VXLAN Tenant Routed Multicast

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Tenant Routed Multicast (TRM) offers multicast delivery efficiency to VXLAN overlays. It is based on the next-generation control plane (ngMVPN) specifications published in IETF RFCs 6513 and 6514. TRM provides the efficient and resilient delivery of customer Layer 3 multicast traffic in a multitenant network.

As seen in the picture below, BGP EVPN provides a control plane for unicast routing, whereas ngMVPN provides scalable multicast routing features. It employs a "always route" strategy in which every edge device (VTEP) equipped with distributed IP Anycast Gateway for unicast becomes a designated router (DR) for multicast. 

Only on the edge devices (VTEP) is bridged multicast forwarding present, where IGMP snooping enhances multicast forwarding to interested receivers. Other than local delivery, all other multicast traffic is efficiently routed.

VXLAN Tenant Routed Multicast
Fig 1.1- VXLAN Tenant Routed Multicast

When TRM is enabled, the underlay's multicast forwarding is used to reproduce VXLAN-encapsulated routed multicast traffic. Per VRF, a Default Multicast Distribution Tree (Default-MDT) is created. This is an addition to the Layer 2 VNI broadcast, unknown unicast, and Layer 2 multicast replication groups that already exist. 

For replication and transport, the individual multicast group addresses in the overlay are mapped to the corresponding underlay multicast address. The benefit of employing a BGP-based strategy is that TRM can function as a completely distributed overlay rendezvous point (RP), with the RP present on every edge device (VTEP).

A data center fabric that supports multicast is often part of a larger multicast network. Multicast sources, receivers, and even the multicast rendezvous point may be located within the data center, but they may also be located on campus or remotely accessible via the WAN. TRM enables for the smooth integration of existing multicast networks. It can use multicast rendezvous locations outside of the fabric. 

TRM also supports tenant-aware external connectivity via Layer 3 physical interfaces or subinterfaces.

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