Junos OS Fundamentals
Architecture: two planes
The single most important idea is that Junos OS, built on FreeBSD, separates the control plane from the forwarding plane. The Routing Engine (RE) runs routing protocols, maintains the routing table (RIB) and runs management processes. The Packet Forwarding Engine (PFE) forwards packets in hardware using the forwarding table (FIB). rpd builds the RIB, hands selected next hops to the kernel, and the kernel programs the PFE.
The daemons
Names you should recognize: mgd (management and CLI), rpd (routing protocols), chassisd (chassis), dcd (interface/daemon control), snmpd, mib2d, alarmd, and the pfed/sfid family handling forwarding. show chassis routing-engine shows RE state, memory, CPU and temperature.
One transactional configuration model
All configuration lives in a candidate configuration. You edit it, then commit; a successful commit validates syntax, activates the configuration, and archives the previous version so rollback is possible. Nothing takes effect until the commit succeeds, whether the change came from the CLI, J-Web, NETCONF or a script. This is the idea most exam questions circle back to.
Platforms and high availability
Junos runs on branch SRX, MX, EX/QFX switching and virtual vSRX/vMX images with a largely identical CLI, but feature support varies by platform and release. Devices with dual REs support Graceful Routing Engine Switchover and nonstop routing so forwarding survives a failover.
Traps
Confusing RE and PFE roles (OSPF never runs on the PFE); assuming a commit is needed for show commands; assuming every feature exists on every platform.
Rule of thumb
"Where does the routing protocol run?" Routing Engine. "Where is the packet actually dropped or forwarded?" Packet Forwarding Engine.