How to find out a VPN link has gone down (or better, come back up)?

[Bernard Higonnet]

I have a machine, FreeBSD 8.1-RELEASE #0, running a VPN (MPD5) server.

Every once in a while, the link goes down, presumably because of some 
third-party event on the network.

When this happens, an element placed in the routing table by hand 
disappears since the node on the VPN server to the far-off client 
machine no longer exists (even though the entry in the routing table is 
marked "permanent" when it does exist). When the network problem goes 
away, the VPN link is automatically restored, but not the routing table.

Can someone tell me if there is a way to be notified that the link has 
been restored so that I can run a script etc. to restore the routing 
table to what I want?

TIA
Bernard Higonnet