In a throwback to the problems I dealt with using AirPlay across VLANs, I recently jumped through similar hoops for Sonos speakers. There are many forum and blog posts out there that describe (or attempt to describe) how to make this work, however all of the ones I read suffered from one or both of these problems: Their instructions had errors (eg, reversing the upstream and downstream interfaces when talking about multicast). They don't have a diagram of traffic flow! Every network engineer ...
