M320, T320 Router, and T640 Routing Node Control Board Location
You can install one or two Control Boards in the router. The Control Boards install into the upper rear of the chassis in the slots labeled CBO and CB1 (see Figure 127).
Each Control Board requires that a Routing Engine be installed in the adjacent slot. CB0 installs above RE0, and CB1 installs below RE1. A Control Board does not function if no Routing Engine is present in the adjacent slot.
If two Control Boards are installed, one functions as master and the other acts as backup. If the master Control Board fails or is removed, the backup restarts and becomes master.
The Control Boards are hot-pluggable.