This week I hit a new oddity while working with a customer. Yes, an Azure customer!
He had setup Express Route (ER) dozens of times before, but this time it just didn't work! He went thru the VPN gateway setup and when tried to save/create the gateway he got this: Set-AzureVnetConfig : BadRequest: The virtual network xxxxxx has a gateway with connection type 'Dedicated', but you are not allowed to use the Express Route feature. Only users who are registered for Express Route can use 'Dedicated' gateways. And without a working gateway, there is nothing to connect the ER to your Azure environment.
After a chat with some internal experts, I learned a little known fact: the ability use ER is actually a setting on your subscription! While most subscriptions allow this by default, occasionally this is NOT the case. When that happens, you just need to put in a request for Microsoft to enable that option and you're good to go!