Hyper-v　Active route not established

Dear 木村 聡行,

Thank you for sharing the details and the routing table comparison. Based on the image, it appears that during the malfunction, certain active routes are missing, which may be causing intermittent communication failures between your applications in Hyper-V on Windows Server 2022. This behavior occurring once every ten attempts suggests a possible timing or initialization issue during virtual network setup. Reviewing the virtual switch configuration and ensuring consistent route propagation across reboots or VM startups may help stabilize connectivity. Additionally, verifying that integration services are up to date and that static routes are correctly defined for both host and guest environments can prevent route loss.

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Best regards,

QQ.

The behavior matches known routing issues that can occur on Windows when routes are not correctly maintained or propagated for a network interface used by virtual machines.

On Windows Server and client versions, there are documented cases where:

• Active routes can be removed or not populated correctly after IP configuration changes on an interface.
• Network connectivity for Hyper‑V virtual machines can intermittently fail under certain conditions (for example, specific virtual switch configurations or heavy traffic) until a fix or update is applied.

For environments using Hyper‑V, the following actions are recommended based on similar issues:

1. Ensure the host OS is fully updated
   • Install the latest cumulative updates for Windows Server 2022.
   • For earlier versions (Windows Server 2012 R2, 2012, 2008 R2, 8.1, 8), Microsoft released hotfixes and update rollups to correct problems where:
     • VM network connectivity is lost or corrupted.
     • Active route entries are deleted incorrectly when IP addresses are added/removed.
   • Windows Server 2022 includes many of these fixes, but staying current with updates is essential to avoid similar routing bugs.
2. Check for patterns that match known issues
   • If the problem appears after adding/removing IP addresses on the VM or host NIC, it resembles the issue where active routes are deleted incorrectly when IPs change. In that case, avoid frequent IP reconfiguration on the affected interface, or script a route refresh after changes.
   • If the problem appears when starting additional VMs, enabling VLAN tagging, or using third‑party virtual switch extensions, verify that the virtual switch configuration is supported and that all relevant updates or hotfixes for virtual switch behavior are installed.
   • If the problem appears under heavy outgoing traffic, compare with the documented Hyper‑V issues where VM network connectivity is lost under load and ensure equivalent fixes are present in the current OS.
3. Operational mitigations
   • When the failure occurs, compare the route table with the normal state and re‑add any missing routes using route add or by restarting the affected network interface inside the VM.
   • Avoid unnecessary IP address changes on the VM NICs and host NICs that back the Hyper‑V virtual switch, as these can trigger route recalculation.
   • If using advanced offload features (such as Large Send Offload v2) or third‑party switch extensions, test with those features temporarily disabled to see if route stability improves.

If the issue persists after applying all current Windows Server 2022 updates and validating the virtual switch configuration, collect detailed route tables and network traces at the time of failure and open a support case so that the behavior can be compared directly with the known routing and Hyper‑V networking issues documented for earlier versions.

References:

• Active Routes table entries are deleted incorrectly when you remove an IP address in Windows 8 or Windows Server 2012
• Virtual machine network fails when you start the second VM on a Windows Server 2012 R2-based Hyper-V server
• Network is corrupted between Guest OS and external network for VMs hosted on Windows 8.1 or Windows Server 2012 R2
• The network connection of a running Hyper-V virtual machine is lost under heavy outgoing network traffic on a Windows Server 2008 R2-based computer