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It appears that router and firewall emulation has been on the rise, lately.

At first, only JUNOS emulators were available in the form of a JUNOS Olive, which is a plain ol' PC running Juniper's modified version of FreeBSD. Next came Dynamips, which is a MIPS emulator that allows the simulation of several of Cisco Systems' routing platforms.

Now, we're seeing emulators for Cisco Systems' PIX firewalls and fully-featured JUNOS emulators, both based on customized versions of QEMU. The PIX emulator, pemu is open-source, however, the JUNOS emulator, JQEMU, does not appear to be.

Here's a small list of your emulation/simulation options for Juniper and Cisco gear:

Cisco 7200/3600/2600 series routers: Dynamips, which works on Linux and Windows under Cygwin
Juniper M/T-series routers
1. Olive on VMware Server or Workstation: installation is tricky, no multicast support on em (e1000) devices, and the dcd process will crash on JUNOS < 8.4
2. Olive on a real PC: need eepro100 NIC's, but works well
3. Olive on JQEMU: closed-source patch, so it only runs on Windows. Slow.
Cisco PIX: pemu

All of them require purchasing the operating system from the vendor, obviously.

Legal, I believe. Also, as a correction to the text above, I don't believe there is any way of obtaining a valid software license without purchasing hardware or a support contract (which would be silly w/out hardware).

Additionally, QEMU for Linux can be patched with an eepro100.c from svn://svn.berlios.de/ar7-firmware/qemu/trunk/hw/. The fxp interfaces on JUNOS will then correctly process multicast frames.