How DVMRP
Supports
IP Multicast
Spanning Tree
Delivery
DVMRP is a distance-vector routing protocol that allows routers to
establish shortest-path, source-rooted, IP multicast delivery trees. While it
is similar to the Routing Information Protocol (RIP), one important
difference is that DVMRP focuses on the previous hop back to a multicast
source, not the next hop to a destination. Multicast routers are concerned
with moving packets away from the source on a loopless path so that
multicast storms do not occur.
DVMRP version 3.x uses the Reverse Path Multicast (RPM) algorithm to
construct a delivery tree that begins at the source and spans out to reach
group members on a loopless path through the network. Hence, DVMRP
seeks to form a source-rooted spanning tree for each source-group pair.
The shape of each tree changes dynamically, depending on the location
of hosts that join and leave the group. As shown in Figure 68, any routing
interface that contains group members is called a leaf interface.
Figure 68 Sample IP Multicast Spanning Tree
Branch
Leaf
Leaf
How DVMRP Supports IP Multicast
Source
Branch
Leaf
= Active multicast group member
= Multicast router
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