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The Central Licensing Server is not redundant.
When CLS is lost, plant operation will be possible but license annoyance will start after some interval depending on version of 800xA.
The possibility to launch engineering tools will probably cease as soon as CLS is lost. Already open engineering tools (to which licenses have been granted) may continue to work (for a while).
I haven't tested myself: but if the CLS node is made to be pointed to by a DNS A-record, you could have the CLS software kit installed to a secondary node. If the primary CLS fail, move the license dongle to the secondary CLS and update the DNS A-record to point to the secondary server.
I suspect that you may have to restart the ABB Client License Provider service and/or flush the DNS cache (ipconfig /flushdns) in all nodes to speed up the selection of the secondary CLS; however this can easily be done with the help of some Command Shell or Power Shell scripts.