In all case the F5 doesn't take my SNAT rule and the traffic take another public IP.
On the peer device (which is not behind an F5) I have a log "Asymmetric Routing". It's normal because he tries to establish the tunnel with an IP and there is another IP that respond to him.
On the F5 I can see it on the logs
16:30:36.409542 IP Y.Y.Y.Y.isakmp > X.X.X**.85**.isakmp: isakmp: phase 1 I ident
16:30:51.939720 IP 10.199.0.1.isakmp > Y.Y.Y.Y.isakmp: isakmp: phase 1 I ident
16:30:51.939732 IP X.X.X**.251**.20251 > Y.Y.Y.Y.isakmp: isakmp: phase 1 ? ident
The peer device seems to successfully contact my firewall on Y.Y.Y.85 but the F5 respond with the Y.Y.Y.251
Is there anything that I forgot in the configuration?