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Initiate the Pairing process as described in the following Knowledge Base article. In doing so, it should be noted, that the pairing process only has to be applied to the master switch. Therefore only the master switch needs an LMC licence.
Pairing -Prozess eines LANCOM Gerätes mit der a LANCOM device with the LMC
3) Konfigurationsmöglichkeiten des Stacking-Verbunds in der Configuration options of the stack in the LANCOM Management Cloud:
3.1) Connect to the LMC, and in the menu Devices click on the stacking group under Name. It is easily recognizeable as a "Layer" symbol with the device count is shown under Model.
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