Cross Subscription or Tenant VNET to VNET VPN Gateway

Scenario 

There are 2 tenants , Tenant A and B.

They both have existing infrastructure in their subscriptions. Due to organisational merge they want to connect VNETs together on Azure Fabric keep both sides secure.

On both the sides they have Express route and doesnt have enough bandwidth on gateway subnet to house a gateway.

How do we do this ? 

they will need to use VNET to VNET VPN gateway is the solution.

In subscription A
VNET 1 VNET2

In subscription B
VNET 3 VNET 4

VNET 1 to VNET 2 are connected via VNET- VNET Peering you choose the option “Use remote gateways”. For vnet 2 you select “Allow gateway transit”.
VNET 3 to VNET 4 are connected via VNET- VNET Peering Do the same for subscription B, except, vnet D will use remote gateway, and C will allow it.

VNET 2 VNET 4 are connected via cross subscription VPN Site-to-Site Gateway

 

Use VPN Gateway between 3 and 4 .