Hi jsven,
I don't know why you do it this way, because your base search is searching for the multiple MsgId but further down the pipe you discard them again....could it be those are multivalve fields and/or your events are not properly line broken? Anyway, probably you have a reason to do so; so let me help you....
try this search:
mysearch... (MsgId=AUT22670 OR MsgId=AUT24414 OR MsgId=AUT22673 OR MsgId=AUT23574 OR MsgId=AUT20915 OR MsgId=AUT22886)
| dedup User
| search NOT MsgId="AUT22673" OR NOT MsgId="AUT23574" OR NOT MsgId="AUT20915" OR NOT MsgId="AUT22886"
| eval Cluster="C"+substr(Node,10,1)
| table MsgId
Keep in mind, try to avoid NOT search, instead search for what you want and need.
Also keep in mind if you have multi value fields, it will still match events which for example holds a value of MsgId="AUT11111, AUT20915" . To remove those events as well use the NOT MsgId="*AUT20915*" , but this will be a bad performer on large searches.
Here is a link to a .conf slide about Search Efficiency Optimisation http://conf.splunk.com/session/2015/conf2015_JHarty_DuncanTurnbull_Splunk_UsingSplunkSearchLanguage_AQuickGuideToSearch.pdf
Hope this helps ...
cheers, MuS
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