RPF Query!!!!!!!!!!

Hey i know the RPF check happens every time when a multicast packet comes to
router to check whether the same interface can be used to reach the source
of the multicast feed.Why does this has been done?? To avoid loops!!!! *WHy
RPF check is mandatory for a multicast packet*?
If to avoid loops Can u give me a classic case of loop formation in
multicasting when RPF is not present

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