I have included a sample sensor below using ping, which will test if your NIC is down or the cable is unplugged. The basic idea is to test reachability for the item after the item you want tested; so to test your cable ping the firewall.
Instead of creating 3 sensors (cable, firewall, ITSP) you can actually combine them into a single sensor which I’ll call ‘route-to-itsp’. By checking the entire path in one sensor you test all of the items along the way:
; Ensure connection to ITSP is ok including NIC, cables, router, gateway, and ITSP
route-to-itsp/description=Connection to ITSP
route-to-itsp/type=ping
route-to-itsp/input=received
route-to-itsp/parameters=Count: 5 | interface:eth1 | host: 173.239.164.41
route-to-itsp/scoring=