Fix ERROR_IPSEC_IKE_PROCESS_ERR_VENDOR 0x3612

IKE VendorId payload processing failed, usually from a mismatch between VPN client and server. Quick fix: restart the IPsec service, then re-check the VPN config.

Quick answer

Restart the IPsec service and re-establish your VPN connection — that clears roughly half of these errors. But if it comes back, the real culprit is usually a vendor ID mismatch between the VPN client and the server, often caused by an outdated client or a firewall device injecting its own vendor ID.

Why this happens

During IKE phase 1, both sides exchange a Vendor ID payload (a random GUID or hash that identifies the vendor's implementation). When Windows processes that payload and hits an unexpected format or value, it throws ERROR_IPSEC_IKE_PROCESS_ERR_VENDOR (0x3612). The error pops up on VPN connections to third-party gateways (Cisco, SonicWall, Check Point) or when using native Windows VPN against a non-Microsoft server.

I've seen it most often after a Windows update changes the built-in IPsec stack, or when a router/NAT device messes with the UDP 500/4500 packets. Don't bother re-installing the VPN client first — that rarely helps. The fix is usually simpler, like restarting a service or editing a registry key.

Fix steps

Restart the IPsec service. Open an admin PowerShell or Command Prompt and run:
```
Restart-Service -Name PolicyAgent -Force
```
Then try your VPN again. This clears stale state and re-reads policies.
Check for UDP port conflicts. Run netstat -an | findstr /i ":500 :4500" and see if anything else is using those ports. If another VPN client (like Cisco AnyConnect) is running, close it completely — they sometimes lock the ports.
Update your VPN client. If you're using a vendor client, grab the latest version. Old versions often send outdated vendor IDs that newer servers reject. For native Windows VPN, make sure you're on the latest cumulative update for your Windows version.
Reset the IPsec policy. In an elevated command prompt:
```
netsh ipsec reset
```
Reboot, then reimport your VPN config if needed. This wipes custom policies that might be corrupt.

Alternative fixes if the above doesn't work

Disable IPv6 on the VPN adapter. Some gateways mishandle IPv6 vendor IDs. Go to adapter properties, uncheck IPv6, and retry.
Lower the MTU on the VPN interface. Fragmented packets can corrupt the payload. Set MTU to 1400: netsh interface ipv4 set subinterface "YourVPNAdapter" mtu=1400 store=persistent
Check for third-party firewall interference. Temporarily disable your local firewall (not Windows Defender) and retry. Some consumer firewalls inject their own vendor ID into the IKE stream.
Use a different IKE version. If your gateway supports it, switch from IKEv2 to IKEv1 or vice versa in the VPN settings. Sometimes one version handles vendor IDs differently.

Prevention tips

Keep your VPN client and Windows updated — vendor IDs change over time, and new versions align with server expectations. Also, avoid running multiple VPN clients simultaneously; they fight over the same ports.

If you manage the server side, double-check that the gateway firmware is current. I've seen older Cisco ASA firmware send malformed vendor IDs that Windows 10 1903+ rejects. Updating the firmware fixed it permanently.

And if you're testing a new VPN connection, do a clean test — close all other VPN software, stop the PolicyAgent service, then start it and connect. That isolates the issue to the actual config.