In GCP, a multi-Nodes Site is deployed with an Internal Passthrough Network Load Balancer (NLB). Load-balanced packets by GCP NLB are received by backend VMs with the packet's source and destination IP addresses. Same for GCP Health-Check, the Destination network interface receives The IP address of the internal forwarding rule.
Therefore all nodes must advertise the Health-Check and App services (HTTP LB) on the GCP VIP. This can be done using LBs advertising on a Site.
To secure CE upgrade mechanism, the solution is to have 2 XC Sites as GCP NLB backends. A Secure upgrade is the flow: drain traffic on 1 XC Site, upgrade 1 XC Site, add the XC Site, observe the traffic, do the same for the second Site. This design requires to have LB advertising on a Virtual Site AND a VIP.
Today,
the advertise policy object allows to advertise on a Virtual Site AND a VIP
an advertise policy can only be referenced by Virtual Host objects
an HTTP object cannot advertise on a Virtual Site AND a VIP, a VIP can be set on a HTTP advertised on a Site only.
Could you add the parameter "IP" in the page "HTTP LB > Other Settings > VIP Advertisement : Custom > Custom Advertise VIP Configuration > Virtual Site" ?
GCP doc :