Which statement is correct about graceful Routing Engine switchover (GRES)?

Graceful Routing Engine switchover (GRES) enables a router with dual Routing Engines to switch from a primary to a backup RE without interrupting packet forwarding. However, GRES alone only preserves interface and kernel state; it does not preserve routing protocol state, meaning the routing protocol process (rpd) must restart upon switchover. When GRES is combined with Nonstop Active Routing (NSR), the backup RE runs its own instance of rpd, which remains fully synchronized with the primary RE's routing state. Consequently, during a switchover, routing protocol adjacencies are preserved, and the new master RE does not need to restart rpd, ensuring seamless routing continuity without relying on neighboring routers.

A: GRES specifically prevents the Packet Forwarding Engine (PFE) from restarting and preserves kernel and interface information to maintain uninterrupted packet forwarding.

B: Helper mode and restarting mode are operational states associated with Graceful Restart (GR), a completely different high availability protocol, not GRES.

D: GRES alone does not preserve routing state. Without NSR, the new master RE must restart the routing protocol process (rpd), causing routing adjacencies to drop.

1. Juniper Networks Official Documentation: Junos OS High Availability User Guide. Section: "Understanding Graceful Routing Engine Switchover".

Locator: Paragraphs 1-3. It explicitly states that GRES preserves interface and kernel information but requires NSR or Graceful Restart to preserve routing state, otherwise rpd restarts.

2. Juniper Networks Official Documentation: Junos OS High Availability User Guide. Section: "Understanding Nonstop Active Routing".

Locator: "How NSR Works" section. It documents that NSR uses the GRES infrastructure and runs the routing protocol process (rpd) on the backup RE to synchronize state, preventing rpd from needing to restart during a mastership change.