When you say "develop a software using C or Tcl, for a CISCO router", do you mean that you are writing a custom firmware from scratch? Or are you wanting to write some sort of plug-in or application that interfaces with the existing firmware?
In either case, I would recommend that you first consult the documentation you have for the router. See what kind of API and driver support you have for the C and TCL languages. If one language looks like it is better supported than the other, then you will likely have an easier time using that language.
In my experiences, it is typically easier to do systems-level programming in C than in TCL. Most embedded systems assume that you are going to work in C and provide C-language interfaces to system functions. You didn't mention anything about the router model, so I can't say anything too specific.