MDT is required when your cloud require to distinguish and isolate multicast traffic from core multicast traffic.mainly used for Lyaer 3 VPN customers in MPLS Core network.
MDT default is required to identfy traffic for a specific customer X using a multicast group So the multicast tree will be careated and handled by multicast core network accordingly.
For example
Customer X is using layer 3 vpn service from an Service provider and would like to have unicast and multicast services using the same VPN(VRF)
In order to handle this multicast traffic Service provider will create a default group (239.255.255.1) So with respect to this group one multicast tree will be created and Core routers will forward traffic accordingly.
MDT default group actualy used by Service provider to handle VRF customer s multicast traffic
Customer will have to handle its multicast traffic and there will be another Multicast tree according to customer specific group configurations
