Once you enable option to "Download repository Index updates on startup", Maven will remember it and it will keep updating index every time you restart the Eclipse.Sometime this makes your Eclipse little bit slower, so you might want to disable it and then do it manually.This will rebuild repository index for local use, and will help you to quickly add any JAR file dependency in your Maven project.
Eclipse maven updating indexes forever day dating and relationship
Eclipse was not able to search dependency in Maven central repository.
I was surprised because I have never faced this issue while I was using M2Eclipse plugin from long time.
Then I quickly checked Maven settings in Eclipse and found that the option which is required to download and build the index was not turned on. To give you some background about how Eclipse search dependency on Maven remote repository, be it default Maven central or internal Nexus repository.
Eclipse search artifact dependency on repository's index file.
Problem is fixed now, you can see that Eclipse is now able to search dependency from Maven repository.