D-Link DI-1750 Reference Manual page 302

Periodically, the source performs what is known as flooding in order to push datagrams downstream.
Initially, DVMRP routers assume that every node on the connected subnets wants to receive data.
Along with the datagrams, a packet called the route report is transmitted (across a time interval). All the
routes known by a given router is sent to all adjacent routers. By using IGMP, the routers are aware of
which hosts are connected to the subnets. On receiving the route reports, a router selects the best
adjacent router through which it can reach the given source network.
After registering that router as the upstream neighbor, a dependency is expressed to the router for
receiving packets. Upon receiving this dependency, the receiving router registers the expressed
dependency as its dependent downstream neighbor.
Hence, an optimal multicast spanning tree is constructed. On receiving a multicast packet, a router
accepts that packet for forwarding if and only if the packet is received from the upstream neighbor.
Otherwise, it will be dropped.
Then, the list of the dependent downstream neighbor is verified and the Lower Layer protocols forwards
the packets to the specific neighbors. If there are no dependent downstream neighbors, PRUNE
message are sent to the upstream neighbor, requesting that the router should stop sending packets
from the specified source network for a specified amount of time.
Thus, datagrams will no longer be transmitted along that path (cutting down on the bandwidth being
used). After a prune interval, packets will be flooded downstream again, along the shortest path trees.
If any other routers expresses some dependencies, GRAFT message are sent to that upstream
neighbor. Upon receiving an acknowledgement, GRAFTACK message are expected back. On
receiving a GRAFT MESSAGE from a downstream neighbor, multicast packets will be sent on that
interface.
The above procedures of Pruning and Grafting could initiate a similar action in the receiving routers.
This means that unnecessary data traffic will be reduced in the network, which is the advantage of
DVMRP.
7.1.2 Basic multicast-group route configuration task list
Basic multicast-group configuration include:
♦ Start multicast-group route
♦ Start multicast-group function on the port
♦ Configure IGMP speciality (optional)
♦ Configure TTL threshold (optional)
♦ Cancel multicast-group fast transmit (optional)
7.1.3 Advance multicast-group route configuration task list
Advance multicast-group configuration include
♦ Configure PIM-DM task list (optional)
♦ Configure DVMRP task list (optional)
♦ Configure multicast-group static route (optional)
♦ Control the transmission speed of IGMP group (optional)
♦ Configure multicast-group boundary (optional)
♦ Configure multicast-group Helper (optional)
♦ Configure Stub multicast-group route (optional)
♦ Watch and maintenance multicast-group route (optional)
Model Name
- 300 -