24-port NWay Ethernet Switch User’s Guide
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Switch Management and Operating Concepts
are densely distributed over the network. Then Protocol
Independent Multicast – Sparse Mode (PIM-SM) protocol which
performs better when group members are sparsely distributed.
Distance Vector Multicast Routing
Protocol (DVMRP)
DVMRP is defined in RFC 1075 and was derived from the
Routing Information Protocol (RIP) with the difference being that
RIP forwards the unicast packets based on the information about
the next-hop toward a destination, while DVMRP constructs
delivery trees based on the information on the previous-hop back
toward the source. The earlier version of this distance-vector
routing algorithm constructs delivery trees based on the TRPB
algorithm. Later DVMRP was enhanced to use RPM.
Standardization of the latest version of DVMRP is being
conducted by the Internet Engineering Task Force (IETF) Inter-
Domain Multicast routing (IDMR) working group.
DVMRP implements the RPM algorithm. The first multicast
packet sent from a particular source to a particular multicast
group is flooded across the network. Then prune messages are
used to truncate the branches that do not lead to a group
member. A new type of message is used to quickly “graft” back a
previously pruned branch of a delivery tree in case a new host on
that branch joins the multicast group. Similar to prune
messages which are forwarded hop by hop, graft messages are
sent back one hop at a time until thy reach a node that is on the
multicast delivery tree. Similar to RPM, DVMRP still
implements the flooding of packets periodically.
In cases where more than one router are present in a
subnetwork, the one that is closest to the source of a multicast
message is elected to be in charge of forwarding multicast
messages. All other routers will simply discard the multicast