Siemens SIMATIC HMI Series System Manual page 67

Requirements
The following conditions can lead to undesirable reactions when operating WinCC:
● The IP address band used in the terminal network is higher than the APIPA address band
(169.254.x.x).
● IP addresses are routed via the default gateway.
● IP addresses from the APIPA band are routed to the firewall.
Cause of the increased load at the firewall
Following a system startup, each WinCC station sends its IP address once to all the other
WinCC stations in the network. The WinCC stations define the WinCC station with the lowest
IP address as the server that coordinates availability of the project.
If a WinCC station does not receive an address from the DHCP server and is therefore missing
in the APIPA process, this station becomes the coordinating server. As a result, all the other
WinCC stations attempt to access this server cyclically to publish the project.
The coordinating server, however, cannot be addressed because the IP address from the
APIPA band is automatically transferred to the firewall. This also causes an increased network
load at the central firewall.
Solution
This reaction can be avoided by assigning a unique IP address to each WinCC station.
Information on uninterruptible power system
Please note the following:
WinCC: General information and installation
System Manual, 02/2017, A5E40840020-AA
WinCC Release Notes
2.2 Notes on operation
67