Multicast Distribution Trees

A multicast distribution tree represents the path that multicast data takes between the routers that connect sources and receivers. The multicast software builds different types of trees to support different multicast methods.

Source Trees

A source tree represents the shortest path that the multicast traffic takes through the network from the sources that transmit to a particular multicast group to receivers that requested traffic from that same group. Because of the shortest path characteristic of a source tree, this tree is often referred to as a shortest path tree (SPT). Figure 6-11 shows a source tree for group 224.1.1.1 that begins at host A and connects to hosts B and C.

  

Figure 6-11 Source Tree

The notation (S, G) represents the multicast traffic from source S on group G. The SPT in this figure is written (192.0.2.1, 224.1.1.1). Multiple sources can be transmitting on the same group.

Shared Trees

A shared tree represents the shared distribution path that the multicast traffic takes through the network from a shared root or rendezvous point (RP) to each receiver. The RP creates an SPT to each source. A shared tree is also called an RP tree (RPT). Figure 6-12 shows a shared tree for group 224.2.2.2 with the RP at router D. Source hosts A and D send their data to router D, the RP, which then forwards the traffic to receiver hosts B and C.

  

Figure 6-12 Shared Tree

The notation (*, G) represents the multicast traffic from any source on group G. The shared tree in this figure is written (*, 224.2.2.2).

Bidirectional Shared Trees

A bidirectional shared tree represents the shared distribution path that the multicast traffic takes through the network from a shared root, or rendezvous point (RP), to each receiver. With Bidirectional PIM (BIDIR-PIM), traffic from the source is forwarded toward the RP by the designated forwarder (DF). On each point-to-point link and every network segment, one DF is elected for every RP of the bidirectional group, and the DF will also be responsible for forwarding multicast traffic received from the source on that network. In BIDIR-PIM, the traffic from the source can be forwarded directly on the branch toward the interesting receivers without first reaching the RP. From the perspective of the receiver, there are no significant changes compared to the regular PIM-SM. The last-hop designated routers will forward (*, G) joins toward the RP serving the group. The only difference is that the DF performs the tasks of the designated router. (More on the designated router and designated forwarder can be found in the “PIM Sparse Mode” and “BIDIR-PIM” sections, later in this chapter.) The advantage of the bidirectional shared tree is shown in Figure 6-13.

  

Figure 6-13 Bidirectional Shared Tree

Multicast traffic flows directly from source host A to receiver host B through routers B and C by designated forwarder B, because the receiver can be reached by DF without reaching RP on the shared tree. In a shared tree scenario, the data from source host A is first sent to the RP (router D) and then forwarded to router B for delivery to host B.

The notation (*, G) represents the multicast traffic from any source on group G. The bidirectional tree in the figure is written as (*, 224.2.2.2).

The forwarding and tree building process in bidirectional shared trees consists of three main stages:

The DF is responsible for sending (*, G) joins toward the RP for the active bidirectional group. Downstream routers address their (*, G) joins to upstream DFs. This is accomplished by putting the IP address of the upstream DF in the upstream router field of a PIM join message.

When the DF receives an (*, G) join, it adds the link to the OIL of the (*, G) entry and joins toward the RP. If the interface exists in the OIL, the interface timer is refreshed.

The DF also has the responsibility of forwarding multicast traffic in BIDIR-PIM. When multicast traffic is received from the source on a link for which the router is the DF, the router must forward that traffic via its RPF interface toward the RP. Furthermore, the DF must forward the received traffic out all other interfaces in the (*, G) OIL, excluding the interface on which the traffic was received.

Note

Steps 1 and 2 describe the bidirectional shared tree building process and step 3 describes the multicast forwarding process in BIDIR-PIM.