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SCB-MX Description

The MX Switch Control Board (SCB-MX) provides control plane functions, chassis management functions, and switch plane functions for MX960, MX480, and MX240 routers. It is also a carrier for the Routing Engine which installs directly into a slot on the SCB-MX. See Figure 1.

Figure 1: SCB-MXSCB-MX

Software release

  • Junos OS Release 12.3 R1 and later

  • Name in CLI: SCB

SCB-MX 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, power distribution module status and control, and the craft interface

  • Provides Ethernet connectivity to all processors in the chassis for control plane communications.

  • Provides bandwidth of up to 160 Gbps per slot with redundant fabric support.

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

SCB-MX Components

  • Control Board and Routing Engine (CB-RE) primary-role mechanism.

  • External clock interface—Allows BITS or GPS clock source input to the centralized timing circuit, or allows centralized timing to be output to BITS or GPS.

  • Switch fabric—Provides switching functions for MPCs.

  • 1000Base-T Ethernet controller—Provides a 1 Gbps Ethernet link between the Routing Engines.

  • Power circuits for the Routing Engine and the SCBE2-MX.

  • LEDs—Provide status of the SCBE2-MX and the clocking interface.

SCB-MX Slot Numbers

For the MX960:

  • You can install up to three SCB-MXs in the MX960 chassis for a total of six switch fabrics and six fabric planes. The SCB-MXs install vertically into the front of the MX960 chassis in the slots labeled 0, 1, and 2/6.


On the MX960, either a SCB-MX or a DPC can be plugged into slot 8. A third SCB-MX is used only for switch fabric redundancy. Therefore, if an application does not require switch fabric redundancy, a DPC can be used in slot 8.

For the MX480 and MX240 routers:

  • You can install one or two SCB-MXs in the MX240 and MX480 for a total of four switch fabrics and eight fabric planes. 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.

Weight and Dimensions

  • Weight: 9.6 lb (4.4 kg) (with Routing Engine installed)

  • Width: 17 in (43.2 cm)

  • Depth: 22 in (55.9 cm)

  • Height: 1.25 in (3.2 cm) height.

SCB-MX Fabric Planes and Redundancy

For MX960 routers:

  • Each SCB-MX provides two switch fabric planes for packet forwarding among the DPCs and MPCs in the MX960.

  • The MX960 chassis may contain up to three SCB-MX's Therefore, six fabric planes are available.

  • The MX960 provides 2 + 1 SCB-MX redundancy when used with DPC line cards.

  • In redundant fabric mode, four fabric planes from the first two SCB-MX’s will be in Active mode, and two fabric planes from the third SCB-MX will be in Spare mode.

  • In a non-redundant fabric mode, all six fabric planes will be in Active mode to increase fabric bandwidth.

  • SCB-MXs installed in slots 0 and 1 provide non-redundant fabric connections. An SCB-MX installed in slot 2/6, in conjunction with SCB-MXs in slots 0 and 1, provides redundant fabrics, but the Routing Engine installed on it receives no power and provides no additional routing functions. If no SCB-MX is installed in slot 2/6, you must install a blank panel in the slot.

For MX240 and MX480 routers:

  • Each fabric ASIC is configured in virtual plane mode, where two virtual planes exist on one fabric ASIC. This means that the MX240 and MX480 require only a single SCB-MX to provide line rate throughput, and an additional SCB-MX to provide 1 + 1 SCB redundancy.

  • In redundant fabric mode, four virtual planes from the first SCB-MX will be in Active mode, and four virtual planes from the second SCB-MX will be in Spare mode.

  • In a non-redundant fabric mode, all eight virtual planes will be in Active mode to provide the maximum fabric bandwidth.

SCB-MX Maximum Power Requirements

185 W at 55° C

160 W at 40° C

155 W at 25° C


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 LED Descriptions







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.

SCB-MX Fabric Plane Scale and Redundancy

Each of the fabric planes on the SCB-MX is able to process 20 Gbps of bandwidth. The MX240 and MX480 use eight fabric planes across two SCB-MXs, whereas the MX960 uses six fabric planes across three SCB-MX’s˙. Because of the fabric plane virtualization, the aggregate fabric bandwidth between the MX240, MX480, and MX960 is different. Refer to Table 2.

Table 2: SCB-MX Fabric Plane Scale and Redundancy Assuming Four PFEs per FPC













Switch Fabrics




Fabric Planes




Spare Planes

4 (1+1 SCB-MX redundancy)

4 (1+1 SCB-MX redundancy)

2 (2+1 SCB-MX redundancy)

Fabric Bandwidth Per Slot

160 Gbps

160 Gbps

120 Gbps

Interoperability With Routing Engines

Routing Engines

Maximum Power Requirement



110 W

RE-S-1300 (EOLed)

RE-S-2000 (EOLed)

RE-S-1800 (all variants)

RE-S-2000-4096 (EOL’d)

90 W