When designing High Availability (HA) for Juniper Service Provider routers, understanding the interaction between the control plane and data plane is vital. The user has already enabledGraceful Routing Engine Switchover (GRES), which synchronizes the interface and kernel state between the primary and backup Routing Engines (REs). However, GRES by itself does not preserve the routing protocol state (like OSPF adjacencies or BGP sessions).
To achieve the preservation of the routing protocol state without relying on external "helper" routers, you must implementNon-Stop Active Routing (NSR). According to Juniper Networks documentation, NSR uses the infrastructure provided by GRES to also synchronize the routing protocol process (rpd) information. Under NSR, the backup RE maintains a "hot" standby state of all routing protocols. If the primary RE fails, the backup RE takes over immediately. Because it already possesses the full routing table and peer session states, the peering neighbors are unaware that a switchover occurred. No protocol adjacency resets occur, and traffic continues to flow uninterrupted.
It is crucial to differentiate NSR fromGraceful Restart (Option C). While Graceful Restart also aims to maintain traffic flow during a switchover, itdoesrequire help from neighboring routers (known as "helper mode"). If the neighbors do not support or are not configured for Graceful Restart, the sessions will drop. Since the user explicitly stated they want to "avoid relying on helper routers," Graceful Restart is not the correct solution.
Non-stop Active Bridging (Option A)provides a similar "hitless" failover but specifically for Layer 2 environments (STP/VLANs) rather than Layer 3 routing protocols.BFD (Option B)is a failure detection protocol used to speed up convergence but does not preserve state during an RE failover; in fact, without NSR, BFD would likely trigger a faster teardown of the session during a switchover. Therefore,NSRis the only feature that meets the requirement for independent control-plane preservation.
