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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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6.5.3 DVMRP tunnels<br />

Some <strong>IP</strong> routers might not be configured to support native multicast routing.<br />

DVMRP provides the ability to tunnel <strong>IP</strong> multicast datagrams through networks<br />

containing non-multicast routers. The datagrams are encapsulated in unicast <strong>IP</strong><br />

packets <strong>and</strong> forwarded through the network. This behavior is shown in<br />

Figure 6-8.<br />

Figure 6-8 DVMRP tunnels<br />

When the packet is received at the remote end of the tunnel, it is decapsulated<br />

<strong>and</strong> forwarded through the subnetwork using st<strong>and</strong>ard DVMRP multicast<br />

operations.<br />

6.6 Multicast OSPF (MOSPF)<br />

MOSPF is a multicast extension to OSPF Version 2 (refer to 5.6, “Open Shortest<br />

Path First (OSPF)” on page 196), defined in RFC 1584. Unlike DVMRP, MOSPF<br />

is not a separate multicast routing protocol. It is used in networks that already<br />

use OSPF for unicast <strong>IP</strong> routing. The multicast extensions leverage the existing<br />

OSPF topology database to create a source-rooted shortest path delivery tree.<br />

MOSPF forwards multicast datagrams using both source <strong>and</strong> destination<br />

address. This contrasts the st<strong>and</strong>ard OSPF algorithm, which relies solely on<br />

destination address.<br />

258 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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