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9. Support for Multicast

9. Support for Multicast​

The EVPN IMET route is used to discover the multicast tunnels among the endpoints associated with a given EVI (e.g., given VNI) for VLAN-Based Service and a given <EVI, VLAN> for VLAN-Aware Bundle Service. All fields of this route are set as described in Section 5.1.3. The originating router's IP address field is set to the NVE's IP address. This route is tagged with the PMSI Tunnel attribute, which is used to encode the type of multicast tunnel to be used as well as the multicast tunnel identifier. The tunnel encapsulation is encoded by adding the BGP Encapsulation Extended Community as per Section 5.1.1. For example, the PMSI Tunnel attribute may indicate the multicast tunnel is of type Protocol Independent Multicast - Sparse-Mode (PIM-SM); whereas, the BGP Encapsulation Extended Community may indicate the encapsulation for that tunnel is of type VXLAN. The following tunnel types as defined in [RFC6514] can be used in the PMSI Tunnel attribute for VXLAN/NVGRE:

  • 3 - PIM-SSM Tree
  • 4 - PIM-SM Tree
  • 5 - BIDIR-PIM Tree
  • 6 - Ingress Replication

In case of VXLAN and NVGRE encapsulations with locally assigned VNIs, just as in [RFC7432], each PE MUST advertise an IMET route to other PEs in an EVPN instance for the multicast tunnel type that it uses (i.e., ingress replication, PIM-SM, PIM-SSM, or BIDIR-PIM tunnel). However, for globally assigned VNIs, each PE MUST advertise an IMET route to other PEs in an EVPN instance for ingress replication or a PIM-SSM tunnel, and they MAY advertise an IMET route for a PIM-SM or BIDIR-PIM tunnel. In case of a PIM-SM or BIDIR-PIM tunnel, no information in the IMET route is needed by the PE to set up these tunnels.

In the scenario where the multicast tunnel is a tree, both the Inclusive as well as the Aggregate Inclusive variants may be used. In the former case, a multicast tree is dedicated to a VNI. Whereas, in the latter, a multicast tree is shared among multiple VNIs. For VNI-Based Service, the Aggregate Inclusive mode is accomplished by having the NVEs advertise multiple IMET routes with different RTs (one per VNI) but with the same tunnel identifier encoded in the PMSI Tunnel attribute. For VNI-Aware Bundle Service, the Aggregate Inclusive mode is accomplished by having the NVEs advertise multiple IMET routes with different VNIs encoded in the Ethernet Tag field, but with the same tunnel identifier encoded in the PMSI Tunnel attribute.

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