iRule to SNAT traffic to LTM in another network

I think there's a few different things going on here. If you need client traffic to shift to site 2 if the pool members in site 1 are down, you can:

1. Do an explicit redirect to the site 2 LTM. You would absolutely need a VIP and pool on the site 2 LTM, regardless of the option.

   when HTTP_REQUEST {
       if { [active_members site1-poolA] < 1 } {
           HTTP::redirect "https://site2-vip.com"
       }
   }
   

2. Have GTM provide this failover automatically.

3. You could technically send client traffic through the site 1 VIP to the site 2 VIP either with an iRule node command or as a priority group-assigned pool member, or even as a separate pool. Your SNAT, if you need it all all, needs to be a site 1 LTM IP address that the site LTM can respond to - not necessarily the automap SNAT.

The big question I think would be what you constitute as a failure. If the entire site goes down, options 1 and 3 won't help you.