Concepts

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 networks.

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

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

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