JNCIA Summary Notes – Day 1

Router Design

Router has two separate engines.

Control Plane – Routing Engine

This is the central control system. It is based on a single  Intel PCI motherboard and prcessor.


  • Software upgrades and maintenance
  • Monitoring the router
  • Router configuration
  • JUNOS software is stored here
  • Operates all routing protocols
  • Performs all routing table decisions
  • Builds the master routing table with the best paths to destinations and stores them in the forwarding table of the Routing engine

Forwarding Plane – Packet forwarding engine

This is the central location for data packet forwarding. The plane is controlled by ASICs. Contains a passive midplane and multiple boards and processors. Main portions of the engine are:

  • Physical Interface Card (PIC)

Physical media in the router connects to the PIC.

  • Flexible PIC Concentrator (FPC)

Connects to both the switching control board and the router’s interfaces within the Packet Forwarding Engine.

It is controlled by a PowerPC CPU which does not participate in data forwarding

Hosts a Juniper Networks ASIC which interacts with the data packets as they enter and exit the router interfaces.

  • Switching control board. contains PowerCPU and RAM.  Static random access memory (SSRAM) contains the forwarding table for the router

Control board is also refered to as:

* Forwarding Engine Board (FEB) – M5 and M10. Contains only 1.

* System Switching Board (SSB) – M20. Can hold 2 but only 1 is operational at a time

* System Control Board (SCB) – M40. only 1 per chassis

*  Switching and Forwarding Module (SFM) – M40e (2 each but only 1 operational at a time) and M160 (4 each working in parallel)

* Memory Mezzanine Board (MMB) – T320 and T640.


  • Forwarding of data packets across any interface in the router

Components of the Routing Engine 

Software Architecture

JUNOS software is based on the FreeBSD Unix operating system. the Kernel is the heart of the JUNOS software.

Common daemons:

* Routing Protocol Daemon (rpd) – controls protocol messages, routing updates and routing policies

* Device Control Daemon (dcd) – Configuration and mainetnance of both the physical and logical properties of router interfaces

* Management Daemon (mgd) – Controls user access

* Chassis Daemon (chassisd) – Controls properties of the router itself

* Packet Forwarding Engine Daemon (pfed) – Controls communication between the Packet forwarding engine and routing engine

Software components:

* jkernel – basic components of JUNOS

* jbase – additions to the JUNOS since the last update of the jkernel

* jroute – software that operates on the Routing Engine.

* jpfe – Embedded OS software that controls the components of the Packet Forwarding Engine.

* jdocs – software documentation

* jcrypto – controls various security functions

* jbundle – contains all the other packages


# help topic ospf area-backbone

# help reference ospf area

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