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MX480 SCB-MX Description


The MX Switch Control Board (SCB-MX) provides Control Plane functions, Chassis management functions, and Switch Plane functions. It is also a carrier for the Routing Engine (RE). Below are various facts about the SCB-MX:

  • Each SCB-MX contains two fabric ASICs, and the same SCB-MX card is usable in MX960, MX480 and MX240.

  • The Control Plane functionality is provided primarily by a RE, an ACBC FPGA, and a gigabit Ethernet switch, which all reside on the SCB-MX.

  • The RE is a Pentium processor subsystem that can be plugged into the SCB-MX. The RE uses two primary interfaces to the SCB-MX: PCI bus and gigabit Ethernet.

  • On the SCB-MX, the RE’s PCI bus interfaces to the ACBC FPGA, which aggregates most of the system I/O and provides Chassis management functions.

  • The RE’s gigabit Ethernet port connects to a gigabit Ethernet switch on the SCB-MX. This gigabit Ethernet switch provides Ethernet connectivity to all of the processors in the Chassis, including the standby RE, for Control Plane communications.

  • There are two SCB-MXs in the system which can operate the control plane function in redundant mode. Each SCB-MX contains the mastership logic that determines which SCB-MX/RE combination is the master in the system. The master is also called the Junos master. The Junos master controls all components in the system, including the standby SCB-MX/RE.

  • Each SCB-MX generates an output mastership signal to each FRU. Each SCB-MX also generates an output operational signal to each FRU. On each FRU, the mastership signal is qualified by the operational signal and combined with local status signals to select the master SCB-MX used for FRU control.

  • Each SCB-MX contains two switch fabric chips (ASICs). However, the switch fabric ASICs are under the control of the current Junos master.

  • For the MX240, each SCB-MX provides four planes of switch fabric for packet forwarding among the DPCs/MPCs.

  • On MX480/MX240, each fabric ASIC is configured in virtual plane mode, where two virtual planes exist on one fabric ASIC. The MX480/MX240 contains a maximum of two SCB-MXs and hence eight logical planes are available.

The Routing Engine installs directly into a slot on the SCB-MX (see Figure 1).

Figure 1: SCB-MX

The MX Switch Control Board (SCB-MX) provides the following functions:

  • Powers on and powers off DPCs, FPCs, and MPCs

  • Controls clocking, system resets, and booting

  • Monitors and controls system functions, including fan speed, board power status, PDM status and control, and the craft interface

  • Provides interconnections to all the DPCs, FPCs, and MPCs within the chassis through the switch fabrics integrated into the SCB-MX

The Routing Engine installs directly into a slot on the SCB-MX (see Figure 2).

Figure 2: SCB-MX

MX480 SCB-MX Slots

You can install one or two SCB-MXs in the MX480. The SCB-MX installs horizontally into the front of the chassis in the slots labeled 0 and 1. If any slots are empty, you must install a blank panel.

MX480 SCB-MX Redundancy

If two SCB-MXs are installed, one SCB-MX functions as the master SCB-MX and the other as its backup. If the master fails or is removed, the backup restarts and becomes the master.


Three LEDs on the SCB indicate the status of the SCB. The LEDs, labeled FABRIC ACTIVE, FABRIC ONLY, and OK/FAIL, are located directly on the SCB. Table 1 describes the functions of the SCB LEDs.

Table 1: SCB-MX LEDs







On steadily

Fabric is in active mode.



On steadily

SCB-MX operates in fabric-only mode.


SCB-MX operates in fabric/control board mode.



On steadily

SCB-MX is online.


On steadily

SCB-MX has failed.


SCB-MX is offline.