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Configuration Guide - IP Multicast

CloudEngine 12800 and 12800E V200R005C10

This document describes the configurations of IP multicast, including IP multicast basics, IGMP, MLD, PIM (IPv4), PIM (IPv6), MSDP, multicast VPN, multicast route management (IPv4), multicast route management (IPv6), IGMP snooping, MLD snooping, static multicast MAC address, multicast VLAN, multicast network management.
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Basic Concepts of Multicast VPN

Basic Concepts of Multicast VPN

The device supports multicast VPN based on multicast domain (MD). In this solution, the public network maintains a multicast distribution tree (MDT) for each VPN instance. An MDT is automatically set up after multicast VPN is configured, regardless of whether multicast services are running on the network. Multicast packets from a VPN site are encapsulated on a PE and forwarded along the MDT to a remote PE bound to the same VPN instance. If the site connected to the remote PE has receivers of the specified group, the PE forwards the multicast packets to the CE. If there are no receivers attached to PE, the PE drops the multicast packets.

On the network shown in Figure 7-2, PE1 is configured with the public network instance and VPN instances VPNA and VPNB, which function as three independent virtual devices. Multicast packets are exchanged between PE and CE devices through the VPN instances, and between PE and P devices through the public network instance. The following describes basic concepts of multicast VPN based on Figure 7-3, using VPNA as an example.

Figure 7-2 Typical BGP/MPLS IP VPN networking
Figure 7-3 Multicast packet forwarding on VPNA
  • MD

    An MD specifies the range within which multicast data from a VPN can be transmitted on the public network. The boundary of an MD is composed of all PEs bound to the VPN instance. Different VPN instances belong to different MDs. As shown in Figure 7-3, the circle in the middle of VPNA represents an MD that serves VPNA. All multicast data on VPNA is transmitted within this MD.

    A VPN instance determines an MD, and an MD belongs to only one VPN instance (one-to-one mapping).

  • PE multi-instance

    The public network instance and multiple VPN instances can be created on a PE, and they separately use Protocol Independent Multicast (PIM). PEs use VPN instances to exchange multicast data with CEs on VPNs, and use the public network instance to exchange multicast data with the P devices on the public network. PEs need to encapsulate VPN packets sent to the public network and decapsulate packets sent from the public network to private networks.

    VPN instances and the public network instance are independent, and they can use different PIM modes.

  • Multicast tunnel (MT) and multicast tunnel interface (MTI)

    On the network shown in Figure 7-3, VPN users only need to know that all PEs are connected to the MT in the MD and transmit VPN data through the MT.

    An MTI functions as the inbound and outbound interfaces of the MT and the MD. A local PE sends VPN data through the MTI, and the remote PE receives data from the MTI. An MTI is the tunnel for data exchange between the public network instance and the VPN instance on a PE. A PE connects to the MT through the MTI. The VPN instances on PEs in the MD establish PIM neighbor relationships with each other through the MTI.

    During packet forwarding, a local PE encapsulates VPN multicast packets into public network multicast data packets and forwards them along the MDT on the public network.


    All interfaces belonging to a VPN, including the MTIs and PE interfaces bound to the VPN instance, must use the same PIM mode.

  • MDT

    MDTs established between PEs connected to sites of a VPN include Share-MDT and Switch-MDT. Packets are forwarded along MDTs so that packets from a VPN are replicated at bifurcation points on the public network, reducing the loads on the public network.

    When packets are forwarded along a Share-MDT on the public network, all PEs connected to sites of the VPN instance receive the packets. When packets are forwarded along a Switch-MDT, only PEs connected to the sites with receivers in the VPN instance receive the packets.

    For details on how a Share-MDT is set up, see Share-MDT Establishment. For details on how a Switch-MDT is set up, see Switch-MDT Switchover.

  • Share-MDT

    A Share-MDT connects to all PEs bound to a VPN instance. All VPN multicast packets transmitted on the VPN are forwarded through the Share-MDT regardless of the PEs from which the packets enter the public network. The Share-MDT uses a Share-Group as the multicast group address. On a VPN, Share-Group identifies a Share-MDT.

  • Share-Group

    Each MD has an independent multicast address called Share-Group address on the public network. A Share-Group address identifies an MD on the public network and is used to establish a Share-MDT for the MD. VPN multicast packets are transparent to the public network. PEs do not distinguish the multicast group to which the packets belong or the packet type (protocol or data). PEs encapsulate the VPN multicast packets into public network multicast data packets and use the Share-Group address of the MD as the group address of the packets.

    A Share-Group corresponds to an MD. That is, an MD has only one Share-Group address, and a Share-Group belongs to only one MD.

  • Switch-MDT

    When a PE with a Switch-Group-Pool configured detects traffic on the Share-Group, the PE sends a switchover notification to the downstream PEs to trigger a switchover from Share-MDT to Switch-MDT. The switchover notification carries the Switch-Group address. After receiving the notification, the downstream PE connected to sites with receivers joins the Switch-Group, and a Switch-MDT is generated. The ingress PE forwards encapsulated VPN multicast data along the Switch-MDT on the public network. When packets are forwarded along the Switch-MDT, only the PEs connected to sites with receivers receive the packets. This improves multicast data forwarding efficiency and reduces the loads on PEs.

  • Switch-Group and Switch-Group-Pool

    A Switch-Group-Pool defines the range of available multicast group addresses. When a PE with a Switch-Group-Pool configured detects traffic on the Share-Group, it triggers a Switch-MDT switchover. A PE selects an idle address from the Switch-Group-Pool as the Switch-Group address. A Switch-MDT is then established from this PE (source) to downstream PEs connected to sites with receivers. The Switch-Group is the group address of the Switch-MDT. Compared to a Share-MDT, a Switch-MDT cuts off remote PEs that do not require multicast data, improving multicast data forwarding efficiency. All VPN multicast packets from the source PE are encapsulated using this Switch-Group address.

    An MD has only one Switch-Group-Pool, and a Switch-Group-Pool belongs to only one MD (one-to-one mapping).

Updated: 2019-04-20

Document ID: EDOC1100074724

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