For Juniper platforms equipped with dualRouting Engines (REs), the fundamental technology required to provide high availability during a hardware or software failure of the primary RE isGraceful Routing Engine Switchover (GRES).
According to Juniper Networks technical documentation, GRES allows the backup RE to stay in a "hot" standby state. When GRES is enabled, the primary RE synchronizes critical state information with the backup RE, specifically thechassis stateand theinterface state. This synchronization includes the Packet Forwarding Engine (PFE) configuration.
When the primary RE fails, the backup RE takes over immediately. Because the PFE (which resides on the line cards) was already synchronized and is not restarted during the switchover, the routercontinues to forward packetsthat are already in flight or part of established flows. This prevents a complete network outage during an RE failover.
Comparison with other options:
NSB (Non-Stop Bridging - Option A):Focuses specifically on maintaining Layer 2 protocol states (like STP) during a switchover.
LAG (Link Aggregation - Option B):Provides redundancy for physical links, not the control plane or the RE.
BFD (Bidirectional Forwarding Detection - Option C):Is a protocol used for rapid detection of link or neighbor failures; it does not protect the RE or maintain forwarding during an internal switchover.
It is important to note that while GRES maintains theforwardingstate, it does not by itself maintain therouting protocolstate (adjacencies). To keep OSPF or BGP sessions from dropping during the switchover, GRES must be paired withNon-Stop Active Routing (NSR). However, as the question focuses on the core requirement of continuing to forward packets,GRESis the foundational technology.
