Zeroconf with Zyxel P-660HN-F1Z

There is an option to purchase a discounted and officially sanctioned Zyxel P-660HN-F1Z when signing up for an ADSL account with Sunrise. Recently I started to play around with Bonjour on MacOS X and Avahi on Linux to better understand the whole mechanism. However I did not get very far with the whole thing as the machines were not able to see each other's multicast traffic when talking to each other. Sniffing the network showed that the mDNS requests were sent on 224.0.0.251 port 5353 but no response was received. I thought there was a configuration issue with Avahi on my debian host but it turned out to be a much more sinister problem. Once I had to link the MacOS X host with the debian box directly over a cross-cable and to my surprise everything was working like a charm. Digging deeper I found the culprit to be the Zyxel P-660HN-F1Z router. Well hidden in one of the Advanced configuration menus is an option to switch on multicast (IGMP). Below is a visual guide how to make Zeroconf work on this baby.