Azure VPN is automatically in Active-Standby mode. Every Azure VPN gateway consists of two instances in an active-standby configuration. By default, VPN Gateway allocates a single IP address from the GatewaySubnet range for active-standby VPN gateways, or two IP addresses for active-active VPN gateways. For any planned maintenance or unplanned disruption that happens to the active instance, the standby instance would take over (failover) automatically, and resume the S2S VPN or VNet-to-VNet connections. However You can have only 1 tunnel when using a PolicyBased VPN.
So you do not have to do anything explicitly for Active-Standby setup if you have a Route Based VPN setup. However, if you want to setup Active-Active VPN, please refer this document for more details- https://learn.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-highlyavailable#highly-available-cross-premises-connectivity
Hope this helps. Please let us know if you have any further questions/concerns. Thank you!
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