Understanding Ip Multicast Routing - Cisco ONS 15454 Software Feature And Configuration Manual

Chapter 10 Configuring Networking Protocols

Understanding IP Multicast Routing

As networks increase in size, multicast routing becomes critically important as a means to determine
which segments require multicast traffic and which do not. IP multicasting allows IP traffic to be
propagated from one source to a number of destinations, or from many sources to many destinations.
Rather than sending one packet to each destination, one packet is sent to the multicast group identified
by a single IP destination group address.
A principal component of IP multicasting is the Internet Group Management Protocol (IGMP). Hosts
identify their multicast group membership by sending IGMP messages to the ML-Series card. Traffic is
sent to all members of a multicast group. A host can be a member of more than one group at a time. In
addition, a host does not need to be a member of a group to send data to that group. When you enable
Protocol Independent Multicast (PIM) on an interface, you will have enabled IGMP operation on that
same interface.
The ML-Series cards support the protocol independent multicast (PIM) routing protocol and the
Auto-RP configuration.
PIM includes three different modes of behavior for dense and sparse traffic environments. These are
referred to as dense mode, sparse mode, and sparse-dense mode.
PIM dense mode assumes that the downstream networks want to receive the datagrams forwarded to
them. The ML-Series card forwards all packets on all outgoing interfaces until pruning and truncating
occur. Interfaces that have PIM dense mode enabled receive the multicast data stream until it times out.
PIM dense mode is most useful under these conditions:
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PIM sparse mode assumes that the downstream networks do not want to forward multicast packets for a
group unless there is an explicit request for the traffic. PIM sparse mode defines a rendezvous point,
which is used as a registration point to facilitate the proper routing of packets.
When a sender wants to send data, it first sends the data to the rendezvous point. When a ML-Series card
is ready to receive data, it registers with the rendezvous point. After the data stream begins to flow from
the sender to the rendezvous point and then to the receiver, ML-Series cards in the data path optimize
the path by automatically removing any unnecessary hops, including the rendezvous point.
PIM sparse mode is optimized for environments in which there are many multipoint data streams and
each multicast stream goes to a relatively small number of LANs in the internetwork. PIM sparse mode
is most useful under these conditions:
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Cisco ONS 15454 SONET/SDH ML-Series Multilayer Ethernet Card Software Feature and Configuration Guide, R4.0
78-15224-02
When senders and receivers are in close proximity to each other
When the internetwork has fewer senders than receivers
When the stream of multicast traffic is constant
When there are few receivers in the group
When senders and receivers are separated by WAN links
When the stream of multicast traffic is intermittent
Understanding IP Multicast Routing
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