Abstract: The control plane of a network device comprises a plurality of software processes that manage routing control operations of the device. Through a hypervisor in the control plane, a managing virtual machine controls access to a first virtual machine running a first software system to control a routing communication session between the network device and other network devices. In response to an in-service software upgrade request, the managing virtual machine initializes a second virtual machine. On the second virtual machine, the second software system is loaded. State data maintained by the managing virtual machine can be transferred to the second virtual machine, and the second virtual machine takes control of the routing communication session. During the transfer of control from the first virtual machine to the second virtual machine, techniques of “non-stop forwarding” and “graceful restart” can be implemented to minimize the effect the switchover has on the network.