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Chapter 9

Multicast

data to the next hop toward its final destination. We have also discussed
some of the methods that multicast, in general, uses to distribute multicast
data. Similar to unicast, multicast has a variety of routing protocols, includ-
ing distance vector and link state protocols.

Protocols are used to enhance the efficiency by which multicast applica-

tion data is distributed and to optimize the use of existing network resources.
This section will cover Distance Vector Multicast Routing Protocol
(DVMRP), Multicast Open Shortest Path First (MOSPF), and Protocol Inde-
pendent Multicast (PIM).

Distance Vector Multicast Routing Protocol (DVMRP)

Distance Vector Multicast Routing Protocol (DVMRP) has achieved wide-
spread use in the multicast world. A few years ago, you may have often heard
the term "DVMRP Tunnel" used when discussing the implementation of
multicast feeds from an ISP or a feed from the MBONE. As the name indi-
cates, this protocol uses a distance vector algorithm. It uses several of the fea-
tures that other distance vector protocols (such as RIP) implement. Some of
these features are a 32 max hop-count, poison reverse, and 60-second route
updates. It also allows for IP classless masking of addresses.

Just as with other routing protocols, DVMRP-enabled routers must estab-

lish adjacencies in order to share route information. Once the adjacency is
established, the DVMRP route table is created. Route information is
exchanged via route reports. It is important to remember that the DVMRP
route table is stored separately from the unicast routing table. The
DVMRP route table is more like a unicast route table than the multicast
route table that was shown earlier in this chapter. A DVMRP table contains
the layer 3 IP network of the multicast source and the next hop toward the
source.

Because the DVMRP table has this form, it works perfectly in conjunction

with source tree distribution, as discussed earlier. Using the information in
the DVMRP table, the tree for the source can be established. In addition, the
router uses this information to perform the Reverse Path Forwarding check
to verify that the multicast data coming into the interface is coming in an
interface that leads back to the source of the data. DVMRP uses SPT for its
multicast forwarding.

Figure 9.16 gives a description of how DVMRP works. You can see that

not every router in the network is a DVMRP router. You should also notice
that the adjacencies are established over tunnel interfaces. DVMRP informa-
tion is tunneled through an IP network. On either end of the tunnel, infor-
mation is learned and exchanged to build a multicast forwarding database or
route table.

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