NAGIOS' usual approach to monitoring link-state is somewhat simpler, at least as regards its status map and the concept of host dependencies: it wants to be able to contact the device immediately at the other end of the link. The definition of "contact" is usually ICMP echo-request (ping), but it's arbitrarily configurable.
Yes, you could instead devise some test for openwrt that tells you the router's view of the link-state, but: let us assume that such a test can be written.
- Either it's always going to give the same
results as testing connectivity to
the other end of the link, in which
case, why did you bother doing a complex thing when a simple one sufficed,
- or it can occasionally give a
different result to the test of
connectivity to the other end of the
link, in which case I submit that the
test is broken; what use is it that
the router assures you that the link
is up if it won't carry traffic.
I'm a great believer in testing what you actually want to know. In this case, I assume you want to know if the links will carry traffic; if so, then don't delegate the answer to another device - test it for yourself.
If that doesn't convince you, look at the openwrt code and find out how it determines if the link is up - and repeat that test from NAGIOS.