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IP Multicast Overview
calculates the checksum based on the whole Register packet including the data portion. When
the XSR receives a Register packet, it accepts both partial and whole checksum methods.
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The XSR permits configuration of the CRP value and sets the default priority value to 192, as
required by the RFC. The industry-standard router uses a CRP of 0 - the highest priority - as
the default value, and offers no command to change the priority value.
IP Multicast Overview
IP Multicast reduces traffic by simultaneously delivering a single stream of data to thousands of
recipients. It is especially useful for video conferencing and corporate communications where
traffic is significantly reduced.
IP Multicast traffic begins at its source with a single copy. Packets then flow down a Multicast
Distribution Tree (MDT) and are replicated by routers in the network where branches split. The
alternative approach requires a source to send multiple copies of the same data but traffic
consumption can become unwieldy when the number of receivers grows. Since many duplicates
share the same common path, it is sensible to send a single copy over this path and duplicate
packets only when necessary.
The IP multicast architecture is distinguished by these components: multicast group management,
multicast routing, and multicast traffic forwarding.
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Multicast group management records current users desirous of getting traffic addressed to a
particular multicast group. PIM users or hosts have the flexibility to join or quit a multicast
group at any time. IGMP operates within IPv4 networks for this purpose and IGMP version 3
users or hosts can specify the multicast group as well as the sources from which they want to
receive particular multicast traffic.
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Multicast routing (MR) generates and maintains a loop-free multicast distribution tree for each
multicast group over which multicast traffic flows. Further, the multicast routing module
interacts with multicast group management such that when users or hosts want to join a
group, the MR module will build a new or update an existing distribution tree to include
them. DVMRP, PIM-DM and PIM-SM function for this purpose.
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Multicast traffic forwarding duplicates and forwards traffic according to the MDT for a
particular multicast group.
Defining Multicast Group Addressing
IP multicast traffic is recognized by destination IP addresses within the Class D address range.
Each IP multicast address represents a multicast group. IPv4 multicast addresses are assigned
directly by the Internet Assigned Numbers Authority (IANA). The current assignment of the IPv4
multicast addresses is located in RFC-3171. Some address ranges within Class D are reserved for
special multicast services, such as the following.
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IP addresses ranging from 224.0.0.0 to 239.255.255.255 (Class D addresses) are designated
multicast addresses.
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A multicast IP packet reaches a subset of all hosts on the network which have expressed an
interest in the multicast group address.
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Addresses between 224.0.0.0 and 224.0.0.255 are reserved as link local addresses for use by
network protocols on a local network segment and are never forwarded by any router.
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Addresses between 224.0.1.0 and 238.255.255.255 can be delivered throughout the Internet.
7-2 Configuring PIM-SM and IGMP
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Troubleshooting
