Ever since I upgraded from a Vigor 2820 up to a 2830 router I've not been able to get things like itunes remote or Airplay working with my iPhone - it seems to be an issue with multicast traffic? Which potentially affects quite a lot of local network discovery protocols such as DLNA.
Now my network setup uses the internal wireless of the 2830 as a 5GHz AP and I use a separate mini-AP for 2.4GHz - unfortunately my iPhone only does 2.4GHz and I found whenever using this AP it would be unable to do Bonjour / mDNS discovery. Then I changed my setup around a bit and found something very interesting, I had to use the 2820 as a switch for a few days with the Airplay server (an XBMC right now) and the AP connected into it and somehow, this allowed the discovery to work and I could use Airplay / Bonjour which was nice

So I wanted to put the setup back to normal with the AP & XBMC connected into the 2830 as I don't like having network strewn everywhere and started playing with the settings to see if I could get it to work with it - since it's multicast traffic I eventually checked out the Applications - IGMP menu on the router and enabled IGMP snoop - which got it working!
Now, this isn't quite right imo as if you read the text for IGMP snoop, it says that enabling it should Block multicast traffic between the LAN ports - this also did not need to be enabled with the Vigor 2820 afaik?

Still, if anyone else has problems with bonjour, airprint / airplay or DLNA and is using a Vigor 2830, 2850 or 2920 router (afaik they're the similar hardware because they run the same firmware) and the problem is with multicast going between access points / devices plugged into the router rather than the router's wireless to wired lan, try that and see if it helps?

haha, and now it broke again - it's weird, I think it's the switch doing something? I think if I put a dumb switch inbetween the XBMC and the AP, I bet that will work.

Did you find a solution? I have the same issue.

I replaced my Netgear DGND3300 for a Draytek 2830VN+ and immediately after installing we have multicast issues, since the new router we loose all our airplay devices, napster stops working and we can not see any devices which are usually visible using multicast, we have tried rebooting every device and the only fix is when we reboot the router, so we decided to be cleaver and have the router reboot every morning at 4am, which fixed the issue, however in the evenings it stops finding any device, i have since reinstalled my Netgear and all my devices are discoverable and have been for many days, it definitely the Draytek Router as I have the same problem at another location, although strangely at the over location when connected via the AP800 every device is discoverable.

I need to know what can be done to fix this as I need to order another router for a client for the VPN however cant specify Draytek until I know this works.

turn on the "Enable IGMP Proxy" under applications/igmp