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The Routing Engine is an Intel-based PC platform that runs Junos OS. 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 front of the chassis in horizontal slots in the SCBs labeled 0 and 1. 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.

The Routing Engines are hot-pluggable. Each Routing Engine must be installed directly into an SCB. A USB port on the Routing Engine accepts a USB memory card that allows you to load Junos OS.

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 2.) Junos OS supports USB version 1.0.

  Figure 2: USB Memory Device in a Routing Engine

  

• CompactFlash card—Provides primary storage for software images, configuration files, and microcode. The CompactFlash card is fixed 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.
• Interface ports—The AUX, CONSOLE, and ETHERNET provide access to management devices. 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.
• Online/Offline button—Takes the Routing Engine online or offline when pressed.
• Extractor clips—Used for inserting and extracting the Routing Engine.
• Captive screws—Secure the Routing Engine in place.

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

Routing Engine Interface Ports

Three ports, located on the right side of the routing engine, connect the Routing Engine to one or more external devices on which system administrators can issue Junos OS command-line interface (CLI) commands to manage the router (see Figure 1).

The ports with the indicated labels function as follows:

• AUX—Connects the Routing Engine to a laptop, modem, or other auxiliary device through a serial cable with an RJ-45 connector.
• CONSOLE—Connects the Routing Engine to a system console through a serial cable with an RJ-45 connector.
• ETHERNET—Connects the Routing Engine through an Ethernet connection to a management LAN (or any other device that plugs into an Ethernet connection) for out-of-band management. The port uses an autosensing RJ-45 connector to support 10-Mbps or 100-Mbps connections. Two small LEDs on the top of the port indicate the connection in use: the LED flashes yellow or green for a 10-Mbps or 100-Mbps connection, and the LED is light green when traffic is passing through the port.

Routing Engine Boot Sequence

The Routing Engine boots from the storage media in this order: the USB device (if present), then the CompactFlash card, then the hard disk, 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.

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

If the Routing Engines are configured for graceful switchover, the backup Routing Engine automatically synchronizes its configuration and state with the master Routing Engine. Any update to the master Routing Engine state is replicated on the backup Routing Engine. If the backup Routing Engine assumes mastership, packet forwarding continues through the router without interruption. For more information about graceful switchover, see the Junos System Basics Configuration Guide.

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