Steps for Adding Multicast Support to MPLS L3VPN

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When adding multicast support to an existing MPLS L3 VPN, a service provider should consider the following two steps:

1. Selecting the appropriate MDT (Multicast Distribution Tree) to use:

Multicast Distribution Trees (MDTs) are used to forward multicast traffic between PEs in an MPLS L3 VPN network. There are two types of MDTs: Default MDT and Data MDT.

• Default MDT is used for control traffic, such as Protocol Independent Multicast (PIM) Join/Prune messages and Rendezvous Point (RP) advertisements. Using the Default MDT reduces the amount of multicast routing state in the core, but it may result in bandwidth wastage on PEs, as all multicast traffic is forwarded via the Default MDT.

• Data MDT is used for forwarding customer multicast traffic. Using Data MDT allows for optimized PE utilization as only the customer multicast traffic is forwarded via the Data MDT. Using SSM (Source-Specific Multicast) for the Data MDT helps to minimize the number of group addresses needed in the core.

2. Configuring RPs (Rendezvous Points) for each customer VRF:

When configuring multicast support for an MPLS L3 VPN, a service provider must consider the RPF (Reverse Path Forwarding) check that is performed on incoming multicast traffic at each PE. This check is done to ensure that the multicast traffic is received from the expected source. By default, each VRF is a separate multicast domain, and the RP must be configured inside each customer VRF to minimize the impact of the RPF lookups.

In summary, when adding multicast support to an existing MPLS L3 VPN, a service provider should select the appropriate MDT (either Default or Data MDT) and configure RPs for each customer VRF to minimize the impact of RPF lookups.