A system and method can support subnet manager (SM) level robust handling of an incoming unknown management key in an InfiniBand (IB) network. A local SM in a fabric is adaptive to receive an incoming subnet management packet (SMP) from a remote SM, wherein the SMP contains a first management key. Furthermore, the local SM operates to check whether the first management key is consistent with a configuration associated with the local SM, and can silently drop the incoming SMP if the first management key is not consistent with the local SM configuration.