher on the pin layouts and wiring but it wasn't necessarily my point. I was concerned as to why the console jack on the switch/router would be any different than a port jack since they're both using the RJ45 connector and permit direct communication between unlike devices. I know the Console & Aux ports have their own purposes apart from the net ports, but I figure It would make connection via a laptop a bit easier since just about every laptop has the RJ45 network jack and a CATx cable is pretty common to have around and you wouldn't need an adapter. As someone said earlier, I guess it might be a better question for an electrical engineer to answer. Was just curious since I don't think that's exam material.

smashedpumpkins · June 2011

JohnnyBiggles wrote: »

I appreciate the refresher on the pin layouts and wiring but it wasn't necessarily my point. I was concerned as to why the console jack on the switch/router would be any different than a port jack since they're both using the RJ45 connector and permit direct communication between unlike devices. I know the Console & Aux ports have their own purposes apart from the net ports, but I figure It would make connection via a laptop a bit easier since just about every laptop has the RJ45 network jack and a CATx cable is pretty common to have around and you wouldn't need an adapter. As someone said earlier, I guess it might be a better question for an electrical engineer to answer. Was just curious since I don't think that's exam material.

I have no idea why they use another cable. I'm not sure what pins are used or if all are used. It probably has something to do with that. When it comes to connecting into your laptop you have to use a USB adapter. I have a rollover to serial cable that I attach a USB adapter to.

instant000 · June 2011

JohnnyBiggles wrote: »

I appreciate the refresher on the pin layouts and wiring but it wasn't necessarily my point. I was concerned as to why the console jack on the switch/router would be any different than a port jack since they're both using the RJ45 connector and permit direct communication between unlike devices. I know the Console & Aux ports have their own purposes apart from the net ports, but I figure It would make connection via a laptop a bit easier since just about every laptop has the RJ45 network jack and a CATx cable is pretty common to have around and you wouldn't need an adapter. As someone said earlier, I guess it might be a better question for an electrical engineer to answer. Was just curious since I don't think that's exam material.

I hear you ...if you ever have to do Xmodem, you'd wish the console connection was faster than it is.

Xmodem Console Download Procedure Using ROMmon - Cisco Systems

instant000 · June 2011

smashedpumpkins wrote: »

I have no idea why they use another cable. I'm not sure what pins are used or if all are used. It probably has something to do with that. When it comes to connecting into your laptop you have to use a USB adapter. I have a rollover to serial cable that I attach a USB adapter to.

Hahahaha, on this topic, dug up this post:

http://www.techexams.net/forums/ccna-ccent/41847-cisco-usb-console-cable.html

Look at this one:

pwjohnston wrote: »

So I’m starting to build out my lab gearing up for my CCENT/CCNA. When it hit me, I only have one really OLD pc that has a serial DB-9 on it for the console cable. I’m like WTF? why hasn’t cisco made a USB console cable?

^^^ Hahahahaha, classic!

Qord · June 2011

The purpose was to create a standard of cabling for certain devices. This link has a good introduction to it: Yost serial device wiring: RS232 on RJ45

JohnnyBiggles · June 2011

Qord wrote: »

The purpose was to create a standard of cabling for certain devices. This link has a good introduction to it: Yost serial device wiring: RS232 on RJ45

WHOOMP, there it is

Rollover vs. Straight through

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