Traceroute shouldn't work. The incoming interface should decrement the TTL, see that it became 0 and report back to the source. The external interface doesn't even see the packet. So, the first answer should be from the internal interface of your router. The second answer should then be from the router one hop away, ie. the ISP equipment. If it does work, the broadband router does strange things. However, some equipment might report back it's "main" IPaddress instead of said interface address, but don't count on it.

And anyway, you should use the -m (maximum ttl) flag, or it's win32 equivalent, to not let the probes wander of to far.