As i know, the st0 interface is used when the gateways are configured with route based VPN.
But my question is, say below is the topology
A and D are hosts, B and C are the routers/gateways.
Now, if we want to reach D from A, we can directly have a route from B with the next hop being C.
The same is being done when using st0. st0 is configured on B and C and when A has to reach D, the next hop for B will be st0. And the same is when D has to reach A, the next hop will be st0 for C.
So, cant we just have their gateway IP address added as the next-hop instead of mapping them to st0? Aren't we doing the same thing?
If not, can anyone please help me understand it better.