This might be what you are looking for:
nagios.sourceforge.net/docs/3_0/ … .html#host
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.