We used to have 1 VPN tunnel over 1 Internet connection, connecting 2 datacenters into 1 virtual LAN (10.x.x.x). It was very simple with route tables etc.
Now that we added another tunnel on the second independent physical connection it becomes very difficult to handle routing. It's easy enough to route everything the new connection, but what if we want to route only specific host to host or subnet to subnet??
e.g. client1-site1 <--> vpn-on-site1 <--tun1--> vpn-on-site2 <--> service1-on-site2 client2-site1 <--> vpn-on-site1 <--tun2--> vpn-on-site2 <--> service2-on-site2
Note that there is only 1 vpn node on each side. What we manage to do is for packets to be routed properly from client2-site1 to service2-on-site2 but the returning packets go sometimes to tun1...