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You are here: Home / Juniper / Configure 3G modem as backup WAN link

Configure 3G modem as backup WAN link

June 5, 2016 by Marques Brownlee

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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Filed Under: Juniper

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