I've established a site to site VPN with two Zyxell routers. Site A (LAN: 192.168.16.x) is the main office, and site B (LAN: 192.168.17.x) is a branch office. Both sites are able to reach each other, and things work as they should.
Now, what I don't really understand is how it really works!
How does the router at site A know that requests going to 192.168.17.x should go to site B? I'm thinking that, upon establishing the connection, the router at site A tells the router at site B that it's IP range is within 192.168.16.x and then the router at site B modifies its routing table to reflect this - and vice versa. However, this is just as assumption. Is that really how it works?
Furthermore, what happens if I add more branch offices? Say I'd like to establish a site to site connection between site C (192.168.18.x) and site A. Will machines at site C be able to reach machines at site B through site A? This would of course mean that site C must know about site D. Will I have to create custom routing policies for this, or is this also "automagically" taken care of?