Cisco console to Mac?

Cisco console to Mac?

16/07/12 | by admin [mail] | Categories: Mac OS X

So, Cisco being an old and venerable company, have a few quirks. One of these is the use of serial console cables. Basically, an RS-232 dealio. You remember RS-232? If you owned a computer in the 80s or early 90s, likely you had an RS-232 port and probably used it once or twice. They used to have lots of cool stuff, including Modems. Which was a good thing, since it meant that you didn't need to worry too much about what flavour computer you had. RS-232 is pretty simple, easy to implement in hardware and software and thus ubiquitous.

On the Amiga, I used the serial port for two things: MIDI (it's nearly the same thing as RS-232, except a slightly odd clock and some optical isolation, and a different pin out), and hooking up the dial up modem.

The serial port was also popular among hobbyists and in industrial automation. Many computers also used the serial to output debug info during POST. Sun boxes used (still use?) serial for initial set up and configuration.

So, a venerable history. The downside is that its large form factor and low speed mean that it has lost favour with computer manufacturers. It has become a challenge to find a laptop with a real Serial port.

But Cisco still use it as the connection of last resort on the bulk of their products. They've designed their system with the assumption that if you can get access to the serial port, then you are legit. This means that if you've stuffed up your conf, or need to unlock someone elses (perhaps you like 2nd hand gear), then you may well have to resort to that serial cable.

So, you can hook it up to a PC (or something else) with a 'real' serial port, or use one of the known working USB to RS-232 dongles.

From memory, the only 'Mac' products Apple ever supplied with genuine RS-232 ports were the Xserves. And yes, you can use the serial port on an Xserve to hook up a Cisco console cable. And why not? (Apart from the fact that the newest Xserve is years old). In all likelihood they are living together in a rack somewhere.

It also possible to use one of the known working USB to RS-232 dongles with a Mac.

And how to fire it up and get it working? Well, on an Xserve running 10.5:


/usr/libexec/serial/SerialTerminalSupport stop
screen /dev/cu.serial

Or if you have a Keyspan USB to RS-232 adaptor:

screen /dev/tty.KeySerial1 9600

Of course, there's more.

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