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Multicast message sent to router

Posted on 2013-01-12
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Can you explain it for me? Source router can send multicast message over several routers to destination router. How does the server send the message(for example, video play in server) to the source router and how does the other PC receive the message from the destination router ? It needs some software or some configuration ? Thank you.
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Question by:davidy2001
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by:giltjr
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A multicast packet is just a packet that uses an IP address that is within the defined multicast range.  The server sends out packets with a multicast address and clients listen for that same address.

Clients need to "register" themselves as listening for the specific multicast address, this way if you are in a switched environment switches know to forward the packets over the correct ports.

You can read more about multicast here http://en.wikipedia.org/wiki/Multicast.
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by:davidy2001
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Thank you for your reply! I know how Multicast packets travel from the source router to destination router. But I do not know how the client PC register to the destination router. Can you give some suggestion?
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by:giltjr
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The "registration" process is nothing more than issuing a ARP with the multicast IP address and the corresponding multicast Ethernet MAC address (or whatever L2 protocol you are using MAC address).  This is cached in the L2/L3 device that is local to the client.
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by:giltjr
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You can search 'How does multicast work" with your favorite search engine.  Here is one referece from MS that is fairly well written:

http://technet.microsoft.com/en-us/library/cc759719%28v=ws.10%29.aspx
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by:davidy2001
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Great ! i think it can solve my problem. I am reading it. Thank you.
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by:giltjr
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Are you writing your own application that uses multicast?
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by:davidy2001
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I am just trying to understand Multicast completely. Most papers discuss it a lot, but they never talk about its connection with client PC and that server
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by:giltjr
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Typically the client will talk to the server using unicast.  The server will tell the client what multicast IP address it should use.

In some cases a server will periodically send out a MAC level broadcast with a IP multicast address and client software will listen for this.  IIRC this is how ghost works.

In both cases the client can use IGMP join to join the multicast group.
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by:davidy2001
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I use command "ip igmp join-group 224.x.x.x" at destination router. How can i let the client PC receive these multicast message from the router?
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What brand and model router do you have?  What level of software is it running?

If you have a Cisco router this may help:

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfmulti.html
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