I have created a Splunk app that resides on splunkbase (https://splunkbase.splunk.com/app/3779/) that was specifically meant for easily integrating a CMDB with the Splunk ITSI product suite. The whole goal was that you could filter the CMDB topology based on classes and relationships and only insert those CI’s that are relevant for the Splunk ITSI usecases.
This app, although still on splunkbase, is no longer supported for Splunk installations at version 8.x. I do have a much better solution that is more flexible and reliable and one that even offers functionality for Splunk Enterprise (core) itself.
See the other products.