Passive monitoring eliminates the problem of polynomial growth of monitoring sessions (and consequentialy, monitoring traffic).
However, the problem is moved inside hosts that perform passive monitoring: the number of packets they examine grows with the square of the number of sites. This is probably preferable.
Such hosts might use the content of the domain description database to infer which traffic is representative of the performance of the network infrastructure between two domains.
In this view, active monitoring tools are helpful only when passive monitoring does not find useful traffic patterns.
In such event, hosts performing passive monitoring might invoke the execution of active monitoring sessions that will either produce observations, or induce significant traffic patterns.
Such request should be authenticated, for security reasons.