You need to configure the default route on Vnet2 and Vnet3. The solution must meet the virtual networking requirements. What should you use to…

In a hub-and-spoke topology where spoke VNets (Vnet2, Vnet3) need to use a central gateway in the hub VNet (Vnet1), the most scalable and dynamic method to propagate a default route (0.0.0.0/0) is via BGP. The hub's VPN or ExpressRoute gateway advertises the default route to its BGP peers. When the VNet peering from the spokes to the hub is configured with the "Use the remote virtual network's gateway" option, the spokes automatically learn this advertised route. This directs traffic from the spokes through the hub gateway without requiring manual User-Defined Routes (UDRs) on every spoke subnet.

A. Route filters are used with ExpressRoute Microsoft peering to selectively allow specific Azure service prefixes; they do not configure default routes within a VNet.

C. A user-defined route on the hub's GatewaySubnet controls traffic originating from the gateway, not traffic coming to the gateway from peered spoke VNets.

D. Spoke VNets in a hub-spoke model with gateway transit do not have their own GatewaySubnet. UDRs would be applied to workload subnets, not a gateway subnet.

1. Microsoft Documentation | About BGP with Azure VPN Gateway: Under the section "Frequently asked questions (FAQ)"

the question "Can I advertise default route (0.0.0.0/0) to my virtual networks?" is answered with "Yes. You can advertise 0.0.0.0/0 to your on-premises networks via BGP." This route is then learned by peered VNets using gateway transit.

2. Microsoft Documentation | Hub-and-spoke network topology in Azure: In the "Gateway transit" section

it states

"Gateway transit allows spoke virtual networks to share the hub gateway... Routes to the on-premises network are propagated to the routing tables of the subnets in the spoke virtual network." BGP is the protocol that facilitates this dynamic route propagation.

3. Microsoft Documentation | Virtual network peering: In the "Gateways and on-premises connectivity" section

it explains that when "Use remote gateways" is enabled on a peering

"Routes that the gateway propagates are propagated to the routing tables of the subnets in the peered virtual network." This automatic propagation is the result of the BGP route exchange.