EDIT : It works now.
On forti100D, i had to make policy rule to open access to remote site.
Here's a tuto from sysprobs to make a site to site VPN with forti and SonicWall
I try to make a tunnel VPN Ipsec between Fortigate 100D and SonicWall TZ215.
In Ipsec Monitor, i see my VPN down.
In debug i see that. I searched but i didn't found what it means.
ike 0: IKEv1 exchange=Informational id=fe1672c1360bd050/62fcffc73bda85a0:923f2f08 len=84
ike 0: in FE1672C1360BD05062FCFFC73BDA85A008100501923F2F0800000054A36166DB127E8FE805F4420FDF2E77A676E4D823D8AC1DACF424603564C0A1708863BFA82107593CBCFE8E49AE1650DBF466111471CAAFBA
ike 0: comes addr_ext:500->addr_int:500,ifindex=26....
ike 0: IKEv1 exchange=Identity Protection id=c0f71e2eb6c2f5de/ec3b9b88d1777808 len=92
ike 0: in C0F71E2EB6C2F5DEEC3B9B88D177780805100201000000000000005CFBF0AB07021D5F362BC6F6F9BFA16AA4BF551CAD1B29880790D63E3641C3925E3F873F18CE156FB9C693F0037CA7FF8295834D847B2706665E1000DBE1E68685
ike 0: malformed responder cookie c0f71e2eb6c2f5de/ec3b9b88d1777808 from addr_ext:500->addr_int 26 exchange-type Identity Protection, drop
Can you help me?