Each BIER domain can be divided into sub-domains. A BIER domain supports sub-domains numbered from 0 to 255. Each BIER domain must contain at least one sub-domain, and sub-domain 0 is the default. If a BIER domain contains more than one sub-domain, each BFR in the domain must be provisioned with the set of sub-domains to which it belongs.
A BIER domain is an IGP area, and sub-domains are the different topologies within that area. In IS-IS and OSPF, each topology must also have its own sub-domain ID. For example, in IS-IS a sub-domain is an IS-IS multi-topology. SR OS supports two sub-domains in IS-IS: IPv4-multicast and IPv4-unicast MTs. A sub-domain creates the least traffic engineering in a BIER domain. A user can use separate L3 interfaces into IPv4-unicast MT and a set of disjointed interfaces into the IPv4-multicast MT. This creates separation and traffic engineering for different multicast streams.
For each sub-domain to which a specific BFR belongs, if the BFR is capable of acting as a BFIR or a BFER, it must be provisioned with a BFR ID that is unique within the sub-domain. If a given BFR belongs to more than one sub-domain, it may have a different BFR ID for each sub-domain but this is not required.