Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for switch virtualization by a switch proxy controller. In an aspect, a method includes receiving, by a switch proxy controller, a first request from a first switch fabric, where the first request indicates a first identifier that identifies the first request from other requests from the first switch fabric, generating a second request that indicates a second identifier that identifies the second request from other requests sent from the switch proxy controller to a switch, providing the second request to the switch, receiving, by the switch proxy controller, a first reply that indicates the second identifier indicated in the second request, generating, based on the second identifier indicated in the first reply, a second reply that indicates the first identifier, and selecting the first switch fabric to receive the second reply based on the second identifier.