IPsec VPN with Strongswan - How to multicast?

I've got an IPsec VPN - OpenWRT <=> Vigor router.

All is working well and I can ping across both networks.

I'm struggling to forward multicast traffic to and from both routers. I've been trying to use forecast plugin but without any success. Is forecast only for IKEv2 links? As I can only use IKEv1.