Thanks to bemowski, http://jmatrix.net/dao/case/case.jsp?case=7F000001-15FB38-10C1D24524A-C4

Here is how to connect a cisco router to a PC running Linux:

1) You have a Cisco router, with a flat, blue console cable. The console cable will have an RJ45 connector at one end, and a 9 pin din serial connector on the other.

2) You have a computer running Linux (I’m using RedHat/Fedora), which has a 9 pin din serial port available.

First, make sure that the serial ports are detected at boot time. I do this with dmesg and grep:

bemo@booker:/home/bemo>dmesg |grep ttyS
ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A

3) Connect the router to one of the Linux serial ports.

4) You should have a communications program. I use minicom, which is available by default in RedHat 9, as well as Fedora Core up to 4 (and probably beyond). You must run minicom as root.

Set the communication parameters to 9600 N81. The router will not work any faster.

5) Start minicom, and set the serial port and comm parmaters. That should do it. The next thing you see on the console should be the router prompt:

Router>