Graceful Routing Engine Switchover
As noted previously, GRES is a feature that permits Juniper routers with dual REs to perform a switch in RE mastership without forcing a PFE reset. This permits uninterrupted dataplane forwarding, but unless combined with GR or NSR, does not in itself preserve control plane or forwarding state.
The foundation of GRES is kernel synchronization between the master and backup routing engines using Inter-Process Calls (IPC). Any updates to kernel state that occur on the master RE, for example to reflect a changed interface state or the installation of a new next-hop, are replicated to the backup RE as soon as they occur and before pushing the updates down into other parts of the system, for example, to the FPCs. If the kernel on the master RE stops operating, experiences a hardware failure, a configured process is determined to be thrashing, or the administrator initiates a manual switchover, mastership switches to the backup RE.
Performing a switchover before the system has synchronized leads to an all-bets-off situation. Those PFE components that are synchronized are not reset, while the rest of the components are. Junos enforces a GRES holddown timer that prevents rapid back-to-back switchovers, which seems to be all the rage in laboratory testing. The 240-second (4-minute) timer between manually triggered GRES events is usually long enough to allow for complete synchronization, and therefore helps to ensure a successful GRES event. The holddown timer ...