This read-only Counter64 counts the total IGMP join (membership report) messages received on this PM instance. A steady climb is expected on a busy multicast/IPTV access port, but a sudden step change usually means a channel-change storm or a set-top box repeatedly rejoining; because it is drawn from the most recently completed short-interval historical bin, it is a closed record of the interval right before the current one, useful for checking whether a problem seen right now had already started in the previous window. On an uplink toward the multicast-enabled core router, a count of 1,684 over the last completed collection window is the kind of number an admin would trend against the join/leave rate on the same port to judge whether it's normal channel-surfing or an anomaly worth a closer look.