You configure the routers with a routing protocol which is really just a language. Think of it like English, French, German, I mean all the different languages that are out there. So as long as these two are speaking the same language like let's say OSPF, that's one of the languages that exist, and they are compatible, and we'll learn what compatible means and we look specifically about OSPF, router 2 will communicate over to router 1 and say, "Hey router 1, I know about 192.168.2.0 and 3.0, why? 'Cause I'm plugged into them.