JUNOS High Availability

Graceful Routing Engine Switchover

Graceful Routing Engine Switchover (GRES) takes advantage of the separation between the control and forwarding in JUNOS software to provide system redundancy. GRES allows the control plane, in this case the RE, to switch over to its backup RE without any interruption to the existing traffic flows in the PFE. When GRES is configured on the router, the kernel state on both REs is synchronized to preserve routing and forwarding state on both REs. Any changes to the routing state on the primary RE result in an automatic incremental update of kernel state on the backup RE. As you can see, the GRES concept is very simple.

However, the limitation of GRES is that, by itself, it cannot provide router redundancy. Even though traffic continues to flow through the router during a switchover between REs, the flow occurs for only a limited time. As soon as any of the protocol timers expire, the neighbor relationship between routers is dropped and traffic is stopped at the upstream router. To maintain high availability, networks must quickly discover when a neighbor goes down and must implement the lowest possible timers. Because GRES provides an intact forwarding plane, it is to our advantage not to drop any of the adjacencies, but rather to continue sending traffic toward the failed router.

The solution to this limitation is to use the Graceful Restart (GR) protocol extension. GR signals all supporting protocols that the failing router is capable of forwarding ...