DVMRP PIM Multicast Routing

235

10.1.5.2 DVMRP

DVMRP is, as the name suggests, a distance vector protocol. It was the first dynamic, multicast routing protocol. As such, it is missing many useful features and optimizations that are available in later protocols. However, it is simple and easy to configure in most networks, especially for networks that use another distance vector protocol such as RIP or IGRP, for regular IP routing. It may be the most natural choice in these cases. DVMRP uses IGMP as one of its basic tools. When an end device joins a multicast group, it informs its local router using IGMP. This router then uses IGMP to tell all of its neighbors that it, too, is a member of this group. Then, to eliminate loops, DVMRP takes advantage of the fact that the path back to the source is unique. It assumes that this same path can be used in the forward direction as well. Using it in the forward direction allows each router to calculate the best path back to the source. It can then simply request multicast packets for this group from whatever router is one hop closer to the multicast source. Unfortunately, DVMRP suffers from many of the same scaling problems as other distance vector protocols. It is probably not the best choice in a large network.

10.1.5.3 PIM

PIM can operate either in dense or sparse mode. Dense mode means that routers send all group information to all neighbors. They then prune back the links that do not require particular groups. Dense mode is efficient when there are relatively few groups and when membership is widespread throughout the network. However, if the network supports a large number of dynamic multicast applications, dense mode is extremely inefficient. Technically, DVMRP is also considered a dense-mode protocol. In sparse mode, on the other hand, individual routers send their neighbors explicit messages asking that they be included or excluded from forwarding particular groups, as downstream devices join or leave these groups. Protocol Independent Multicast—Sparse Mode PIM-SM is defined in RFC 2362. This protocol is much more complex than either MOSPF or DVMRP. It includes the ability, for example, to switch from a semistatic forwarding structure based on rendezvous points to a dynamic shortest-path tree depending on traffic volume. This switch can be made on a group-by-group basis, according to a locally configured volume trigger. PIM-SM scales very well to large networks, although setting it up is complicated. This protocol is a good choice for a large network whose unicast IP routing protocol is not OSPF. EIGRP networks, for example, are good candidates for PIM-SM multicast routing.

10.1.5.4 BGMP