IGMP join-group vs static-group

Answered Question
May 9th, 2008

I am having difficulty understanding what the difference is between these 2 commands. All the docs that I read say that they both allow the router to forward multicast packets but the "join-group" command will actually cause the router to accept the packets. Can anyone help me understand what is meant exactly by "accept" the packets?

Correct Answer by cisco_lad2004 about 8 years 9 months ago

harold,


I just need to clarify something.

when igmp-join is used, would this be more cpu taxing if testing high volume of multicast. and in which case, should be used with caution.


TIA


Sam

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Harold Ritter Fri, 05/09/2008 - 17:44

Izack,


"ip igmp static-group" configures a static connected member for a specific group. Traffic to that group will be fast-switched to the interface where this comamnd is configured rather than process switched. This command is usually used to forward mcast traffic down an interface.


"ip igmp join-group" causes the router to send an IGMP membership report on the interface where it is configured. The mcast packets will therefore be received and process switched by the router. This command is usually used for test purposes.


Regards,

Correct Answer
cisco_lad2004 Fri, 05/09/2008 - 22:30

harold,


I just need to clarify something.

when igmp-join is used, would this be more cpu taxing if testing high volume of multicast. and in which case, should be used with caution.


TIA


Sam

Harold Ritter Sun, 05/11/2008 - 04:39

Sam,


Good point. I certainly wouldn't recommend using that command to enable a receiver for a multicast group with high bandwidth stream.


Regards,

izackvail Sat, 05/10/2008 - 05:16

Thank you for your response, that helps out a lot. I am still a little confused why a router would process a multicast packet? You mentioned it is for testing but is that the sole purpose of the command join-group? When the router processes the multicast packet what is really happening?

Harold Ritter Sat, 05/10/2008 - 06:52

Izack,


It could be as simple as pinging a multicast group from a given source and the router on which the "ip igmp join-group" is configured for that same group will respond to that ping. I often seen that as an easy way to test a new multicast network.

izackvail Sat, 05/10/2008 - 19:52

Thanks again Harold. The join-group sounds to me like a command reserved mostly for a lab environment. It sounds like there isn't much practical use for it besides simulating a multicast source.

Harold Ritter Sun, 05/11/2008 - 04:36

Izack,


In your last statement you say simulating multicast source but this command rather simulate a multicast receiver.


I would tend to disagree with you last statement. Although I agree that one wouldn't use this command to perform capacity testing, I have used it many times in the past to perform functional tests and troubleshoot new multicast networks and it has helped me tremendously as it creates receivers without having to involved any workstations and requires very little configuration.


Regards,

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