Could you please elaborate on what you mean by "plugging in an additonal youview box?" Do you have multiroom from BT or are you just plugging an additional box for things such as catch up etc?
One piece of advice I would give is that the disc to which my Youview box is connected has a strong signal from my main disc, which I believe helps to main a flawless picture on multicast channels. So in your case, the disc which you are connecting your additonal box to must have a stong signal with your Smart Hub 2.
Intially, I did contact the BT Whole Home helpdesk, who advised that multicast wasn't supported. I, however, tested this out and found that it did support multicast. Subsequent to this, I receieved this response from BT, "The Whole Home Wi-Fi discs are designed to work with multicast traffic, by converting it into unicast traffic over the Wi-Fi then back to multicast. This enables services using multicast streaming that are connected to the system to continue to work as normal."
Additionally, I have read about another user successfully using multicast over these dics on another forum page.
Just to add, I did not configure anything. Simply, I purchased the Whole Home Kit and updated the firmware to Build 12 and ever since have been using the BTTV box connected to disc by an ethernet couples with no issues.
That’s great thanks very much for the info, I think I’ll order it and try.
To expand yes I have BT multi room the first box is connected directly to the hub and the second (upstairs) currently using the BT plug in mini connectors which I wanted to get rid of ideally and put the disc next to it instead.