Oct 27, 2007

Multicast VPNs(MVPN)

Multicast VPNs (mVPNs) provide a scaleable architecture to enable multicast in an RFC2547 Layer 3 Multiprotocol Label Switching (MPLS) VPN environment.
Originally derived from tag switching, MPLS uses labels to combine the intelligence of routing with the high performance of switching. MPLS VPNs are a natural extension of MPLS and are often by service providers to offer VPN services over a shared infrastructure. MPLS VPNs operate based on label stacks.
Despite the advantage of label stacking and the ability to decouple routing from forwarding for unicast traffic, MPLS VPNs did not address how to handle multicast traffic. As a result, the only available solution for delivery of IP multicast video, voice, and data over a deployed Layer 3 MPLS VPN was to statically configure point-to-point GRE tunnels between Customer Edge (CE) routers. As the number of CE routers increased, the number of point-to-point GRE tunnels required to maintain a full mesh of CEs quickly became unmanageable. A more scalable solution was required.
Cisco IOS Multicast VPNs address the inherent scalability issues of using fully meshed point-to-point GRE tunnels by introducing the concept of Multicast Tunnel Interfaces (MTIs) and Multicast Distribution Trees (MDTs).
MTIs use GRE encapsulation; however they fundamentally differ from traditional point-to-point GRE tunnels in that they use multicast-rather than unicast-destination addresses. The multicast destination address used by a MTI is what allows a Provider Edge (PE) router to map Customer multicast traffic (C-packets) to Provider multicast traffic (P-packets).

Figure 21. Example of MTI Encapsulation


MVPN uses two types of MDTs in the MPLS core. Each serves a different purpose:
• Default-Multicast Distribution Tree (MDT): nailed tree used for maintaining PIM adjacencies between PE routers and carrying low-rate multicast traffic.

• Data-MDT: dynamic tree used for high-rate multicast traffic; unlike the Default-MDT, this tree is built only as needed between the source PE and PEs with interested receivers.

Figure 22. Example of Default-MDT



Figure 23. Example of Data-MDT

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