The relationship between the routing table (Routing Information Base or RIB) and the forwarding table (Forwarding Information Base or FIB) is the cornerstone of the Junos OS architectural separation between the control and data planes. The routing table resides in the control plane on the Routing Engine (RE). Its primary role is to aggregate all reachability information from every configured protocol and select the best route for each destination prefix based on criteria like preference and metrics.

Once the Routing Engine has identified the single " active " best path for a destination, it distills this complex routing data into a simplified, streamlined version called the forwarding table . The forwarding table contains only the essential information needed to move a packet: the destination prefix, the egress interface, and the Layer 2 next-hop MAC address. The RE then " pushes " this forwarding table to the Packet Forwarding Engine (PFE) in the data plane. By having the PFE use a table built from the routing table , Junos ensures that transit traffic can be processed at wire speed without the overhead of complex routing logic. This one-way flow—from the comprehensive RIB on the RE to the optimized FIB on the PFE—allows the device to maintain high performance even while routing protocols are recalculating or the management plane is under heavy load.