IP Multicast over ATM Point-to-Multipoint VCs is a feature that dynamically creates ATM point-to-multipoint switched virtual circuits (SVCs) to handle IP multicast traffic more efficiently.
This feature can enhance router performance and link utilization because packets are not replicated and sent multiple times over the ATM interface.
Traditionally, over NBMA networks, Cisco routers would perform a pseudobroadcast to get broadcast or multicast packets to all neighbors on a multiaccess network. For example, assume in the figure that Routers A, B, C, D, and E were running the Open Shortest Path First (OSPF) protocol. Router A must deliver to Routers D and E. When Router A sends an OSPF Hello packet, the data link layer replicates the Hello packet and sends one to each neighbor (this procedure is known as pseudobroadcast), which results in four copies being sent over the link from Router A to the multiaccess WAN.
Figure 1. Environment for IP Multicast over ATM Point-to-Multipoint VCs
With the advent of IP multicast, where high-rate multicast traffic can occur, the pseudobroadcast approach does not scale. Furthermore, in the preceding example, Routers B and C would get data traffic they do not need. To handle this problem, PIM can be configured in NBMA mode using the ip pim nbma-mode command. PIM in NBMA mode works only for sparse mode groups. Configuring PIM in NBMA mode would allow only Routers D and E to get the traffic without distributing to Routers B and C. However, two copies are still delivered over the link from Router A to the multiaccess WAN.
If the underlying network supported multicast capability, the routers could handle this situation more efficiently. If the multiaccess WAN were an ATM network, IP multicast could use multipoint VCs.
To configure IP multicast using multipoint VCs, Routers A, B, C, D, and E in the figure must run PIM sparse mode. If the Receiver directly connected to Router D joins a group and Router A is the PIM RP, the following sequence of events occurs:
Router D sends a PIM Join message to Router A.
When Router A receives the PIM join, it sets up a multipoint VC for the multicast group.
Later, when the Receiver directly connected to Router E joins the same group, Router E sends a PIM Join message to Router A.
Router A will see there is a multipoint VC already associated with the group, and will add Router E to the existing multipoint VC.
When the Source sends a data packet, Router A can send a single packet over its link that gets to both Router D and Router E. The replication occurs in the ATM switches at the topological diverging point from Router A to Router D and Router E.
If a host sends an IGMP report over an ATM interface to a router, the router adds the host to the multipoint VC for the group.
This feature can also be used over ATM subinterfaces.