With multicast multi-topologies, multicast services can run in a specified topology, which isolates multicast services from unicast services and eliminates the configuration conflict when both multicast services and unidirectional Traffic Engineering (TE) tunnel services are deployed. In deploying multicast multi-topologies, you can specify either a multicast topology instance or a unicast topology instance for running multicast services.
Currently, multicast services highly depend on unicast routes. During multicast routing, a router searches a unicast routing table for an optimal route. If the unicast optimal route, shared by both multicast services and unicast services, is faulty, the multicast services and unicast services will be interrupted and users will fail to receive the multicast data. With multicast multi-topologies, multicast services can run in a specified topology, which isolates multicast services from unicast services.
With both multicast services and unidirectional TE tunnel services deployed, if a unidirectional TE tunnel interface is chosen during multicast routing, the multicast data will fail to pass the Reverse Path Forwarding (RPF) check. The local multicast-topology (MT) feature can address this problem. However, in this situation, a separate Multicast Interior Gateway Protocol (MIGP) routing table has to be maintained, which consumes network resources. With multicast multi-topologies, the system searches for routes only in the unicast routing table, the static multicast routing table, and the MBGP routing table specified by the multicast topology. These routing tables contain no unidirectional TE tunnel interfaces, avoiding the problem that multicast data fails to pass the RPF check when a unidirectional TE tunnel interface is chosen. In addition, no separate MIGP routing tables have to be maintained, so network resources can be properly planned.
When configuring multicast multi-topologies, you can configure a multicast topology instance or a unicast topology instance. A multicast topology can be used in both PIM-SM intra-domain and inter-domain scenarios, but a unicast topology instance can be used only in a PIM-SM intra-domain scenario.