Azure ExpressRoute
An Azure service that provides private connections between Azure datacenters and infrastructure, either on premises or in a colocation environment.
419 questions
configuring express route coexisting with the VPN Gateway for high availability
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BUDHADITYA BANERJEE
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We already do have a VPN Gateway configured in Australia East region and now we are planning to configure Express Route circuit as the Primary link with VPN Gateway as secondary. Want to check for the configuration of the Express Route for this scenario.
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Sai Prasanna Sinde 4,260 Reputation points Microsoft External Staff2025-03-06T05:03:09.87+00:00 Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.
We understand that you already have a VPN gateway and now you are planning to configure express route circuit as the primary link with VPN gateway as secondary.
To configure coexisting connections for an already existing virtual network: -
- If you have a virtual network with only one virtual network gateway (for example:a Site-to-Site VPN gateway) and you want to add ExpressRoute gateway), check the gateway subnet size.
- If the gateway subnet is /27 or larger you can create a expressroute circuit with your required resiliency type and follow this documentation to Connect a virtual network to ExpressRoute circuits using the Azure portal.
The virtual networks that you connect to your Azure ExpressRoute circuit can either be in the same subscription or part of another subscription.
- The virtual networks that you connect to your Azure ExpressRoute circuit can either be in the same subscription or part of another subscription.
- If the gateway subnet is /28 or /29, you must first delete the virtual network gateway and increase the gateway subnet size.
- Delete the existing ExpressRoute or Site-to-Site VPN gateway.
- Delete and recreate the GatewaySubnet with a prefix of /27 or shorter.
- Configure a virtual network with a Site-to-Site connection and then Configure the ExpressRoute gateway.
- Once the ExpressRoute gateway is deployed, you can link the virtual network to the ExpressRoute circuit.
Note: Deleting and recreating your gateway will result in downtime for your cross-premises connections. However, your VMs and services can still communicate out through the load balancer during this process if they're configured to do so.
- You can add a Point-to-Site configuration to your coexisting set by following the instructions in Configuring Point-to-Site VPN connection using Azure certificate authentication.
Limits and Limitations:
- Only route-based VPN gateway is supported: Use a route-based VPN gateway. You can also use a route-based VPN gateway with a VPN connection configured for 'policy-based traffic selectors' as described in Connect to multiple policy-based VPN devices.
- ExpressRoute-VPN Gateway coexist configurations are not supported on the Basic SKU.
- BGP Communication: Both the ExpressRoute and VPN gateways must communicate via BGP. Ensure that any UDR on the gateway subnet doesn't include a route for the gateway subnet range itself, as doing so interferes with the BGP traffic.
- Transit Routing: For transit routing between ExpressRoute and VPN, the ASN of Azure VPN Gateway must be set to 65515. Azure VPN Gateway supports the BGP routing protocol. To work together, keep the ASN of your Azure VPN gateway at its default value, 65515. If you change the ASN to 65515, reset the VPN gateway for the setting to take effect.
- Gateway Subnet Size: The gateway subnet must be /27 or a shorter prefix (such as /26 or /25), or you receive an error message when adding the ExpressRoute virtual network gateway.
- If you want to Configure a Site-to-Site VPN as a Failover Path for ExpressRoute, please go through this documentation for reference.
- To achieve the highest resiliency and availability, configure a zone-redundant Azure ExpressRoute virtual network gateway.
For reference: 1. Configuring for high availability
2 . About zone-redundant virtual network gateway in Azure availability zones
Kindly let us know in the comments below if the above helps or you need further assistance on this issue.
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Sai Prasanna Sinde 4,260 Reputation points Microsoft External Staff
2025-03-07T01:27:40.77+00:00 Just checking in to see if you had a chance to see my response to your question. Please tell us if it was helpful and feel free to reach out to us if you have any queries.
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