Systems and methods for on-demand unicast forwarding in a high performance computing environment, in accordance with an embodiment. An exemplary method can provide, at a computer comprising one or more microprocessors, a plurality of switches, a plurality of host channel adapters, and a plurality of hosts. The method can provide, at two rails, redundant connectivity between the plurality of hosts, wherein each rail comprises a set of the plurality of switches. The method can detect a disruption i connectivity between two hosts of the plurality of hosts. The method can re-establish communication between the two hosts of the plurality of hosts via unicast packets on an inter-rail link, the inter-rail link being provided between the two rails.