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 220.127.116.11 <<< 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.