Fortigate asks about ARP in an IPSEC tunnel?

Hi all ¡

I have several fortigate IP sec tunnels but one of them is acting strangely. The tunnel is stablished with another fortigate correctly.

Internal network 10.10.10.0/24 --> Fortigate 1

Internal network 11.11.11.0/24--> Fortigate 2.

I have static routes in both fortigates etc... 

If internal network of fortigate 1 sends traffic to internal network of fortigate 2, fortigate 1 ask about the ARP of the destination IP:

in arp who-has 11.11.11.1 tell Fortigate 1

If the internal network of fortigate 2 sends traffic to internal networ of fortigate 1, it works.

I'm sure that I have not previous sessions to 11.11 network, only one route and no other routes or connected interfaces. If I do a route lookup the destination interface is the correct VPN Interface...

Could you help me? Why Fortigate 1 ask about ARP and it not sends traffic to the tunnel interface? I have checked routing, phase 2 selectors etc.. I can't find the problem.

Thanks.