How to set default route for the VPN

I have been able to set up a site-to-site IPSec/IKEV1 VPN between a CISCO gear and my droplet at DO. The VPN is up. But there is not traffic. When I do a simple ping on the remote Private IP from my droplet, I get nothing:

ping 192.168.100.213
PING 192.168.100.213 (192.168.100.213) 56(84) bytes of data.
^C
--- 192.168.100.213 ping statistics ---
21 packets transmitted, 0 received, 100% packet loss, time 20159ms

But if I try to ping the remote Private IP by specifying the interface to use as *eth1, I can reach it smoothly :

ping 192.168.100.213 -I eth1
PING 192.168.100.213 (192.168.100.213) from 10.XXX.XXX.XXX eth1: 56(84) bytes of data.
64 bytes from 192.168.100.213: icmp_seq=1 ttl=63 time=169 ms
64 bytes from 192.168.100.213: icmp_seq=2 ttl=63 time=168 ms
^C
--- 192.168.100.213 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 168.838/169.150/169.462/0.312 ms

How can I solve that ? It feels like when I’m trying to reach out, the default interface is eth0. Is there a way to change the default interface ? or is there a configuration that can help me tell that all the packets that are coming from my private IP of the droplet must pass trough eth1 ?