Dog "bully" - dog smells at another dog's rear

I'm not sure about the chasing part as it seems aggressive or territorial. But, the rear smelling is normal dog behavior. I think this is how they greet each other and figure each other out. It is sort of like when humans ask each other their name, where they you from, what type of work do you do. They pick up so much information from scent.

My sister's dog stays with me sometimes. I will take my dog for a walk to give her dog a break. When we return, my sister's dog is all over my dog smelling her rear end. I suspect she is getting all sorts of information about where we went from that sniffing.