We Need to Talk About VNETs – Azure Back to School 2022

Virtual Networks are arguably one of the most common resources in Azure. You will find them in the vast majority of environments facilitating some form of private or static network functionality. However, they haven’t always been around.

For those that remember the older Azure days, the ASM model didn’t have the same network concept. We have only had Virtual Networks since the introduction of ARM and they have changed drastically over the years.

Don’t get me wrong, at it’s core a Virtual Network is still just an address range. A private network of at least one subnet that sets your connectivity boundary. However, it has been a long time since I have seen a Virtual Network only operate in its basic capacity.

As a result of this ever growing list of services a Virtual Network offers, I think it is about time we talk about VNETs!

But aren’t VNETs straight forward? I deploy them all the time etc. Yes they can be, but they also offer an enormous range of network services. As an abstract piece of evidence, did you know that if you download the Virtual Network documentation page on Docs to a PDF, it is 848 pages!?

First off, what this article is not – an explanation of the basic elements of a VNET. For example subnets and address spacing. So presuming you have some familiarity, let’s discuss what I like to call secondary services.

So, what is a secondary service? For me, it’s a service that cannot exist, or serves no purpose without requiring a VNET. Think Bastion, Route Server, NSGs etc. they all serve specific purposes but commonly enhance the functionality of a VNET. Some of these, like Bastion, I feel would be better if included within a VNET resource, like Service Endpoints. However, that is for another post!

They are also drastically different in their complexity. For example, a Service Endpoint can be deployed without much effort and barely any planning (just double check those routes). However, Route Server requires significant elements of both.

While this list of secondary services is ever growing, I do not necessarily think this is a bad thing. I am always all for extra functionality. However, understanding that you cannot simply deploy a VNET and have the majority of network features that most people use is something that should be clearer. There are new services released like Network Manager that will help with management, but none offer a single view of everything.

To both convey the complexity, but help simplify things (weird I know) I thought it best to pick two services, create a test environment so you can try them, and discuss some of the components. So I’ve chosen two of the newer services:

  • Azure Route Server
  • NAT Gateway

Azure Route Server

Route Server is an excellent addition to routing services within Azure. Previously, there could be some routes originating from Virtual Network Gateways via BGP, the system routes and everything else would be via Route Table. While this works, it can be cumbersome and management at scale of Route Tables is almost non-existent. Route Server solves some of those problems by allowing BGP interaction between NVAs, Virtual Network Gateways and your VNETs system routing table. It’s also nice that it is a managed service, and HA out-of-the-box.

The objective of Route Server is to simplify and centralise routing management. This is helped by using a default peering process, meaning if your NVA supports BGP – it should work with Route Server. It also natively supports peering of VNETs with the same switch as “use remote gateway” meaning it slots very neatly into Hub-Spoke designs. Including the use of Virtual Network Gateways as peers (note VPN VNGs have to be configured in active-active mode).

As with many secondary services, Route Server requires a dedicated subnet in your VNET and each VNET can only have a single Route Server. The subnet does not allow the addition of an NSG or a UDR. This may flag as a concern as Route Server now requires a Public IP, however, this is only to guarantee access to management services and does not open the VNET (according to Microsoft). Also, no data traffic is sent between Route Server and your NVAs.

However, it’s important to note some of the configurations where Route Server alone is not the answer and in some cases begs the question that if I still have to use UDRs for that, why should I bother with Route Server? For example, ExpressRoute will advertise routes that will be preferred over Route Server routes, meaning you would need to overwrite this with a UDR. You cannot simply turn off the ER advertisement as this runs over the same peering functionality. A nice fix here would be to split that choice into two switches. One for VNGs, one for Route Server.

Another element that may be important is price. VNETs are free, UDRs are free, Route Server is far from that. On many large environments, this may be a negligible cost. However, you should weigh up the benefits vs the cost with introducing Route Server.

So to help, as promised, here is a repo that will build a test footprint for you. I’ve taken the Route Server tutorial using Quagga and integrated it with theother services from this article. You can follow the steps to complete the configuration and confirm you have a functioning peer. You should see output similar to the below from Cloud Shell:

Learned routes in Route Server

NAT Gateway

Implicit internet outbound is potentially one of the Azure network features that surprise most people. Deploy a VM into a VNET and you will be able to reach the internet with a random IP from the region deployed. Not exactly a dream scenario for many admins!

However VMs are not where I see this used most often. That doesn’t mean it’s not a good solution for VMs, it works exactly the same and works well. I just more commonly see this to facilitate static outbound IPs for PaaS resources. Like an App Service that requires a static IP due to a vendor allow list.

One interesting piece here, NAT Gateway when configured on a subnet, will take precedent over locally attached Public IPs and Standard Load Balancer Outbound NAT rules. However, UDRs will still overwrite this when advertising 0/0. Another item of note, no ICMP support, only TCP/UDP.

To try out NAT Gateway, I have again included it within a repo. This will also deploy a Ubuntu VM, which you can use Bastion to connect to and login. This VM has a Public IP locally attached but is deployed to a subnet with a NAT Gateway. So, use Bastion to connect, then simply copy the ipcheck script and paste it into the command line, it will give you an output similar to the below which you can then verify against your NAT Gateway resource. Proving NAT Gateway is taking precedence over the locally attached IP.

Locally attached IP
NAT Gateway seen as the outbound IP publicly

Roundup

In closing, I think that more and more secondary services does two things.

  1. Makes networking in Azure ever more complex
  2. Solidifies VNETs as the most important core resource

Now, everyone should agree with number one. However, two may cause some concern, but hear me out. Regardless of your resource deployment, your application architecture etc. 99/100 you will deploy a VNET and 9/10 you will need at least one secondary service. This means that getting it right, having it well designed for deployment, management etc is crucial. Not everyone loves networking, but within Azure at the moment – you’ve gotta learn it!

Speaking of learning, if networking is your thing, check out the most appropriate Azure exam – Microsoft Certified: Azure Network Engineer Associate.

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