The advisory field dates back to the old NSFnet. It was used there (and later
by ANS) to determine the appropriate carrier to whom a packet would be given
after the death of the PRDB and before routing policies were well established
in the fullness of BGP4. I believe ANS stopped using this field some time ago
and we stopped worrying about it. I believe the field is as dead as ASS690
will be in a week or two (or maybe already is).