Abstract: Techniques are disclosed for performing an In-Service Software Upgrade (“ISSU”) of a first packet forwarding component (PFC) of a virtual router configured to forward traffic flows for a plurality of session instances within a cloud-based data center. The techniques described herein may retain flow state information throughout the ISSU process without interrupting network traffic flow. In one example, a processor of a plurality of compute nodes within the data center receives a request to perform an ISSU of the first PFC. The processor spawns a second virtual routing agent and a second PFC. The second virtual routing agent synchronizes flow state information with a first virtual routing agent for the virtual router. After synchronizing the flow state information, the virtual router switches from forwarding traffic flows by the first PFC to forwarding traffic flows by the second PFC. The ISSU process deactivates the first virtual router and the first PFC.