Packet Guide to Routing and Switching

Chapter 8. Multicast

Most of the host-based traffic seen on IPv4 networks is the result of unicast (node-to-node) or broadcast (node-to-everyone) frames or packets. The third type of traffic is called multicast, which is used to contact a subset of the nodes on the network or to create a one-to-many transmission. Perhaps the most famous use of multicast comes to us from the MBone, the Multicast Backbone project. While interest in the MBone project is somewhat quiescent, multicast still has its uses in networks today, including video streams, protocols such as spanning tree, and vendor-specific communication. With IPv6, we also see much greater dependence on multicast as broadcast addressing is no longer used. Thus, it becomes important for a network administrator to know something of multicast operation and its impact on the network.

Multicast has two parts: that which occurs between devices on the same network and a component that enables multicast over a collection of networks. Routers handle traffic traveling between networks and by default, routers do not forward multicast traffic. They have to be configured to forward multicast and like most destinations, the routers also have to be given instructions for the multicast transmissions. This is accomplished via Protocol Independent Multicast (PIM).

As we will later in this chapter, another way to think of multicast traffic is traffic that goes to a “host group.” Locally, end hosts signal their interest in a multicast transmission ...