Aviatrix customer has applications running in various VPCs, these applications need to be able to communicate with FiServ securely via BGP over IPSec tunnels. FiServ also has lots of its own customers that most likely use RFC1918 as internal address space. To avoid conflicts, FiServ assigns non-RFC1918 IP Prefixes to FiServ customers, the incoming connection must be Source NAT (SNAT) to these non-RFC1918 IP Prefixes, and FiServ also only allows incoming traffic from these non-RFC1918 IP Prefixes on FiServ firewalls.

FiServ reference architecture: https://developer.fiserv.com/product/FirstVisionEMEA/docs/?path=docs/Support/Client-Onboarding.md&branch=main#connectivity-overview-diagram

This blog intends to walk through various considerations that affects final design.