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M120 Routing Engine Description


The Routing Engine runs the Junos OS. The software processes that run on the Routing Engine maintain the routing tables, manage the routing protocols used on the router, control the router interfaces, control some chassis components, and provide the interface for system management and user access to the router.

You can install one or two Routing Engines in the router. The Routing Engines install into the rear of the chassis in vertical slots directly into the CB labeled CB0 and CB1. If two Routing Engines are installed, one functions as the master and the other acts as the backup. If the master Routing Engine fails or is removed, and the backup is configured appropriately, the backup takes over as the master. For detailed information, see Taking the M120 Host Subsystem Offline.

If the host system is redundant, the backup Routing Engine is hot-removable and hot-insertable, but the master Routing Engine is hot-pluggable. A Routing Engine that is not redundant is hot-pluggable. Each Routing Engine requires a CB to be installed in the adjacent slot. RE0 installs below CB0, and RE1 installs below CB1. A Routing Engine does not power up if it is not installed into the CB.


If two Routing Engines are installed, they must both be the same hardware model.

There is a USB memory device that connects directly into the front of Routing Engine. The USB port allows you to plug in a USB keychain device.

Figure 1: M120 Routing Engine
M120 Routing Engine
Figure 2: RE-A-1800x2 Routing Engine
RE-A-1800x2 Routing Engine

Routing Engine Components

Each Routing Engine (shown in Figure 1) consists of the following components:

  • CPU—Runs Junos OS to maintain the router's routing tables and routing protocols. It has a Pentium-class processor.

  • DRAM—Provides storage for the routing and forwarding tables and for other Routing Engine processes.

  • USB port—Provides a removable media interface through which you can install the Junos OS manually. See Figure 3. Junos supports USB version 1.0.

  • CompactFlash card—Provides primary storage for software images, configuration files, and microcode. The disk is a fixed compact flash and is inaccessible from outside the router.

  • Hard disk—Provides secondary storage for log files, memory dumps, and rebooting the system if the CompactFlash card fails.

  • LED—Indicates disk activity for the internal IDE interface. It does not necessarily indicate routing-related activity.


    The LEDs that report host module status (including Routing Engine status) are on the craft interface rather than the Routing Engine faceplate.

  • HDD LED—Indicates disk activity for the hard disk drive.

  • Interfaces for out-of-band management access—Provide information about Routing Engine status to devices (console, laptop, or terminal server) that can be attached to access ports located on the craft interface.

    Each Routing Engine has one 10/100-Mbps Ethernet port for connecting to a management network, and two asynchronous serial ports—one for connecting to a console and one for connecting to a modem or other auxiliary device.

  • EEPROM—Stores the serial number of the Routing Engine.

  • Reset button—Reboots the Routing Engine when pressed.

  • Offline button—Takes the Routing Engine offline when pressed.

  • Extractor clips—Control the locking system that secures the Routing Engine.


For specific information about Routing Engine components (for example, the amount of DRAM), issue the show chassis routing-engine command.

Figure 3: USB Memory Device in an M120 Routing Engine
USB Memory Device in an M120 Routing

Routing Engine Boot Sequence

The Routing Engine boots from the storage media in this order: the USB device, then the CompactFlash card (if present), then the hard disk (or two solid state drives (SSD) in the case of the RE-A-1800x2, which appear as DISK1 and DISK2), and then the LAN. The disk from which the router boots is called the primary boot device, and the other disk is the alternate boot device.


If the router boots from an alternate boot device, a yellow alarm lights the LED on the router’s craft interface.