Understanding Backup Routers
The purpose of the backup router is not to forward transit traffic. It is for local management of the routing device, by way of the out-of-band management interface (fxp0 or me0, for example).
The Junos OS process responsible for establishing routes is known as the routing protocol process (rpd). The backup router feature allows the routing device to install a route to a management network, before the routing protocol process is up and running. A backup router can be used during the initial boot process of Junos OS, before any routing protocols have converged. It allows the device to establish a Layer 3 connection quickly, thus keeping management unavailability to a minimum. In selecting a backup router, it is common practice to choose the default gateway of the management network that is directly connected to your routing device.
When a routing device is booting, the routing protocol process is not running. Therefore, the router or switch has no routes. To ensure that the router or switch is reachable for management purposes while it boots or if the routing protocol process fails to start properly, configure a backup router, which is a router that is directly connected to the local router or switch (that is, on the same subnet) through its private management interface (for example, fxp0 or me0).
It is important to make sure that the specified backup router address is reachable and directly connected. The backup router address should be an address that is directly connected to the management interface.
Router A can be the backup router for Router B, and Router B can be the backup router for Router A if the management interface of each router is connected to an interface on the other router, thus providing the necessary reachability.
When the routing protocol process starts, the backup route (the route created by the backup router) is removed, and any default, static, or protocol-learned routes are installed.
Backup-router configurations can alter the backup routing engine kernel forwarding table even with nonstop active (NSR) enabled.