Yes, i understand what they recommended in the class. If you start with clustering and when you have the additional indexers, the old data will be replicated. If you add clustering option later, then old data will not get replicated.
This approach will work, although you will be better of having both the machines available from the start. The other thing to consider is the time gap between the addition of new machines. If it is few days is one thing, if it is few months then the replication traffic to catch up the old data might saturate your network.
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