IPsec with non-existing local subnet

[ooswald]

What is the general approach when a remote IPsec site expects our traffic to originate from a subnet which does not exist on our side?

I've established an IPsec tunnel to the remote site (10.100.0.0/16). The local subnet is configured as 10.100.8.96/28 (which is from where they expect our traffic). Tunnel is up:

Code Select Expand

        con2[188]: ESTABLISHED 4 hours ago, *.*.*.*...*.*.*.*
        con2{554}:  INSTALLED, TUNNEL, reqid 2, ESP SPIs: *** ***
        con2{554}:   10.100.8.96/28 === 10.100.0.0/16

I added an outbound NAT with translation target set to 10.100.8.97.

Is this enough or do I need to configure anything else, such as a VIP?