When you install a new indexer, you have to run a configuration at search head to make the indexer available to searching. Following the steps below:
Log into Splunk Web on the search head and click Settings at the top of the page.
Click Distributed search in the Distributed Environment area.
Click Search peers.
On the Search peers page, select New.
Specify the search peer, along with any authentication settings.
Note: You must precede the search peer's host name or IP address with the URI scheme, either "http" or "https".
Click Save.
Repeat for each of the search head's search peers.
For further information, check this document
https://docs.splunk.com/Documentation/Splunk/8.0.0/DistSearch/Configuredistributedsearch
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