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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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The disadvantage to the center-based tree algorithm is that it might build a<br />

suboptimal path for some sources <strong>and</strong> receivers.<br />

6.4.3 Multicast routing protocols<br />

A number of multicast routing protocols have been developed using these<br />

algorithms:<br />

► Distance Vector Multicast Routing Protocol (DVMRP)<br />

► Multicast OSPF (MOSPF)<br />

► Protocol Independent Multicast (PIM)<br />

The remainder of this chapter describes these protocols.<br />

6.5 Distance Vector Multicast Routing Protocol<br />

(DVMRP)<br />

DVMRP is an established multicast routing protocol originally defined in RFC<br />

1075. The st<strong>and</strong>ard was first implemented as the mrouted process available on<br />

many UNIX systems. It has since been enhanced to support RPF. DVMRP is an<br />

interior gateway protocol. It is used to build per-source per-group multicast<br />

delivery trees within an autonomous system (AS).<br />

DVMRP does not route unicast datagrams. Any router that processes both<br />

multicast <strong>and</strong> unicast datagrams must be configured with two separate routing<br />

processes. Because separate processes are used, multicast <strong>and</strong> unicast traffic<br />

might not follow the same path through the network.<br />

6.5.1 Protocol overview<br />

DVMRP is described as a broadcast <strong>and</strong> prune multicast routing protocol:<br />

► DVMRP builds per-source broadcast trees based on routing exchanges.<br />

► DVMRP dynamically prunes the per-source broadcast tree to create a<br />

multicast delivery tree. DVMRP uses the RPF algorithm to determine the set<br />

of downstream interfaces used to forward multicast traffic.<br />

Neighbor discovery<br />

DVMRP routers dynamically discover each neighbor by periodically sending<br />

neighbor probe messages on each local interface. These messages are sent to<br />

the all-DVMRP-routers multicast address (224.0.0.4). Each message contains a<br />

list of neighbor DVMRP routers for which neighbor probe messages have been<br />

254 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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