How are the Routing Engine and the Packet Forwarding Engine components connected?

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.