Same-subnet Private Endpoint access fails with UDR policies enabled in secured vWAN

Question

Same-subnet Private Endpoint access fails with UDR policies enabled in secured vWAN

Viktor Garai 0

Hello,

In an Azure Virtual WAN setup with a secured hub and Azure Firewall routing intent for private and internet traffic, same-subnet VM-to-Private Endpoint connectivity fails with the following configuration.

Each application has its own subnet containing VMs and private endpoints. Each subnet has two UDRs:

VNet address range -> Azure Firewall
Own subnet address range -> Virtual network

The goal is to keep same subnet traffic local while inspecting traffic between subnets.

With Private Endpoint route-table policies enabled:

Same-subnet VM > VM works.
Same-subnet VM > Azure Files PE fails.
Adding an explicit PE /32 > VnetLocal doesn’t help.
Adding PE /32 > Firewall works, provided a firewall allow rule exists.

Disabling PE route-table policies restores access, but also lets traffic from other subnets bypass firewall inspection. Adding the PE /32 to the secured hub’s Private Traffic Prefixes didn’t resolve the local-access issue. The source VM’s effective routes showed the PE’s system /32 InterfaceEndpoint route as Invalid, while the own-subnet VnetLocal route remained Active.

Is this expected behaviour?

Is there a supported way to keep same-subnet VM-to-PE traffic local while forcing traffic from other subnets through the firewall, without maintaining per-PE /32 routes and firewall allow rules for connections that should remain local?

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2 answers

Your answer

Answer 1

Thanks @Andriy Bilous Then based on my understanding would an aggregated version of this approach also work, to avoid maintaining a route for every other subnet?

Assume a VNet 10.0.0.0/15 containing 50+ application subnets, each /24.

For the source subnet 10.0.0.0/24, replace the VNet-wide firewall route and own-subnet VirtualNetwork exception with these destinations, all pointing to Azure Firewall:

10.0.1.0/24
10.0.2.0/23
10.0.4.0/22
10.0.8.0/21
10.0.16.0/20
10.0.32.0/19
10.0.64.0/18
10.0.128.0/17
10.1.0.0/16

These cover the entire VNet except the source subnet. Each subnet would receive its own calculated exclusions, with PE route-table policies remaining enabled.

The intention is:

Same-subnet VM-to-VM and VM-to-PE traffic stays local using system routes.

Traffic to other subnets traverses the firewall.

Internet and inter-VNet inspection remains governed by vWAN routing intent.

Would this be equivalent to your suggested destination-subnet routing approach, including for PEs? It would require nine UDRs per table in this example, without adding routes whenever another subnet or PE is deployed.

Answer 2

Hello Viktor Garai

Yes, the observed behavior is expected.

With Private Endpoint route-table policies enabled, a UDR covering the PE address can override the normal /32 InterfaceEndpoint route. UDR with Next hop = VirtualNetwork does not restore Private Endpoint reachability, while Next hop = NVA/Azure Firewall does.

That explains:

PE /32 -> VirtualNetwork → fails
PE /32 -> Firewall → works
disabling PE route policies → restores the InterfaceEndpoint path

However, you do not necessarily need per-PE /32 routes.

Instead of:

VNet CIDR -> Firewall Own subnet -> VirtualNetwork

use routes only for the other subnets that must be inspected:

Other-app-subnet-A -> Firewall Other-app-subnet-B -> Firewall

and have no UDR covering the local subnet.

Then:

same-subnet VM → VM uses the normal VNet system route;
same-subnet VM → PE keeps the /32 InterfaceEndpoint route;
traffic to a PE in another subnet is overridden by that destination-subnet UDR and sent through Azure Firewall.

So this same pattern is typical for forcing traffic between subnets in the same vWAN-connected VNet: use UDRs for the specific remote subnet prefix, rather than a VNet-wide route plus a VirtualNetwork exception.

Also, adding the PE /32 to the secured hub Private Traffic Prefixes does not create a same-subnet local exception. Microsoft states that same-VNet PE inspection is controlled with subnet UDRs and Private Endpoint network policies.

And the recommended design is:

Do not use VNet CIDR -> Firewall + own subnet -> VirtualNetwork when VMs and PEs share that subnet. Use destination-subnet routes to the firewall only for the subnets that require inspection.

For PE traffic that actually traverses Azure Firewall, also review SNAT requirements; Microsoft recommends SNAT when inspecting Private Endpoint traffic to avoid asymmetric return-path issues.Yes, the observed behavior is expected.

https://learn.microsoft.com/en-us/azure/private-link/disable-private-endpoint-network-policy
https://learn.microsoft.com/en-us/answers/questions/2183987/network-policy-fails-for-private-endpoint