M320 Component Redundancy
A fully configured router is designed so that no single point of failure can cause the entire system to fail. Only a fully configured router provides complete redundancy. All other configurations provide partial redundancy. The following major hardware components are redundant:
- Switch Interface Boards (SIBs)—A fully configured router has four SIBs. Each FPC has a dedicated ASIC with four high-speed links that connect to the SIBs (one link per SIB). In the event of a complete SIB failure, the traffic is forwarded across the remaining three planes with a slight degradation in performance. When the failed SIB is replaced, the router regains full forwarding capacity.
- Host subsystem—The host subsystem consists of a
Routing Engine functioning together with a CB. The router
can have one or two host subsystems. If two host subsystems are installed,
one functions as the master and the other functions as the backup.
If the master host subsystem (or either of its components) fails,
the backup can take over as the master. To operate, each host subsystem
requires a Routing Engine installed in an adjacent slot to the CB.
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.
- Power supplies—A router with a minimum of three AC power supplies or four DC power supplies is fully power redundant. In the AC configuration, all power supplies share the load evenly. In the DC configuration, two power supplies share the load to some of the components, and two other power supplies share the load to the remaining components. In both configurations, if one power supply fails, the remaining power supplies can provide full power to the router.
- Cooling system—The cooling system has redundant components, which are controlled by the host subsystem. If one of the fans fails, the host subsystem increases the speed of the remaining fans to provide sufficient cooling for the router indefinitely.
