This video from Hack in The Box 2012 introduces the architecture, special properties of configurations and services as well as how to reverse engineer the OS. Obviously, this is done only to ensure compatibility with router products of other vendors routers might be still hurt in the process.
About Felix FX Lindner
Felix is the leader of the Phenoelit group and looks back at around fifteen years of (legal) hacking with only a couple Cisco IOS and SAP remote exploits, tools for hacking HP printers and protocol attacks lining the road. Felix runs Recurity Labs GmbH, a security consulting and research company in Berlin, Germany.