This attribute specifies the domain number associated with this bridge
port. Domain numbers are used for confining frames to a domain.
Domain based filtering provides a complete segregation of traffic based
on domain number associated with frames. There can exist only sixteen
domains when a spanning tree is desired specific to each domain. When
there is only a single spanning tree spanning all the domains the
maximum number of domains possible are 4294967293.
This attribute is of no relevance when associated with backbone bridge
port. Its value is ignored in such a situation.