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1. Introduction To Junos OS

posted 28 Sep 2013, 12:42 by Santosh Kumar

Juniper Junos operating System is Robust and modular operating system which is based on Free BSD UNIX OS. In junos OS for complete function of device there are multiple process running on it and each process handles a portion of that device functionality and each process is having its own protected memory space so processes can’t interfere directly and if one process fails it’s not necessary that entire device fails. This modular approach also provide flexibility to add new features without any problem. All Junos based juniper devices share the same source code but they may have minor changes in Junos image depending on their network role. In this OS we have clear separation in Control Plane and Forwarding Plane


As shown in above picture Control Plane runs Routing Engine (RE) which is acting as brain of the platform and responsible for protocol updates, System Management and to maintain Routing Table (RT), Bridging Table and Primary Forwarding Table (FT). RE connects to Packet Forwarding Engine (PFE) through an internal link and sends Forwarding Table (FT). PFE runs in Forwarding Plane and responsible for Forwarding transit traffic through the device, in many platform PFE uses Application-Specific Integrated Circuits (ASICs) for increase performance. PFE gets FT from RE through internal link, FT updates are a high priority updates for Junos kernel and performed incrementally.

                Whenever a packet comes to Junos device in general PFE deals with packet and forward them as per FT provided by RE but some exceptional traffic like routing updates, management connections, traffic destined to device local IP or packet which requires ICMP message generation are processed by RE and such type of exceptional traffic is rate-limited on the internal link to protect RE from potential DOS attack.

                Junos OS runs on Routing, Switching & Security Devices and these devices are suitable for small to large Enterprise/Service Provider Network. Few example of Junos devices are

M Series Multiservice Routers

T Series Core Routers

J Series Services Routers

MX Series Ethernet Services Routers

EX Series Ethernet Switches

SRX Series Services Gateways.

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