You didn't make clear whether you set up the static NAT via the automatic technique (going to NAT tab of the object itself) or using a manual NAT rule by directly adding it to the NAT rule base.
If you configured it using the automatic method, all you need to do is place that object in the destination of a firewall rule permitting access into it and install policy. The firewall will automatically proxy ARP for the NAT address as needed (this assumes that the "Automatic ARP configuration" checkbox is set in the NAT global properties), you can verify this with the "fw ctl arp" command. If you see your NAT address listed there, the active member of a ClusterXL cluster will automatically proxy ARP for it.
If you used the manual rule setup on an R77.30 or earlier gateway, you will have to add a static *proxy* ARP (NOT just a static ARP - big difference) to both cluster members via the Gaia web interface, and make sure that the "merge manual proxy ARP configuration" checkbox is selected (it is not enabled by default) in the global NAT properties of SmartDashboard.
If you configured the manual rule NAT on R80.10 gateway you can do the same static proxy ARP procedure, or you can follow the steps in this SK to enable automatic proxy ARPing for NAT addresses used in manual NAT rules:
sk114395: Automatic creation of Proxy ARP for Manual NAT rules on Security Gateway R80.10
This is a brand new feature for R80.10 but is disabled by default. I have seen it in action and it works well.
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