The Junos operating system is a robust, modular operating system that provides industry-leading performance and scalability. It uses a single software train across all Juniper platforms for ease of management. The Junos OS separates the control plane and forwarding plane functions, with the routing engine maintaining routing/forwarding tables and controlling the packet forwarding engine. Platforms running Junos span switching, routing, and security roles for networks of all sizes.

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Junos Operating System
Fundamentals

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The Junos OS
 Robust, modular operating system
•Provides industry-leading performance and scalability
•Based on the FreeBSD UNIX operating system

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Single Software Train
 A single software train for all platforms running the
Junos OS
•Eases management overhead by providing a consistent set
of features that are implemented in a consistent manner
12.0 12.1 …
TX Matrix
J2320

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Separation of Control and Forwarding
 All platforms running the Junos OS share a common
design goal:
•Clean separation of control and forwarding functions
Packet Forwarding Engine
Routing Engine
Frames/Packets In Frames/Packets Out
FT
RT The
Junos OS
FT
Control Plane
Forwarding Plane
Internal Link

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Routing Engine
 Maintains routing and forwarding tables
 Controls and monitors the chassis
 Manages the PFE
Packet Forwarding Engine
Routing Engine
FT
RT The
Junos OS
Control Plane
Forwarding Plane

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Packet Forwarding Engine
 Uses Layer 2 and Layer 3 forwarding tables, provided
by the RE, to forward traffic toward its destination
 Implements various services such as policing,
stateless firewall filtering, and class of service
Packet Forwarding Engine
Routing Engine
Frames/Packets In Frames/Packets Out
FT
Control Plane
Forwarding Plane

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Transit Traffic Processing
 Transit traffic is forwarded through the local system
•PFE uses the forwarding table provided by the RE
•Examples of transit traffic include unicast and multicast
traffic
Packet Forwarding Engine
Routing Engine
Frames/Packets In Frames/Packets Out
FT
Control Plane
Forwarding Plane
CPU

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Exception Traffic Processing (1 of 2)
 Exception traffic is processed by the local system
•Traffic destined for the local system is processed by RE CPU
•In most cases, the PFE processes traffic requiring the
generation of ICMP messages, such as TTL expired
Frames/Packets In
Frames/Packets Out
Packet Forwarding Engine
Routing Engine
Control Plane
Forwarding Plane
CPU

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CPU
Exception Traffic Processing (2 of 2)
 Exception traffic is rate-limited on the internal link to
protect the RE from potential DoS attacks
•Control traffic is given preference when congestion exists
Frames/Packets In
Built-In Rate Limiting
Packet Forwarding Engine
Routing Engine
Control Plane
Forwarding Plane

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Overview of Junos Devices
 Platforms running the Junos OS span switching,
routing, and security roles and are suited for small to
large networks in both enterprise and service provider
environments
Switching
Routing
Security

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Junos Routing Devices
ACX Series LN Series M Series
MX Series PTX Series T Series

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Junos Switching Devices
EX Series QFX Series

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Junos Security Devices
J Series
SRX Series