multicast ping

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Jun 20th, 2006
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When you set an ip igmp join-group on an interface, are you simulating a multicast client or server?

I guess a client, but if what I'm stating is true, what's the ping doing exactly?

The only explanation I can think is that the ping is simultation the source.

But again, how could the source know whether the client has received the packet or not (considering multicast is sending udp)?

Thanks to read

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leighharrison Wed, 06/21/2006 - 02:33
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Hi there,

When you put "ip igmp join-group" on an interface, it acts not as a server or a client, but as a member of the group.

When the multicast address is ping'd, then any node listening on that address should respond. Try to ping in an ospf environment.

There is not really a concept of server and clients in multicast, just members of the group (nodes litening on that IP address) and people sending to that address, could be a server streaming media, could be a ping ;)



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bellocarico Wed, 06/21/2006 - 03:14
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Thanks for the quick answer, I'm still confused on the following points:

- Is the sender joining the group too?

- Does it do it in the same way as a receiver joins?

(I know you told me that there is not such client/server concept in multicast, but still to understand...)

- If the sender needs to join the group,.is it receiving the traffic it is sending ?!?

- If a PC joins a multicast group, it will receive data, but also be able to send data to that group.

Isn't this a security hole?

Thanks to read!

leighharrison Wed, 06/21/2006 - 05:02
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Hello again!

The sender does not join the group. You only need to join a group if you want to have the data passed to you. An IP address is only needed to recieve traffic. When a PC boots up it send out a broadcast for a dhcp address to the network before it has an address.

If the sender is a member of the group, then, depending on the setup, it could recieve it packets back. But the sender is not usually in the group.

As for a security hole, I'm pretty sure that PC's sending data to eachother is not a security hole ;-)

In the "sh ip mroute" on a cisco router, the multicast routes have an S and a G entry. S is source ang G is group.

From CCO "The special notation of (S,G), pronounced "S comma G," enumerates an SPT in which S is the IP address of the source and G is the multicast group address. Using this notation, the SPT for the example in Figure 43-7 would be (, "

Check out this link for some good diagrams:



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bellocarico Wed, 06/21/2006 - 06:43
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> The sender does not join the group

If the sender does not join the group, anybody can send traffic, am I right?

If so, I really miss to understand what kind of communication there is between the source and the directly connected router (hopefully running PIM).

For the receiver (joining the group) there is IGMP, but what does the source PC do? just send a stream? Is the PIM router able to recognize automatically a source for a specific group?

> As for a security hole, I'm pretty sure that PC's sending data to eachother is not a security hole ;-)

Hum, I still think that this is a security hole, as anybody can interfer with the data received by other multicast clients.

Isn't this the reason why IGMP v3 was initially designed (SSM)?



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