Configuring Fault-Tolerant S7 Connections on Virtualized WinCC OS Servers (VMware vSphere)

In SIMATIC PCS 7 running on VMware vSphere, configuring a Fault-Tolerant S7 Connection between a virtualized WinCC OS Server and redundant S7-400H CPUs requires provisioning two separate virtual network adapters (VMXNET3) mapped to two distinct IE General modules in NetPro. If an engineer attempts to configure redundant sub-connections using only a single IE General adapter, NetPro detects an IP address conflict and forces the secondary connection to revert to Layer 2 Industrial Ethernet (MAC addressing). Configuring dual IE General cards on separate IP subnets resolves the conflict, enabling 100% TCP/IP fault tolerance.

Architecture of Fault-Tolerant S7 Connections (S7-400H) in Virtual Environments

In high-availability SIMATIC PCS 7 plants, process controllers operate as redundant pairs (S7-400H, such as CPU 417-4H 6ES7417-4HT14-0AB0 or CPU 410-5H 6ES7410-5HX08-0AB0). To guarantee uninterrupted monitoring during hardware failovers:
* The Fault-Tolerant S7 Connection: A specialized logical connection defined in NetPro between the WinCC OS Server and the S7-400H controller.
* Dual Physical Sub-Paths: The logical connection consists of two underlying physical sub-connections:
* Sub-connection 1: OS Server to CPU 0 (Rack 0).
* Sub-connection 2: OS Server to CPU 1 (Rack 1).
* The Virtualization Shift:
In traditional physical systems, dedicated CP 1623 communications processors managed these sub-connections using ISO Layer 2 (MAC-based) transport. When migrating to VMware vSphere ESXi, Siemens standard virtualization architecture replaces physical CP cards with software-based Basic Communication Ethernet (BCE) running over standard TCP/IP (ISO-on-TCP, RFC 1006).

Why NetPro Reverts Redundant TCP/IP Connections to MAC Addressing

A common and baffling error occurs when engineers attempt to configure a Fault-Tolerant S7 Connection on a virtualized OS Server in NetPro:
1. The engineer opens the S7-400H fault-tolerant connection properties.
2. In the connection path table, the engineer selects TCP/IP for Sub-connection 1 and TCP/IP for Sub-connection 2.
3. The engineer clicks OK.
4. When the dialog is re-opened, Sub-connection 1 remains set to TCP/IP, but Sub-connection 2 has automatically reverted to 'Industrial Ethernet' (MAC address)!

The Technical Cause (IP Conflict on Single Adapter):
* In NetPro, an IE General module represents a single physical/virtual network adapter holding one unique IP address.
* A fault-tolerant S7-400H connection over TCP/IP strictly requires that Sub-connection 1 and Sub-connection 2 traverse two distinct, physically separated IP subnets (e.g., Subnet A: 192.168.1.x and Subnet B: 192.168.2.x).
* If the PC Station in NetPro has only a single IE General card, NetPro cannot bind two simultaneous redundant TCP/IP sockets on different subnets to the same IP adapter. To prevent compile failure, NetPro automatically forces the second path back to MAC-based Layer 2 transport, which fails in virtualized environments lacking ISO transport bridges.

The Dual IE General Design Pattern for Virtualized OS Servers

To resolve this conflict and establish genuine fault tolerance over TCP/IP, engineers must implement the Dual IE General Pattern:

Virtual Hardware Layer (VMware ESXi):
The virtual OS Server VM is provisioned with two independent virtual network adapters (both configured as VMXNET3):
Network Adapter 1: Bound to Plant Bus Subnet 1 (e.g., IP: 192.168.1.10).
Network Adapter 2: Bound to Plant Bus Subnet 2 (e.g., IP: 192.168.2.10).
Station Configuration Editor Layer:
Inside the guest Windows OS:
Slot 1: WinCC Application
Slot 3: IE General (Adapter 1)
Slot 4: IE General (Adapter 2)
NetPro Engineering Layer:
When NetPro sees two separate IE General adapters connected to two separate IP subnets, it permits selecting TCP/IP for both sub-connections without reverting to MAC addressing.

Virtual Switch (vSwitch) and Uplink Mapping in VMware vSphere

For full physical redundancy, the hypervisor network topology must mirror the controller architecture:

Two Dedicated Virtual Switches:
vSwitch_PlantBus_A: Connects physical Server NIC 1 to SCALANCE Switch A (Subnet 1).
vSwitch_PlantBus_B: Connects physical Server NIC 2 to SCALANCE Switch B (Subnet 2).
Port Group Isolation:
VM Network Adapter 1 connects to Port Group PG_PlantBus_A.
VM Network Adapter 2 connects to Port Group PG_PlantBus_B.
Zero Common Failure Points:
A severed cable, blown switch, or failed physical server NIC isolates only one sub-path. The SIMATIC NET SOFTNET-IE S7 Redundant channel seamlessly redirects communication telegrams over the surviving adapter in under 100 milliseconds.

Comparison Table: Single NIC vs Dual IE General vs PCIe Passthrough

The following table compares the different network architectures for virtualized S7-400H communication:

Engineering ParameterSingle IE GeneralDual IE General (Recommended)PCIe Passthrough (CP 1623)Underlying ProtocolReverts to MAC (Fails on IP)100% TCP/IP (ISO-on-TCP RFC 1006)ISO Transport (Layer 2 MAC)Virtual NICs in VM1 x VMXNET32 x VMXNET3DirectPath I/O PCI DeviceFault ToleranceDegraded / Non-functionalComplete physical redundancyComplete physical redundancyvSphere vMotion / HASupportedFully supportedDisabled (PCI passthrough locks VM)Host Memory LockingDynamic / StandardDynamic / Standard100% Locked RAM requiredDriver PackageSoftnet-IE S7Softnet-IE S7 RedundantHardnet-IE S7 REDCONNECTLicensing Requirement6GK1704-1CW14-0AA06GK1704-0HB14-0AA06GK1716-0HB14-0AA0

Step-by-Step Procedure to Configure Redundant S7 Connections in NetPro

Follow this verified engineering procedure to configure redundant S7 connections in NetPro:

Add Second Network Adapter in VMware vSphere:
* Power down the virtual OS Server.
* Edit VM Settings > Add New Device > Network Adapter.
* Adapter Type: VMXNET3.
* Network Connection: Connect to vSwitch_PlantBus_B.
* Power on the VM and assign static IP address (e.g., 192.168.2.10) inside Windows.
Update Station Configuration Editor on OS Server:
* Open Station Configuration Editor inside the VM.
* Verify Slot 3 contains IE General (Adapter 1).
* In Slot 4, click Add > select IE General > bind to Adapter 2.
Update PC Station in SIMATIC Manager HW Config:
* Open the PC Station in HW Config.
* Add a second IE General module into Slot 4.
* Connect Slot 3 to PlantBus_1 subnet (IP: 192.168.1.10).
* Connect Slot 4 to PlantBus_2 subnet (IP: 192.168.2.10).
* Save and compile HW Config.
Configure Fault-Tolerant Connection in NetPro:
* Open NetPro.
* Select WinCC Application on the PC Station.
* Insert New Connection > Target: S7-400H CPU > Type: S7 connection fault-tolerant.
* In Properties, inspect the sub-connections:
- Sub-connection 1: Interface IE General (Slot 3) -> Subnet 1 -> CPU 0 (TCP/IP).
- Sub-connection 2: Interface IE General (Slot 4) -> Subnet 2 -> CPU 1 (TCP/IP).
- Confirm that both paths remain strictly set to TCP/IP without reverting to MAC.
Download Network Routing:
* Save and compile NetPro (Check and compile all).
* Download connection routing to the PC Station and S7-400H CPUs.

Verification

Verify that fault-tolerant communication operates reliably:
1. Open WinCC Explorer on the virtual OS Server and activate Runtime.
2. Open WinCC Channel Diagnosis (WinCCChannelDiagnosis.exe):
* Select Named Connections.
* Verify that the S7-400H connection displays a green checkmark with status "Ready".
* Expand the connection; verify that both Sub-connection 1 and Sub-connection 2 show status "Connected".
3. Perform a physical failover test: Disconnect physical Ethernet cable from Switch A (Subnet 1); verify that WinCC graphic values continue updating without interruption as communication automatically routes via Subnet 2.
4. Open the S7-400H Diagnostic Buffer; verify zero connection abort errors.

Limitations and Operational Precautions

Subnet Separation: The two redundant plant bus subnets must reside on completely separate IP subnets (e.g., 192.168.1.0/24 and 192.168.2.0/24). Never configure both network adapters with IP addresses in the same subnet, as Windows TCP/IP routing will cause packet looping.
VMware NIC Teaming Caveat: Do not attempt to use VMware software NIC teaming as a replacement for NetPro fault tolerance. PCS 7 Redconnect operates at the application session layer, providing deterministic CPU switchover that hypervisor link teaming cannot replicate.
Redundancy Software Licensing: Virtualized fault-tolerant communication requires the SIMATIC NET SOFTNET-IE S7 REDCONNECT license package (6GK1704-0HB14-0AA0).
Commercial Accuracy: Ensure that the S7-400H hardware (6ES7417-4HT14-0AB0 or 6ES7410-5HX08-0AB0) and SIMATIC NET Redconnect licenses are verified against official Siemens compatibility lists. Obtain quotation verification from an authorized Siemens distributor.

Engineer FAQ

Can I use ISO transport protocol (MAC addressing) in a VMware virtual machine?

While possible using PCIe DirectPath passthrough with physical CP 1623 cards, standard VMware virtual switches (vSwitch) cannot bridge ISO Layer 2 packets across enterprise routed networks. Modern virtualized PCS 7 standardizes on TCP/IP (ISO-on-TCP, RFC 1006).

Why does Softnet-IE Redconnect require two IE General adapters in NetPro?

Because S7 fault tolerance requires two independent network paths. Since each IE General card represents one IP address, establishing two independent IP routes requires two distinct IE General instances.

What happens during an S7-400H CPU switchover on a virtualized OS Server?

The SIMATIC NET Redconnect driver detects that the active CPU has stopped responding and transparently switches cyclic polling to the standby CPU over the secondary sub-connection in under 100 ms, with zero data loss on WinCC.

Do virtual OS Clients also require dual IE General adapters?

No. OS Clients communicate with OS Servers across the Terminal Bus, not the Plant Bus. OS Clients require only a single standard VMXNET3 network adapter.