A static route is necessary to ensure that traffic is going via the correct interface.
In the VPN setting, for phase2 when you add a local subnet and a remote subnet, this ensures that traffic between these two subnets can flow over the VPN tunnel. For routing, you need to have a static route configured.
It is also advised to configure a black hole route to ensure that VPN traffic does not get routed towards the internet incase your VPN tunnel is down.
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