Shane, Many moons ago ANS used the advisory attribute for transit across their network and they required all their route objects to have such a field. Other than that use I never heard of anyone using it for anything other purpose. To my knowledge, ANS was the reason for the advisory attribute. When we converted to RPSL we dropped support for the advisory attribute and it was filtered out of all route objects.
As for how the attribute is actually used, I found 2726 route objects with the attribute. Of these, there are only 111 unique origin AS. As for the AS actually listed as the target of the advisory, there are only 13 unique AS! The actual frequencies are:
This suprises me since to my knowledge we no longer supported this attribute. So I grep'd our production DB cache for 'advisory' and it did not show up. So where are you finding all these occurences? --jerry