Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 280
Protocol-Independent Multicast (PIM)

DVMRP provides great mechanisms for multicasting. Using the latest versions of IGMP and DVMRP, a fully functional multicast tree can be dynamically built and utilized for voice, video, and data. There is, however, a disadvantage to DVMRP: It does not scale well. DVMRP is a distance-vector multicast routing update protocol. DVMRP broadcasts the first multicast packet it receives from the source and then prunes the tree based on feedback from other routers. DVMRP is known as a dense mode multicast protocol, and is most efficient when the clients are densely located on the network. It does not scale well when the protocol is being used over WAN links, or when there are simply a few clients scattered throughout the customer’s network. This broadcast and prune mechanism, along with multicast routing updates, causes unnecessary overhead over low-bandwidth media types. Furthermore, DVMRP routing tables are based on a RIP-like update. DVMRP also requires the routers to keep state information. This includes group and source information that is used to calculate a tree.

If all the members of a multicast group are located in a bandwidth-rich region (supported by high-speed LANs and not low-speed WANs), then it should be supported by a dense mode protocol such as DVMRP, MOSPF, or PIM-Dense Mode (DM). This can be limiting in that the scope of the group cannot include any members beyond the scope of the domain without placing the unnecessary burden of Broadcast and Prune messages and possibly multicast routing updates over the link that includes that remote receiver.

PIM offers two versions for multicast routing: dense mode and sparse mode.