Configure 3G modem as backup WAN link

This article explains how to configure 3G wireless modem as a backup to WAN connection.

Use 3G wireless modem as a back up to Internet/WAN connection and also achieve fallback to the primary link when Internet connectivity is back on the primary link. In a usual setup we would want to monitor the primary Internet/WAN connection and fail-over to 3G modem if there is no Internet access .
The main challenge would be to detect the failure, when a non-direct link failure happens.

Example : In case of ADSL connection, when telephone line goes down,the Ethernet link would still stay up but there would be no internet.

The configuration would be divided into 3 steps:

Step 1: Configure 3G wireless modem

To configure 3G wireless modem please refer: 3G Wireless Modem Configuration Overview
To trigger dialing of 3G modem interface you may use watch-list or dialer-filter

Step 2: Configure static default routes via both primary-link and dialer interface

In this step we would configure 2 default routes one pointing out via primary link and the other via dialer interface with qualified-next-hop option.
Note that we would configure the route via primary internet link with a preference of 5 and route via the dl0 interface with preference of 4.

Reason for the above is explained in the Expected Behavior section.

set routing-options static route 0.0.0.0/0 next-hop 10.0.0.2
set routing-options static route 0.0.0.0/0 qualified-next-hop dl0.0 preference 4

Step 3: Detect failure of primary Internet/WAN connection

This is achieved by using rpm and ip-monitoring

Basic configuration :

Configure rpm-probes and test :

set services rpm probe example test test-name probe-count 3
set services rpm probe example test test-name probe-interval 15
set services rpm probe example test test-name test-interval 10

set services rpm probe example test test-name thresholds successive-loss 3
set services rpm probe example test test-name thresholds total-loss 3

set services rpm probe example test test-name target address 8.8.8.8  <<< Any internet address/ could be your gateways IP as well 
set services rpm probe example test test-name destination-interface fe-0/0/0.0  <<< Your Primary WAN interface
set services rpm probe example test test-name next-hop 10.0.0.2

For further details please refer: IP Monitoring with Route Fail-over Configuration and Behavior

Configure ip-monitoring :

set services ip-monitoring policy test match rpm-probe example
set services ip-monitoring policy test then then interface dl0.0 enable

Expected Behavior :

As long as the rpm probes are successful, the dl0.0 (dialer interface) would stay administratively down.
This would cause the route via dl0 interface to stay down
Once the primary internet connection goes down, this would trigger rpm to fail and IP-monitoring would cause dl0.0 interface to administratively go up.
This would in turn cause via dl0 interface to active in the forwarding table, as it has better preference that the route via primary internet link.
Fail-over to backup 3G link has been achieved.
Now when the internet is up via the primary internet link, the rpm test passes, this would trigger
IP-monitoring to bring down the dl0 interface down.
This would in turn cause the route via dl0 to be removed from the forwarding table.
Fail-over back to primary link is achieved.

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