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Configuration Guide - IP Multicast

CloudEngine 8800, 7800, 6800, and 5800 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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Concepts

Concepts

This section describes PIM-related concepts based on the network shown in Figure 4-1.

Figure 4-1 PIM network

Multicast Distribution Tree

On a PIM network, a point-to-multipoint (P2MP) multicast forwarding path is established for each multicast group on separate routers. The multicast forwarding path looks like a tree, so it is also called a multicast distribution tree (MDT).

Two types of MDTs are available:

  • Shortest path tree (SPT): uses the multicast source as the root and multicast group members as leaves. SPT applies to both PIM-DM and PIM-SM networks.

    In Figure 4-1, the MDT, RouterE→RouterD→RouterA/RouterB, is an SPT, which uses the source as the root and HostA and HostB as leaves.

  • Rendezvous point tree (RPT): uses a rendezvous point (RP) as the root and multicast group members as leaves. RPT applies to PIM-SM or Bidir-PIM networks.

    For details about RP and RPT, see PIM-SM (ASM Model).

    NOTE:

    Leaf hosts of an RPT on a Bidir-PIM network may not only receive multicast data as group members, but also forward multicast data as multicast sources. That is, multicast data is forwarded bidirectionally on the RPT, so the RPT on a Bidir-PIM network is also called a Bidir RPT. For details about Bidir RPT, see Bidir-PIM.

PIM Router

Routers with PIM enabled on interfaces are called PIM routers. During the establishment of an MDT, PIM routers play the following roles:

  • Leaf router: connects to user hosts, which may not be multicast group members. For example, RouterA, RouterB, and RouterC in Figure 4-1 are leaf routers.
  • First-hop router: directly connects to the multicast source on the multicast forwarding path and is responsible for forwarding multicast data from the multicast source. For example, RouterE in Figure 4-1 is the first-hop router.
  • Last-hop router: directly connects to multicast group members (receivers) on the multicast forwarding path and is responsible for forwarding multicast data to these members. For example, RouterA and RouterB in Figure 4-1 are last-hop routers.
  • Intermediate router: resides between the first-hop router and the last-hop router on the multicast forwarding path. For example, RouterD in Figure 4-1 is an intermediate router.

PIM Routing Entry

Two types of PIM routing entries are generated using PIM: (S, G) and (*, G); S indicates a specific multicast source, G indicates a specific multicast group, and * indicates any multicast source.

  • (S, G) entries are often used to establish an SPT on a PIM network. (S, G) entries apply to PIM-DM and PIM-SM network.
  • (*, G) entries are often used to establish an RPT on a PIM network. (*, G) entries apply to PIM-SM and Bidir-PIM networks.

A PIM router may have both (S, G) and (*, G) entries. When a PIM router receives a multicast packet with the source address S and the group address G and the packet passes the RPF check, the router forwards the packet according to the following rules:

  • If the (S, G) entry exists, the router forwards the packet according to the (S, G) entry.
  • If the (S, G) entry does not exist but the (*, G) entry exists, the router creates an (S, G) entry based on this (*, G) entry, and then forwards the packet according to the (S, G) entry.

PIM routing entries contain the following information to guide multicast packet forwarding:

  • Multicast source address
  • Multicast group address
  • Upstream interface, which receives multicast data on the local router, such as Int3 in Figure 4-1
  • Downstream interface, which forwards multicast data, such as Int1 and Int2 in Figure 4-1
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Updated: 2019-04-20

Document ID: EDOC1100075361

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