Missing tunnel between routed networks

If we have 2 Computes (C1, C2), and 2 Networks (N1, N2).
case 1:
1. Create VM V1 on C1 in N1.
2. Create VM V2 on C2 in N2.
3. Connect the networks with router.
result:
No tunnel between OVSs.

case 2:
1. Connect the networks with router.
2. Create VM V1 on C1 in N1.
3. Create VM V2 on C2 in N2.
result:
No tunnel between OVSs.