You must configure the default route on Vnet2 and Vnet3 to meet the virtual networking requirements. Which mechanism should you use to configure that default route?

BGP route exchange is the correct mechanism because it allows dynamic routing information, including default routes (0.0.0.0/0), to be propagated automatically between Azure VPN Gateways and on-premises devices, or between peered virtual networks with gateway transit enabled. This ensures that Vnet2 and Vnet3 learn the necessary default route to reach destinations outside their own network space, typically via a central hub VNet or an on-premises network. Route filters are used with ExpressRoute to consume a subset of advertised routes, not to define default routes. User-defined routes (UDRs) assigned to GatewaySubnet in Vnet1 would only affect traffic originating from Vnet1's gateway, not Vnet2 or Vnet3. Similarly, UDRs assigned to GatewaySubnet in Vnet2 and Vnet3 would define static routes for those subnets, but BGP is preferred for dynamic and scalable default route propagation in complex hub-spoke or multi-VNet scenarios.