When a router or switch is booting, the routing protocol process (rpd) is not running; therefore, the router or switch has no static or default routes. To ensure the router or switch is reachable over the network while it boots or if the rpd fails to start properly, configure a backup router (running IP version 4 [IPv4] or IP version 6 [IPv6]), 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 (e.g., fxp0).

By default, all hosts (default route) are reachable through the backup router. To achieve network reachability while loading, configuring, and recovering the router or switch, but without installing a default route in the forwarding table, include the destination option, specifying an address that is reachable through the backup router. Specify the address in the format network/mask-length so that the entire network is reachable through the backup router.

Any destinations defined by the backup router are not visible in the routing table, they are only visible in the local forwarding table when rpd is not running. Therefore, a recommended best practice is to also include the destinations of the backup router configured as static routes with the retain flag at the [edit routing-options] hierarchy level. The retain flag is necessary to allow the static route to remain in the forwarding table when rpd stops running, because the routing table does not exist if rpd is not running.

On systems with dual redundant Routing Engines, the backup Routing Engine's reachability through the private management interface is based only on the functionality of the backup-router configuration, it is not based on whether the rpd is running. The backup router adds the destination prefix upon bootup, whereas configuring a static route requires rpd to run first before installing the destination prefix. If rpd is allowed to run on the backup Routing Engine, then a destination can be ad ded in the routing table and the forwarding table by configuring static route with retain.

Note: Active routes and more specific routes take precedence over destination prefixes defined with the backup-router statement.

Note: The Routing Engine on the backup router only supports 16 destination addresses. If you configure more than 16 destination addresses, the Junos OS ignores destination addresses after the sixteenth address and displays a commit-time warning message to this effect.

Configuring a Backup Router Running IPv6

To configure a backup router running IPv6, include the inet6-backup-router statement at the [edit system] hierarchy level: