We have an on premises site (lets call this A), and two Azure tenants (lets call them B and C).
A and B are connected via a private express route connection, and a redundant site to site VPN connection.
C is connected to B via a VNET to VNET connection, with BGP enabled.
In this scenario, transit routing is working. Traffic flows between A and C via B. However it doesn't use the express route connection between A and B, which is preferred. It's using the VPN connection instead.
I suspect this is because the VNET to VNET connection between B and C terminates on the same gateway as the VPN connection between A and B.
So my question is, how do we get transit routing working between A and C so that the traffic flows via the express route connection, with the VPN only to be used as a backup? Bearing in mind, we can't use VNET peering between B and C because these are separate tenants.