Simple MULTICASTING question

Answered Question
Mar 4th, 2010

Hi mates : )

I am trying to understand multicasting. It is IP based (Layer 3) soo looks i need a layer 3 device for multicast traffic. Can i realize multicast communication without routers?

i'll appreciate if you can answer the below questions ;

1. Lets say I have a LAN with 10 hosts all are connected to a layer 2 switch. I want to make a multicast group consists of 3 hosts. and send video data from a server to that multicast group without disturbing other hosts.  Is that possibble?

2. What is the configuration on switch side to enable the multicasting?

3. How can I make hosts (PCs with windows XP) member of a multicast group.

thanks in advance for helping : )

ohalnet

I have this problem too.
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Correct Answer by Jon Marshall about 6 years 9 months ago

ohalnet53 wrote:

Hi mates : )

I am trying to understand multicasting. It is IP based (Layer 3) soo looks i need a layer 3 device for multicast traffic. Can i realize multicast communication without routers?

i'll appreciate if you can answer the below questions ;

1. Lets say I have a LAN with 10 hosts all are connected to a layer 2 switch. I want to make a multicast group consists of 3 hosts. and send video data from a server to that multicast group without disturbing other hosts.  Is that possibble?

2. What is the configuration on switch side to enable the multicasting?

3. How can I make hosts (PCs with windows XP) member of a multicast group.

thanks in advance for helping : )

ohalnet

ohalnet

1) Yes it's possible but you need some way to record the multicast mac-address to the switch ports ie. the switch needs to learn which hosts want to receive the multicast stream

This can be done on a L2 switch by enabling something called the IGMP snooping querier function. Put simply a L2 switch works out which ports want to receive a multicast stream by using IGMP snooping. IGMP snooping allows the switch to listen to responses from hosts to IGMP queries. IGMP queries are usually sent from L3 devices enabled for multicast routing. But if you don't have a L3 device you can tell the switch to generate it's own IGMP queries and that is what the IGMP snoopign querier function does.

Not all switches support this function so if they don't you can always manually add the multicast mac-address in the mac-address table on the switch to the ports you want to receive the traffic. Note this is not a flexible or scalable solution if you have many mulitcast groups or if the end hosts who want the stream dynamically change.

2) Depends on your switch. IGMP snooping is enabled by default on most switches. If you then have the querier function you enable that, otherwise you can use manual mappings.

3) You need specific software to do this. VLC is one example. This is application specific on the PC and nothing to do with the actual network itself.

One final point. If you don not have an IGMP snooping querier function and you don't want to manually map multicast mac-addresses to ports then you can turn off IGMP snooping and the switch will simply treat the multicast packet like a broadcast ie. it sends the packet to all ports other than the one it was received on.

Jon

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Correct Answer
Jon Marshall Thu, 03/04/2010 - 08:35

ohalnet53 wrote:

Hi mates : )

I am trying to understand multicasting. It is IP based (Layer 3) soo looks i need a layer 3 device for multicast traffic. Can i realize multicast communication without routers?

i'll appreciate if you can answer the below questions ;

1. Lets say I have a LAN with 10 hosts all are connected to a layer 2 switch. I want to make a multicast group consists of 3 hosts. and send video data from a server to that multicast group without disturbing other hosts.  Is that possibble?

2. What is the configuration on switch side to enable the multicasting?

3. How can I make hosts (PCs with windows XP) member of a multicast group.

thanks in advance for helping : )

ohalnet

ohalnet

1) Yes it's possible but you need some way to record the multicast mac-address to the switch ports ie. the switch needs to learn which hosts want to receive the multicast stream

This can be done on a L2 switch by enabling something called the IGMP snooping querier function. Put simply a L2 switch works out which ports want to receive a multicast stream by using IGMP snooping. IGMP snooping allows the switch to listen to responses from hosts to IGMP queries. IGMP queries are usually sent from L3 devices enabled for multicast routing. But if you don't have a L3 device you can tell the switch to generate it's own IGMP queries and that is what the IGMP snoopign querier function does.

Not all switches support this function so if they don't you can always manually add the multicast mac-address in the mac-address table on the switch to the ports you want to receive the traffic. Note this is not a flexible or scalable solution if you have many mulitcast groups or if the end hosts who want the stream dynamically change.

2) Depends on your switch. IGMP snooping is enabled by default on most switches. If you then have the querier function you enable that, otherwise you can use manual mappings.

3) You need specific software to do this. VLC is one example. This is application specific on the PC and nothing to do with the actual network itself.

One final point. If you don not have an IGMP snooping querier function and you don't want to manually map multicast mac-addresses to ports then you can turn off IGMP snooping and the switch will simply treat the multicast packet like a broadcast ie. it sends the packet to all ports other than the one it was received on.

Jon

ohalnet53 Thu, 03/04/2010 - 21:50

thanks jon,

whenever i see your name on the reply, i am sure the question is answered in a way everybody understands.

Jon Marshall Fri, 03/05/2010 - 07:45

ohalnet53 wrote:

thanks jon,

whenever i see your name on the reply, i am sure the question is answered in a way everybody understands.

Ohalnet

Many thanks for the kind words and the rating. Much appreciated.

Jon

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