Build A Network Router On Linux
Idean writes "Zebra is open source TCP/IP routing software that is similar to Cisco's Internetworking Operating System (IOS). Flexible and powerful, it can handle routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and all of their various flavors. This article shows how our authors set up Zebra and used it to manage routes dynamically in conjunction with real Cisco hardware."
What is great about this is that is allows you to create a routing lab that seems to very closely resembles a cisco device.
I bought 3 2500 routers on eBay for 700 bucks, had I known about this software, I could have spent that money on something else.