Siemens SCALANCE X-400 Operating Instructions Manual page 20

Introduction to Industrial Ethernet Switches
The two devices connected in the configuration exchange data frames with each
other to synchronize their operating statuses (one device is master and the other
slave). If there are no problems, only the link from the master to the other network
segment is active. If this link fails (for example due to a link-down or a device fail-
ure), the slave activates its link as long as the problem persists.
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Operating Instructions for SCALANCE X-400 Industrial Ethernet Switches
C79000-G8976-C186-03