A multicast distribution tree (MDT) 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.
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 1-12 shows a source tree for group 239.1.1.1 that begins at host A and connects to hosts B and C.
The notation (S, G) represents the multicast traffic from source S on group G. The SPT in Figure 1-12 is written (172.16.1.1, 239.1.1.1) and called “S comma G.” Multiple sources can be transmitting on the same group.
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 1-13 shows a shared tree for group 239.1.1.1 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.
The notation (*, G) (“star comma G”) represents the multicast traffic from any source on group G. The shared tree in Figure 1-13 is written (*, 239.1.1.1).
A bidirectional shared tree represents the shared distribution path that the multicast traffic takes through the network from a shared root, or rendezvous point, to each receiver. Multicast data is forwarded to receivers encountered on the way to the RP. The advantage of the bidirectional shared tree is shown in Figure 1-14. Multicast traffic flows directly from host A to host B through routers B and C. In a shared tree, 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.
Figure 1-14 Bidirectional Shared Tree
The notation (*, G) represents the multicast traffic from any source on group G. The bidirectional tree in Figure 1-14 is written (*, 239.0.0.1).
Cisco NX-OS supports multicasting with Protocol Independent Multicast (PIM) sparse mode. PIM (PIMv2) is an independent IPv4 routing protocol, and PIM6 is an independent IPv6 routing protocol. In PIM sparse mode, multicast traffic is sent only to locations of the network that specifically request it. PIM dense mode is not supported by Cisco NX-OS.
You need to enable the PIM or PIM6 feature before configuring multicast. Multicast is enabled only after you enable PIM or PIM6 on an interface of each router in a domain. You configure PIM for an IPv4 network and PIM6 for an IPv6 network. By default, IGMP and MLD are enabled on the NX-OS system.
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