Compatibility between peer nodes and search heads
Starting with 6.3, the peer nodes and search heads can run different versions from each other.
The search heads must run the same or a later version from the peer nodes.
Search head clusters participating in an indexer cluster have the same compatibility requirements as individual search heads. For information on other search head cluster version requirements, see "System requirements and other deployment considerations for search head clusters" in the Distributed Search manual.
More details here
http://docs.splunk.com/Documentation/Splunk/7.1.1/Indexer/Systemrequirements#Splunk_Enterprise_version_compatibility
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