Basic Concept of Multicast VPN (MVPN )

Basic Concept of MVPN

The basic concept of MVPN is as follows:

•The Service Provider has an IP Network with its own unique IP multicast domain (ie: P-Network).

•The MVPN customer has an IP Network with its own unique IP multicast domain (ie: C-Network).

•The Service Provider MVPN network forwards the customer IP multicast data to remote customer sites. To achieve this, customer traffic (C-packets) is encapsulated at the Service Provider PE inside P- packets. The encapsulated P-packet is then forwarded to remote PE sites as native multicast inside the P-Network

•During this process, the P-Network has no knowledge of the C-Network traffic. The PE is the device that participates in both networks. Note there may be more than one Customer Network per PE. 

Multicast Routing Inside the VPN Versus Multicast Routing Inside the Provider Network

A PE router an MVPN network has multiple multicast routing tables, as well as multiple instances of PIM, IGMP, and MSDP. There is one global table and a table per MVRF.

Multicast Domains is based on the principle of encapsulating multicast packets from a VPN in multicast packets to be routed in the core. As multicast is used in the core network, PIM must be configured in the core.

PIM-SM, PIM-SSM, and PIM-BIDIR are all supported inside the provider core for MVPN.

PIM-SM or PIM-SSM is the recommended PIM option in the provider core, because PIM-BIDIR is not yet supported by all platforms, PIM-SM, PIM-SSM, PIM-BIDIR and PIM-DENSE-MODE are supported inside the MVPN.

MVPN has the concepts of Multicast Distribution Trees (MDT). An MDT is sourced by a PE router and has a multicast destination address. PE routers that have sites for the same MVPN will all source to a Default-MDT and also join to receive traffic on it.

There is a distinction between Default-MDTs and Data-MDTs. A Default-MDT is a tree that is `always-on' and will transport PIM control-traffic, dense-mode traffic and rp-tree (*,G) traffic. All PE routers configured with the same default-MDT will receive this traffic.

Data-MDTs are trees that are created on demand and will only be joined by the PE routers that have interested receivers for the traffic. They can be created either by a traffic rate threshold and/or source-group pair.

Default-MDTs must have the same group address for all VRFs that comprise a MVPN. Data-MDTs may have the same group address if PIM-SSM is used. If PIM-SM is used, they must have a different group address, as providing the same one could result in the PE router receiving unwanted traffic. This is a PIM-SM protocol issue, not an implementation issue. 

Reference: Multicast VPN Design Guide