# Redundancy Features

This section describes system redundancy features.

## SRP Modules

The router uses a 1:1 redundancy scheme for the SRP module. When two SRP modules are installed in the router, one acts as the primary (active) and the second as a redundant (standby) module. Both SRP modules share a single SRP IOA located in the rear of the chassis. After you install two SRP modules, the modules negotiate for the primary role. A number of factors determine which module becomes the primary; however, preference is given to the module in the lower-numbered slot. The SRP modules record their latest roles and retain them the next time you power up the system.

If the standby SRP module detects that the primary SRP module is not active (and high-availability mode is not enabled), it reboots the system and takes control. If high-availability mode has been enabled, automatic switchover occurs with near hitless failover. For information about configuring and managing SRP module redundancy, see JunosE System Basics Configuration Guide, Chapter 6, Managing Modules.

### NVS Cards

Each SRP module has two NVS cards (0, 1). The NVS cards in the active SRP module are designated disk0 and disk1. The NVS cards in the redundant SRP module are designated standby-disk0 and standby-disk1. After you install new NVS cards or SRP modules, you must issue the synchronize command to match the file system of the NVS card on the redundant SRP module with the file system of the NVS card on the active SRP module. See Replacing an NVS Card for more information.

## Power

The routers provide a power architecture that distributes redundant –48 VDC feeds through the router to each line module, IOA, SRP module, SFM module, and fan module where DC-to-DC converters provide local conversion to the required secondary voltages.

## Fans

The E320 router employs a bottom-to-top cooling system to keep the temperature of the modules and components within normal operating limits. Eight cooling fans are located in a tray at the top of the router. (See Figure 1 and Figure 2.) Air is pulled in from the front of the router at the bottom and is exhausted out the top.

The E120 router employs a right-to-left cooling system. Nine cooling fans are located in a tray at the left of the router. (See Figure 3.) Air is pulled in from the right of the router and is exhausted out the left.

The system monitors the temperature of each module. If the temperature of a module exceeds the maximum limit, the system immediately goes into thermal protection mode and the LMs and SFM modules are powered off. The system controllers remain active and respond on all management interfaces. All other modules remain in a power-off condition. The failure of any two components (fan or converter), or the absence of the fan tray, causes the chassis to enter thermal protection mode to prevent hardware damage. For information about troubleshooting high operating temperatures, see Troubleshooting.

The E320 fan tray has two primary converters that power four fans each. If a primary converter fails, a third redundant converter takes over. The E120 fan tray has dual converters that load share for redundancy. If one converter fails, the other redundant converter takes over. The system software reports an alarm if any of the fans or converters fail.