Server2016 Cluster network traffic coming from host ip rather than role ip

Hello

I have two 2016 vm's in a hyper-v environment that are clustered. Each VM is on a separate physical host.

Each VM only has 1 nic. My clusters ip's are as follows:

I've added the Role IP address (172.18.1.113) to an ipsec tunnel on my firewall, but my firewall see's the traffic as coming from either of the 2 host ip addresses (.115 or .116).  If I ping the remote end of the ipsec tunnels host from the either host A or B and source it as the .113 the ping works, but by default it always takes host ip and fails. 

How do I get the clusters nodes to always send traffic out of the role ip no matter which node is active? 

Thanks

Dan