Tenant Routed Multicast (TRM) brings the efficiency of multicast delivery to VXLAN overlays. It is based on standards-based next-gen control plane (ngMVPN) described in IETF RFCs 6513 and 6514. TRM enables the delivery of customer Layer 3 multicast traffic in a multitenant fabric, and this in an efficient and resilient manner.

While BGP EVPN provides a control plane for unicast routing, as shown in Figure 3-6, ngMVPN provides scalable multicast routing functionality. It follows an “always route” approach where every edge device (VTEP) with distributed IP Anycast Gateway for unicast becomes a designated router (DR) for multicast. Bridged multicast forwarding is present only on the edge devices (VTEP) where IGMP snooping optimizes the multicast forwarding to interested receivers. All other multicast traffic beyond local delivery is efficiently routed.

Figure 3-6 Tenant Routed Multicast (TRM)

With TRM enabled, multicast forwarding in the underlay is leveraged to replicate VXLAN-encapsulated routed multicast traffic. A Default Multicast Distribution Tree (Default-MDT) is built per VRF. This is an addition to the existing multicast groups for Layer 2 VNI broadcast, unknown unicast, and Layer 2 multicast replication group. The individual multicast group addresses in the overlay are mapped to the respective underlay multicast address for replication and transport. The advantage of using a BGP-based approach is that TRM can operate as a fully distributed overlay rendezvous point (RP), with the RP presence on every edge device (VTEP).

A multicast-enabled data center fabric is typically part of an overall multicast network. Multicast sources, receivers, and even the multicast rendezvous point might reside inside the data center but might also be inside the campus or externally reachable via the WAN. TRM allows seamless integration with existing multicast networks. It can leverage multicast rendezvous points external to the fabric. Furthermore, TRM allows for tenant-aware external connectivity using Layer 3 physical interfaces or subinterfaces.