Configuring IP Monitoring with a DHCP Backup Interface

Many backup links like Ethernet and 3G/4G LTE links receive their IP address dynamically. This means that you might not know their IP address beforehand. Hence, it is hard to configure static routes such that the preferred route is through the backup interface (in the case of a real-time performance monitoring (RPM) probe failure). Also, most dynamic interfaces receive a 0.0.0.0/0 route from the DHCP server.

When a 0.0.0.0/0 route is installed in the routing table, it has a preference of 11. If the system has any static routes that are configured to use the primary WAN link, these routes have a preference of 5. Thus, if an RPM probe fails while using IP monitoring with interface failover and the backup interface is enabled, traffic might still be routed out of the primary interface through the static route (unless the interface is physically down). This is because the route has a preference of 5 which takes precedence over the default route the system obtained through DHCP (a preference of 11). If the system received its routes through a dynamic routing protocol, there is no problem as most routes have a preference value greater than 100. Hence, if the system has any static routes, we recommend adding a preference value greater than 12 to these static routes. In the Configuring IP Monitoring with Interface Failover example, if fe-0/0/2 is a dynamic interface, you need to modify the static route configuration to reach IP subnet 5.1.1.0/24. To get this result, enter the following configuration: