Nagios Support Forum

I have a site that is behind two ADSL lines - i am currently using the main link to monitor the on premises equipment - if/when the main link goes down - the on site router moves traffic from the internal network out the second line - i use NRPE to monitor everything internally (currently) through the main link.

I am monitoring both links, so i know when one is gone, but all the monitored devices that are on site are listed as unreachable as they are linked to the main link, even though i can get to them via the other link.

How would i go about tying both links together so that the internal devices can be monitored by either link. I am aware this must be a common scenario, but so far i have been unable to find an example with my searches.

I am using Nagios Core VM Version 4.0.7.

While the definition for parents doesn't make it ultimately clear, Nagios does understand redundant parents. Therefore if you just add a line like this to your host config it should achieve what you want:
parents: This directive is used to define a comma-delimited list of short names of the "parent" hosts for this particular host. Parent hosts are typically routers, switches, firewalls, etc. that lie between the monitoring host and a remote hosts. A router, switch, etc. which is closest to the remote host is considered to be that host's "parent". Read the "Determining Status and Reachability of Network Hosts" document located here for more information. If this host is on the same network segment as the host doing the monitoring (without any intermediate routers, etc.) the host is considered to be on the local network and will not have a parent host. Leave this value blank if the host does not have a parent host (i.e. it is on the same segment as the Nagios host). The order in which you specify parent hosts has no effect on how things are monitored.