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Distance Vector Multicast Routing Protocol
DVMRP is based on RIP and is widely used. Just like RIP, it uses a distance vector (hops) and sends out
periodic route updates. It’s different from RIP in that it is classless and has a hop limit of 32 rather than 16.
When DVMRP is first implemented on a router, it sends a probe packet with its IP address out all the
interfaces. Another DVMRP router receives the probe and adds the IP address to its list of DVMRP neighbors
on that interface. It then sends back a probe with its IP address and the first router’s IP address. When the first
router receives a probe with its own IP address, it knows that it has a two−way adjacency between itself and
the other router. The two routers will then begin to exchange routing information.
As stated earlier, the router uses RPF to ensure that a packet arrives on the correct interface. If the packet fails
the RPF check, it is discarded. When it passes the RPF check, the packet is forward out all other interfaces. As
the multicast traffic is sent out, it reaches every point in the network. That’s great if every host on the network
wants the multicast—but if part of the network does not need the multicast, the traffic is just taking up
bandwidth. To overcome this problem, the multicast routers use a process called pruning. If the router is not
connected to a recipient (multicast destination), it sends a prune message to the upstream router to stop the
flow of unnecessary multicast traffic. The upstream router responds by pruning the connection to the router. If
the interface on the upstream router is connected to another router, it also must send a prune message before
the upstream router will prune traffic from that interface. As the prune message traffic flows from one router
to another, the multicast traffic is only sent to the portion of the network that wants it. If the router wants to
receive multicast traffic after being pruned, it must send a graft message to the upstream router. When the
upstream router receives the graft message, it sends out a graft−ack message. If that router is not receiving the
multicast traffic, it will also send a graft message to its upstream router. The graft message will be sent until it
reaches a router that is receiving the multicast traffic.
DVMRP sends out periodic updates. It cannot be used in a network with a diameter greater than 31 hops, and
it does not consider the bandwidth of the link. It was the first multicast routing protocol, so, despite its
limitations, it is deployed in the Internet Multicast Backbone (MBONE). The MBONE is used primarily to
transmit desktop video conferencing and—due to the use of DVMRP as its protocol—it has been responsible
for several Internet meltdowns when the volume of traffic has become overwhelming.
Protocol Independent Multicast
Protocol Independent Multicast (PIM) is an IP multicast routing protocol that is independent of the unicast
routing protocol on the router. It does not maintain a multicast routing table, but instead uses the unicast
routing table. The routing table can be populated in many ways: OSPF, static routes, Enhanced IGRP
(EIGRP), Border Gateway Protocol (BGP), and so on. By using the existing routing table to perform reverse
path forwarding checks, PIM sends out no updates and the overhead on the router is significantly reduced.
PIM operates in two modes:
Dense mode (DM)—Utilizes the source tree distribution model. It is designed to operate in an
environment where bandwidth is plentiful and the multicast traffic is destined for all LANs. DM is
suitable for environments with a small number of senders and many recipients. DM is also a good
choice in high−traffic networks with a constant stream of multicast traffic. PIM−DM enabled routers
that have no local members of a multicast group will prune themselves from that group with prune
messages sent to neighboring IP multicast routers. The neighbors that receive the prune messages stop
sending multicast messages and start a timer. When this timer expires, they begin sending multicast
messages to the pruned group again. This process is known as the broadcast and prune cycle.
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Sparse mode (SM)—Uses the unidirectional shared tree design. It is designed to operate in
environments where group members are distributed across many regions of the Internet and
bandwidth may not be widely available. SM does not imply that the group has few members, just that
they are widely dispersed across the Internet. The objective of SM is to prevent situations in which a
relatively small number of devices want to participate in a multicast environment and cause the
multicast traffic to overwhelm the network. SM is designed to limit multicast traffic so that only those
routers interested in receiving traffic for a particular group participate in it. Each router that wishes to
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