System Redundancy

The router is designed so that no single point of failure can cause the entire system to fail. The following hardware components contribute to system redundancy:

• Cooling system—When the temperature inside the router is below the acceptable maximum, the cooling system’s components function at less than full speed. If the temperature becomes excessive—for example, because a cooling system component is removed—the MCS automatically increases the speed of the remaining components to reduce the temperature. The cooling system can function at the higher speed indefinitely. For more information, see Cooling System.
• Host module (Routing Engine and MCS functioning together)—The router can have one or two host modules. If two host modules are installed, one (the master) is active and the other is in standby mode. If the master host module (or either of its components) is removed from the chassis, the standby host module becomes active. The Routing Engine and MCS must reside in adjacent slots and be fully operational for the host module to function. For more information, see Host Module.
• PCG—The router has two PCGs. Both PCGs send their clock signals to the other Packet Forwarding Engine components, along with a signal that indicates which clock is the master. If one PCG fails, the other PCG becomes the master system clock. For more information, see Packet Forwarding Engine Clock Generators (PCGs).
• Power supply—The router has two load-sharing, fully redundant power supplies to distribute DC power to the other components. If one power supply fails, the second power supply can provide full power to the router's components indefinitely. For more information, see Power System.
• SFM—The router can have up to four interconnected SFMs. If one SFM fails, the switching and forwarding functions of the failed module are distributed among the remaining SFMs. Total bandwidth is reduced by 1/n, where n is the total number of SFMs installed in the router. For example, in a system with four SFMs, each SFM provides one-fourth of the forwarding capacity. For more information, see Switching and Forwarding Module (SFM).

In the base configuration, the router has one host module and multiple SFMs, PCGs, power supplies, and cooling system components.