Most of the companies use their own house made kernetl for realt time OS. Some of them use VXworks. Also there is anotehr realtime os which is called Chorus. You can customized the kernel as you like. It give very good perfomrance on Motrolla borad. I am not sure about for router. I beleive Vxworks is best best solution.
Thank you for the info. What is the best language is to run on realtime OS. I seen many people recommended C. Is this good language for realtime programming?