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, 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, 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, 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.