Some years ago I did exactly what you need by using virtual machines on my Ubuntu system. I used the "user mode-linux" software (UML, but not the modeling languaje!) to start n linux on my laptop, and then I just had to connect their "virtual" interfaces to create simple and more complex netwoks (playing with the route tables, etc...). I had to do that because, as you said, you can't start multiple BIRD sessions on the same linux.

2009/4/28 ChuYinsu <yst.cyan@gmail.com>

Hi,

I want to do some experiment and study about OSPF protocol. Here is a
simple topology:

+------+ +------+
| RT1 | Interface 1 | RT2 |
| |----------------------------------------| |
+------+ Interface 2 +------+

However I just have one computer, so I wondered can I simulate 2
routers on one computer? Using each Ethernet Interface for one router:

+-------------+Interface 1
| RT1 |--------------------------
| | |
|---------------| |
| RT2 |--------------------------
| |Interface 2
+-------------+

If BIRD supports multiple processes running, maybe this simulation
would be possible. But when I try to do this, the operating system
says another BIRD is already running.
Any ideas?

Thanks!