The technical requirement states that all outbound traffic from the function apps to the on-premises network must use ExpressRoute circuits. To accomplish this, the Function App hosting plan must support outbound virtual network integration, because the application must send outbound traffic into an Azure virtual network from which routes learned through or directed toward the ExpressRoute gateway can reach the on-premises environment.
Fa1, which uses the Flex Consumption plan, supports virtual network integration. Microsoft documents that Flex Consumption provides outbound virtual network connectivity and can therefore reach resources accessible through the integrated virtual network. Microsoft Learn
Fa3, which uses a Dedicated App Service plan, also supports outbound virtual network integration. Microsoft further documents that VNet-integrated Functions can have outbound traffic affected by BGP routes learned from an ExpressRoute gateway, and route tables can direct traffic through that gateway. Microsoft Learn
Fa2, however, uses the legacy Consumption plan, which does not support outbound virtual network integration. Consequently, it cannot meet the requirement to route its outbound on-premises traffic through ExpressRoute using this architecture. Microsoft Learn
Therefore, only Fa1 and Fa3 support the required networking configuration.
