Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?


Operating System Infrastructure and Processes

Junos OS includes the processes that run the device, including IP routing, Ethernet switching, managing interfaces, and a variety of other functions.

Junos OS runs on the Routing Engine. The Routing Engine kernel coordinates communication among the software processes and provides a link to the Packet Forwarding Engine.

Using the CLI, you configure device features and set the properties of network interfaces. After activating a software configuration, use either the Junos Space or CLI user interface to monitor, manage operations, and diagnose protocol and network-connectivity problems.

Routing Engine and Packet Forwarding Engine

A Juniper Networks router or switch has two primary software processing components:

  • Packet Forwarding Engine—Processes packets; applies filters, routing policies, and other features; and forwards packets to the next hop along the route to their final destination.

  • Routing Engine—Provides three main functions:

    • Maintains the routing tables used by the network device and controls the routing protocols that run on the device.

    • Packet forwarding, which provides route lookup, filtering, and switching on incoming data packets, and then directs outbound packets to the appropriate interface for transmission to the network.

    • Provides control and monitoring functions for the device.

Junos OS Processes

The Junos OS software running on the device consists of multiple processes that are responsible for individual functions.

The separation of functions provides operational stability, because each process accesses its own protected memory space.

The following table describes the primary software processes.

Table 1: Junos OS Processes




Chassis process


Detects hardware on the system that is used to configure network interfaces.

Monitors the physical status of hardware components and field-replaceable units (FRUs), detecting when environment sensors such as temperature sensors are triggered.

Relays signals and interrupts—for example, when devices are taken offline, so that the system can close sessions and shut down gracefully.

DNS server process


Resolves hostnames into addresses.

Dynamic Host Configuration Protocol (DHCP) process


Enables a DHCP server to allocate network IP addresses and deliver configuration settings to client hosts without user intervention.

Ethernet switching process


Handles Layer 2 switching functionality such as MAC address learning, spanning-tree protocols, and access port security.

Manages Ethernet switching interfaces, VLANs, and VLAN interfaces.


This process is not applicable for EX2300 and EX3400 switches.

Firewall management process


Manages the firewall configuration and helps accept or reject packets that are transiting an interface on a switch.

Forwarding process


Defines how routing protocols operate on the partition. The overall performance of the partition is largely determined by the effectiveness of the forwarding process.

Interface process


Configures and monitors network interfaces by defining physical characteristics such as link encapsulation, hold times, and keepalive timers.

Integrated Local Management Interface (ILMI) process


Provides bidirectional exchange of management information between two ATM interfaces across a physical connection.

Link Management Protocol (LMP) process


Establishes and maintains LMP control channels.

Management process


Provides communication between the other processes and an interface to the configuration database.

Populates the configuration database with configuration information and retrieves the information when queried by other processes to ensure that the system operates as configured.

Interacts with the other processes when commands are issued through one of the user interfaces on the partition.

If a process terminates or fails to start when called, the management process attempts to restart it a limited number of times to prevent thrashing and logs any failure information for further investigation.

Multicast snooping process


Makes Layer 2 devices, such as VLAN switches, aware of Layer 3 information, such as the media access control (MAC) addresses of members of a multicast group.

Routing protocol process


Defines how routing protocols such as RIP, OSPF, and BGP operate on the device, including selecting routes and maintaining forwarding tables.

Secure Neighbor Discovery (SEND) protocol process


Protects Neighbor Discovery Protocol (NDP) messages.

Simple Network Management Protocol (SNMP) process


Enables the monitoring of network devices from a central location and provides the switch’s SNMP primary agent.

Tunnel OAM process


Enables the Operation, Administration, and Maintenance of Layer 2 tunneled networks. Layer 2 protocol tunneling (L2PT) allows service providers to send Layer 2 protocol data units (PDUs) across the provider’s cloud and deliver them to devices that are not part of the local broadcast domain.

Virtual Router Redundancy Protocol (VRRP) process


Enables hosts on a LAN to make use of redundant routing platforms on that LAN without requiring more than the static configuration of a single default route on the hosts.