Everything in Azure networking is easy until you connect something to it. It's the joke I open with every time I run a training session on how Azure networking actually behaves behind the scenes, and routing is no exception.
I've already written about routing in Azure with UDRs and the risk of building asymmetric paths (Asymmetrical routing in Azure network). This post goes one level deeper: modifying the BGP advertisements themselves.
That's traffic engineering. It's routine practice in on-premises networking and a new one in Azure. Two features carry most of the weight today: advertised gateway prefixes on a hub VNET, and Route-maps in Virtual WAN and Azure Route Server (in public preview).
Advertised gateway prefixes
Azure virtual networks expose a property called summarizedGatewayPrefixes (public preview at the time of writing). It lets you hand the gateway a list of CIDR blocks to advertise instead of its default advertisement.
