Actually, BGP is also one routing protocol as like OSPF with some features to handle large network. But my doubt is why it is called external routing protocol. I thing we can have a network with OSPF itself, without ASes, BGP etc.
What I wonder, why is it not possible to create two ospf domain and connect those two domain using OSPF itself.Like what we do in BGP.
An ospf domain means many areas connected to backbone area.