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02 # set protocols pimsm4 interface eth1 vif eth1 disable false
03 # set protocols pimsm4 interface register_vif vif register_vif disable false
04 # set protocols pimsm4 static-rps rp 192.168.3.1 group-prefix 224.0.0.0/4
05 # commit
Enabling IGMP
The configuration presented so far makes it possible to transfer data from the source to particular receivers.
The receivers, however, must be able to inform routers that they are interested in receiving multicast
transmissions. As we described earlier, this information passes through the network via IGMP, so you must
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configure IGMP on the routers that have local receivers.
At this point, you might be wondering why it is necessary to configure IGMP through XORP when the kernel
already supports IGMP.
The problem is that the implementation of IGMP that is included in the kernel does not provide the server side
of IGMP, which means that the built-in kernel implementation will not forward information in IGMP
messages to the multicast routing protocol.
As in the case of PIM-SM or OSPF, the IGMP configuration requires activation of a daemon responsible for
the service of the protocol:
# set protocols igmp disable false
# commit
In addition, you must indicate the interfaces serviced by IGMP:
# set protocols igmp interface eth2 vif eth2 disable false
# commit
Putting It Together
Figure 4 shows the whole transaction at a glance. As you can see, the computer on the left starts by sending a
video sequence to the address 239.192.1.1. Router A, directly connected to the source, starts sending data to
the rendezvous point through a unicast tunnel. The application on the receiving end generates an IGMP
Report message. This message is processed by the router, which is followed by a Join message of the
PIM-SM protocol sent toward Router C, which is acting as the rendezvous point RP. On receiving the
message, Router C sends the multicast transmission in the direction of this receiver, plus any other receivers
in the group that will receive the multicast data.
Figure 4: Initiating a multicast connection.
Conclusion
Multicasting is a complex issue, and here we present only the most basic concepts. Nevertheless, this brief
introduction should give you a head start in determining how best to implement multicasting in your own
environment.
INFO
[1] XORP: http://www.xorp.org/
[2] XORP download: http://www.xorp.org/downloads.html
THE AUTHOR
Over the last few years, the authors, Tomasz Bartczak, Maciej Piechowiak, Tomasz Szewczyk , and Piotr
Zwierzykowski have worked with network technologies and Unix/Linux operating systems. Additionally,
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they are interested in research activities focusing on multicast algorithms and protocol optimization.
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