confIfIgmpRobustness sets the IGMP robustness variable for this interface, meaning how many query attempts the router allows for before concluding a group has no more members, per its description. An admin raises this on a lossy link where IGMP query or report packets might be dropped, so the router doesn't prematurely stop forwarding multicast traffic for a group that's still actually in use. For example, increasing confIfIgmpRobustness from 2 to 3 on a wireless-backed link would make the router tolerate one more lost query or response round before pruning a multicast group as inactive.