Systems and methods for supporting unique multicast forwarding across multiple connected subnets in a high performance computing environment. In accordance with an embodiment, by enforcing that incoming (i.e., incoming on a router port of a subnet) multicast packets have SGIDs (source global identifiers) that correspond to a restricted set of source subnet numbers when entering the ingress router ports to a local subnet, it is possible to ensure that multicast packets sent from one subnet are never returned to the same subnet through a different set of connected router ports (i.e., avoid looping multicast packets).