Today software upgrades to have new features or capabilities, or to keep maintenance which is still the primary cause to inaccessibility of network system. The design of In-Service Software Upgrade (ISSU) system of redundant route processors takes advantage of Nonstop Forwarding with Stateful Swit- chover(NSF/ SSO), patching, IPC and hardware redundance to permit true in-service software upgrades or version changes while continuously forwarding user traffic. In the upgrading process of the compatibility check and rollback timer design are considered. So the ISSU system have faster upgrades, minimal impact to service and higher availability.