A read-only counter tracking the byte volume of frames successfully received on this interface whose destination address had the multicast bit set. This lets an admin see how much of the interface's inbound traffic is multicast versus unicast, useful for spotting excessive multicast flooding on a segment; these bytes are also included in the standard ifInOctets total. A sudden spike in etherMulticastBytesIn on a port could indicate a multicast storm or a device generating excessive multicast traffic on that segment.