IS-IS supports multi-topologies (MT), such as ipv4-unicast, ipv4-multicast, ipv6-unicast, and ipv6-multicast. SR OS supports ipv4-unicast and ipv4-multicast MTs for BIER.
A sub-domain is supported within only one topology. The mapping is indicated by the pair <MT, SD>.
For example, the following combination of <MT, SD>, where MT 0 is IPv4 unicast and MT 3 is IPv4 multicast, are valid:
<MT=0, SD=0>
<MT=0, SD =1>
<MT=0, SD =2>
However, the following combination, where MT 0 is IPv4 unicast and MT 3 is IPv4 multicast, is invalid because an SD belongs to more than one MT:
<MT=0, SD=0>
<MT=3, SD=0>
IPv4-multicast imports routes into the multicast RTM and ipv4-unicast imports routes into the unicast RTM.
A BIER forwarding table (BIFT) is identified using a label. A BIER label is assigned per (<MT, SD>, SI, BSL) and therefore, different MTs point to different BIFTs.
The MT can be used to engineer multicast and BIER routes separately from unicast routes.