A system for supporting inter subnet partitions in a high performance computing environment , comprising: one or more microprocessors; a first subnet , the first subnet comprising a plurality of switches, the plurality of switches comprising at least a leaf switch, wherein each of the plurality of switches comprise a plurality of switch ports, a plurality of host channel adapters, each host channel adapter comprising at least one host channel adapter port, a plurality of end nodes (2030-2033), wherein each of the end nodes are associated with at least one host channel adapter of the plurality of host channel adapters, a subnet manager , the subnet manager running on one of the plurality of switches and the plurality of host channel adapters, and an inter subnet manager , the inter subnet manager running on one of the plurality of switches and the plurality of host channel adapters; a fabric manager , the fabric manager residing in the high performance computing environment; wherein a switch port of the plurality of switch ports on a switch of the plurality of switches is configured as a router port ; wherein the switch port configured as the router port is logically connected to a virtual router of the first subnet; wherein the fabric manager defines a range of partition key, "P_Key", values within a plurality of partition key values, the defined range of partition key values comprising an Inter Subnet Partition, "ISP", P_Key range; and wherein the fabric manager communicates to the subnet manager the ISP P_Key range. one or more microprocessors; a first subnet , the first subnet comprising a plurality of switches, the plurality of switches comprising at least a leaf switch, wherein each of the plurality of switches comprise a plurality of switch ports, a plurality of host channel adapters, each host channel adapter comprising at least one host channel adapter port, a plurality of end nodes (2030-2033), wherein each of the end nodes are associated with at least one host channel adapter of the plurality of host channel adapters, a subnet manager , the subnet manager running on one of the plurality of switches and the plurality of host channel adapters, and an inter subnet manager , the inter subnet manager running on one of the plurality of switches and the plurality of host channel adapters; a plurality of switches, the plurality of switches comprising at least a leaf switch, wherein each of the plurality of switches comprise a plurality of switch ports, a plurality of host channel adapters, each host channel adapter comprising at least one host channel adapter port, a plurality of end nodes (2030-2033), wherein each of the end nodes are associated with at least one host channel adapter of the plurality of host channel adapters, a subnet manager , the subnet manager running on one of the plurality of switches and the plurality of host channel adapters, and an inter subnet manager , the inter subnet manager running on one of the plurality of switches and the plurality of host channel adapters; a fabric manager , the fabric manager residing in the high performance computing environment; wherein a switch port of the plurality of switch ports on a switch of the plurality of switches is configured as a router port ; wherein the switch port configured as the router port is logically connected to a virtual router of the first subnet; wherein the fabric manager defines a range of partition key, "P_Key", values within a plurality of partition key values, the defined range of partition key values comprising an Inter Subnet Partition, "ISP", P_Key range; and wherein the fabric manager communicates to the subnet manager the ISP P_Key range. <EOS>