ARM template for generating a vnet with a local gateway that has routes back to a supernet that contains the vnet.
Sample parameters file would create a vnet hub with CIDR 10.1.0.0/24 that allows attaching other vnets in the range 10.1.0.0/20. The local gateway includes all routes for 10/8 except 10.1.0.0/20.
cidr_subtract.py is a small tool for calculating the CIDR routes for local gateways.
Note that it takes ~30 minutes for the deployment to finish and you need to run it a second time for the vnet gateway IP to show up in the outputs.