This trigger expression works as follows:
1. It can be triggered if the interface link status is down.
2. `{$NET.IF.CONTROL:"{#IFNAME}"}=1` - a user can redefine the context macro to "0", marking this interface as not important. No new trigger will be fired if this interface link is down.
3. `{TEMPLATE_NAME:METRIC.diff()}=1` - the trigger fires only if the interface link status was up to "1" sometime before.

WARNING: If closed manually, it will not fire again on the next poll because of `diff`.