The Routing Engine (RE) is responsible for maintaining the routing table and the forwarding table.
It runs the routing protocols, computes the best paths, and updates the routing table accordingly.
The routing table information is then used to populate the forwarding table, which is provided to the Packet Forwarding Engine (PFE) to make packet-forwarding decisions.

The fundamental architecture of Junos OS is built upon the clean separation of the Control Plane (Routing Engine or RE) and the Forwarding Plane (Packet Forwarding Engine or PFE). While these two components perform vastly different tasks, they must maintain a high-speed, reliable communication channel to synchronize the state of the device. This connection is achieved through an internal Ethernet link .
This internal link serves several critical functions. First, it allows the Routing Engine to " push " the distilled forwarding table (FIB) to the PFE, ensuring that the hardware knows exactly how to handle incoming transit traffic at wire speed. Second, it acts as a conduit for exception traffic . If the PFE receives a packet that it cannot handle alone-such as an OSPF hello, a BGP update, or an ICMP " Time Exceeded " message-it sends that packet over the internal Ethernet link to the RE for processing. This link is carefully policed by built-in rate limiters to ensure that a flood of transit-related exception traffic cannot overwhelm the Routing Engine ' s CPU, a mechanism known as control plane protection. By using a standard Ethernet-based internal fabric, Junos ensures a scalable and modular design where the RE can focus on complex protocol calculations while the PFE focuses on the heavy lifting of packet switching.
Reference: Junos OS Fundamentals, Control and Forwarding Plane Separation.

Junos OS is built upon a highly stable and secure architecture that distinguishes it from many legacy network operating systems. Two of its most defining characteristics are software modularization and the separation of the control and forwarding planes .
Modularization means that Junos OS runs each major function-such as the routing protocol process (rpd), the management process (mgd), and the interface process (dcd)-as an independent software daemon in its own protected memory space. This ensures that if a single process crashes or needs to be restarted, it does not bring down the entire system or interrupt traffic forwarding.
Furthermore, the physical and logical separation of the Control Plane (Routing Engine) and the Forwarding Plane (Packet Forwarding Engine) is a cornerstone of Junos design. The Routing Engine handles complex intelligence, protocol calculations, and management, while the Packet Forwarding Engine performs high- speed packet switching in hardware. This ensures that a heavy management load or a complex routing recalculation won ' t cause " jitter " or packet loss for transit traffic. Conversely, Junos strictly prohibits unsigned third-party binaries to maintain system integrity, and it maintains a clear functional distinction between routing policies (path selection) and security policies (traffic permit/deny).

Understanding the distinction between collision and broadcast domains is fundamental to Layer 2 and Layer 3 networking. In a modern Ethernet switching environment, a switch effectively eliminates collisions by creating a separate collision domain for each of its physical ports. This is achieved through micro-segmentation and the support of full-duplex communication, which allows for simultaneous transmission and reception of data on a per-port basis.
However, while a switch segments collision domains, it does not inherently segment broadcast domains. By default, all ports on a switch reside within the same broadcast domain, meaning that a broadcast frame (such as an ARP request) received on one port will be flooded to all other ports within that specific VLAN or bridge domain. Segmenting broadcast domains requires either the configuration of Virtual LANs (VLANs) or the use of a Layer 3 device like a router, which does not forward broadcast traffic by default between its interfaces. It is a common misconception that broadcast domains are absent in IPv6; while IPv6 utilizes multicast (via NDP) instead of traditional Layer 2 broadcasts, the concept of the "link-local" scope where these packets are distributed remains functionally equivalent to a broadcast domain. For the JNCIA-Junos standard, the core takeaway is that switches provide per-port collision isolation while maintaining a unified broadcast environment unless explicitly partitioned.