Transit traffic represents the primary " workload " of a Junos device; it is the data that enters one network interface and exits another, destined for a remote host. Unlike exception traffic, transit traffic is forwarded exclusively by the Packet Forwarding Engine (PFE) . The PFE uses specialized Application-Specific Integrated Circuits (ASICs) or programmable NPUs to perform lookups in the hardware-based forwarding table (FIB) at wire speed.

A defining characteristic of transit traffic is that it does not require control plane processing . Once the Routing Engine (RE) has populated the PFE with the necessary forwarding instructions, the RE steps out of the way. The packets pass through the PFE ' s ingress processing, lookups, and egress queuing without ever consuming CPU cycles on the Routing Engine. This bypass is what allows Junos devices to maintain massive throughput and low latency, even if the RE is busy recalculating a complex BGP table. Routing protocol packets (like OSPF updates) and traffic destined for the router ' s own management IP address are explicitly not transit traffic; they are control plane traffic because they terminate at the device ' s " brain. " Transit traffic is strictly " pass-through " data.