Multicast MAC and Unicast IP Address

Is it allowed to have a multicast MAC address linked to a unicast IP
address? If not what standard specifies this?

I know that this mechanism is used for clustering but that certain vendors
do not like it very much. I am interested in knowing what the rules are
because up to now everybody just told me that it is a strange way to use
multicast but some also said that it is illegal. Without proving it.

So let me know if you know about a standard specifying this.

Thanks.
Mat


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