Juniper JN0-106 Exam - Topic 1 Question 15 Discussion

The architecture of a Junos device is bifurcated into two primary functional planes: the Control Plane, managed by the Routing Engine (RE), and the Data Plane, managed by the Packet Forwarding Engine (PFE). The Routing Engine serves as the 'brain' of the device. One of its primary responsibilities is the processing of management traffic, which includes handling CLI sessions (SSH, Telnet), SNMP requests, and system logging. Because the RE runs the Junos OS kernel, it provides the environment for all administrative tasks and system management utilities.

Additionally, the Routing Engine is responsible for the intelligence of the network, which involves running routing protocols (such as OSPF, BGP, or IS-IS) and maintaining the master routing tables. It populates the Routing Information Base (RIB) with all learned paths and then calculates the best paths to build the Forwarding Information Base (FIB). This FIB is then pushed to the PFE for hardware-level packet switching. It is a common misconception that the RE handles transit traffic; however, the RE only handles 'exception traffic' or traffic destined for the device itself. This separation ensures that the control plane remains stable and responsive even during periods of heavy transit load on the forwarding plane. Reference: Junos OS Fundamentals, Architectural Overview, Control Plane vs. Forwarding Plane.