What is – Azure Route Server

Back in March 2021 at Ignite, Microsoft announced a new network feature in Preview, Azure Route Server. As the name suggests, this service is designed to act as a routing service within your Azure footprint. Specifically, it should simplify the management of dynamic routing between your network virtual appliance (NVA) and your Virtual Network. It does so by allowing you to exchange routes via BGP between the NVA and the Azure Software Defined Network (SDN) in your Virtual Network. As it’s a fully managed service, this means you no longer need to implement and maintain multiple route tables.

This service has just gone GA, and as a result, I thought I would share some thoughts relative to its implementation and functionality.

First up, a little bit more on how it works. Microsoft have an excellent graphic to show you the functionality at a high-level:

Diagram showing Azure Route Server configured in a virtual network.

However, the key to understanding this service is having familiarity with implementing an NVA in an environment without it. So, say we build a Palo Alto, or any vendor NVA in our VNET. It is a flat VNET which has several subnets. And we want all our traffic to route via the NVA for filtering. Without Route Server, we have to implement single, or multiple, Route Tables on the subnets and manually manage route modifications on the NVA. Meaning a lot of admin overhead and room for potential user error.

Route Server removes this requirement, which believe me, on complex NVA hub-spoke designs is very welcome. The service leverages BGP to expose the system routing table of the VNET to allow the NVA write directly. Similarly, the NVA can learn routes from the VNET as they update, for example, the addition of a peering.

Creation of the service itself is very simple, a dedicated subnet of /27 or larger named RouteServerSubnet and a Public IP. Essentially a similar creation process to Azure Bastion. Guides for deployment are well documented, and include IaC options (although no Bicep example as yet).

Once Route Server is built, you then peer, and complete configuration as required with your NVA(s). Route Server advertises an ASN and IPs from the dedicated subnet for this, as below

I’ve only mentioned NVAs to this point, but Route Server also works with Virtual Network Gateways, supporting both VPNs and ExpressRoute. Integration here is even tighter, you don’t need to configure the peering, you just enable the switch within Route Server. This switch is called Route Exchange, and when enabled, your NVA and ExpressRoute wil learn each others routes.

One thing I have found so far is that this seems more like a service that should be enabled on the VNET itself, like DNS or Service Endpoints, rather than a resource you have to build and configure. Having it “built into” every VNET would seem like the better option to me, and would definitely be cleaner.

Also, there doesn’t appear to be any integration offered with Azure Firewall as yet, however, I want to dig a bit more on that and will update this post accordingly.

Perhaps someday we will get a VNET Version 2.0 which includes the likes of Route Server, Bastion etc all within the one blade and configuration item!

Until then however, if you want to try out Route Server, the absolute best option is to follow the Docs tutorial which covers everything except Virtual Network Gateway, although you could add that on yourself manually if required as I did (ping me if you need help). The full tutorial is here and is a really clean implementation and removal which gives you the ability to test each feature.

One thing that did catch me out here was advertising the default route, however, while I realised my mistake and corrected it with a UDR, I was glad to see this configuration issue, and others are already documented.

Before deploying to production, consider the FAQ to ensure you understand limits and supported scenarios. Peer and route limits are specifically important here for larger designs!

As always, if you have any questions, get in touch!

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