Azure Route Server VM Limit

Question

Azure Route Server VM Limit

Bottinelli, Stephen 40

I do not understand why the number of VM's in the vnet my route server is configured in (and it's peered vnet's) is listed in the route server limitations. To re-state my question again in a different way, I'm not confused about what the stated numerical limit is, I'm confused about why it's a limitation at all. As far as I can tell by looking as the effective route table of a NIC, it does not include host routes or routes for individual VM's, so how/why does the VM count affect route server? Appreciate any insight into this.

Thanks!

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Answer 1

Hello Bottinelli, Stephen

I can see you're curious about why there’s a limitation on the number of VMs for an Azure Route Server, especially since it appears that the effective route table of a NIC doesn’t include host routes for individual VMs. Let's break this down!

The reason the number of VMs in a virtual network (including peered virtual networks) impacts the Azure Route Server’s limitations is primarily related to the underlying architecture and the way BGP (Border Gateway Protocol) operates. Here’s a summary of the key points:

Capacity: Azure Route Server has a maximum capacity of 50,000 VMs across the virtual network and peered virtual networks. This capacity is based on the number of routing infrastructure units you deploy. By default, it starts with 2 routing infrastructure units supporting 4,000 VMs, but you can scale that up as needed.
Efficiency and Scaling: Each routing infrastructure unit you add provides the Route Server with the ability to manage more VMs. This is crucial when routing traffic as it helps ensure efficient load balancing and operational performance.
BGP Peering: Although the route table does not list individual VM routes, BGP operates on the premise that each VM might send or receive traffic that could influence overall routing decisions. Therefore, even if individual VM routes are not shown, the way they interact with the Route Server and other components in your architecture matters.
Route Management: Each VM may have distinct routing needs based on its application or service requirements. Managing routes effectively for a large number of VMs helps you avoid route advertisement overload, which can lead to session drops if limits are exceeded.

I hope the above answer helps you! Please let us know if you have any further questions.

Please don't forget to be accepting the answer "upvote" where the information provided will help you, this can be beneficial to other members of the community.

Answer 2

The limitation on the number of virtual machines (VMs) that Azure Route Server can support is primarily related to the routing infrastructure and the overall capacity of the Route Server. While the effective route table of a NIC may not include individual VM routes, the Route Server needs to manage and maintain routing information for all connected VMs within the virtual network and its peered networks.

This limitation ensures that the Route Server can efficiently handle routing updates and maintain performance. Each VM generates routing information that the Route Server must process, and having a cap on the number of VMs helps prevent overload and ensures stable operation. The Route Server is designed to support a maximum of 50,000 VMs in the virtual network, which includes those in peered virtual networks, to maintain optimal performance and reliability in routing operations.

In summary, the VM count affects the Route Server because it directly impacts the routing capacity and the management of routing information, ensuring that the service remains efficient and reliable.