Why are network routes asymmetric

Question

I have a generel question on netowrks. I wanted to know why a route from A to B is different from B to A. Why aren't they symmetric always?

Answer 1

Because they depend on how the route table is configured in each individual router or device it pass through from source to target locations.

Given this network:

A - B - C
    | /
    D 

A message can go from A to D by traveling through B (A-B-D) But maybe D is configured to every outgoing packets be routed through C so the return path can be (D-C-B-A).

Probably not the best example but I think it makes a point. Every router is in charge of building and maintain their local route tables so this kind of situations can happen. You can find more information on Wikipedia page for Routing Tables.

Hope this helps!