multicast basics

Unanswered Question


Could someone help with this very basic info.

If multicast host & source are on same vlan but different switches, should we require any configuration in the switch ( eg: igmp or multicast group address). If no multicast related commands configured on the vlan layer 3 interface, will the multicast traffic travel between multiple vlans ( router blocks the traffic like broadcast). 



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Chad Peterson Tue, 08/17/2010 - 11:58
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So if source and clients are in the same vlan, but cross switches, typically you do not need any additional configuration.  However multicast will be handled like broadcasts because there is no IGMP querier on the vlan.  W/o a querier we won't see any IGMP join messages, these can be used by IGMP snooping to contrain MCAST to only the ports that want paticicular groups.  When our IGMP snooping table is empty, we'd say these groups are 'unknown multicast groups'.  Traditionally you flood them.

One caveat here is 6500.  It doesn't flood mcast when the snooping table doesn't have an entry for the group.  Good article about it here:

Also, with no L3 interfaces configured with PIM the multicast will be constrained to the vlan its transmitted in.

Here is mcast quick start guide:

For L2 you really just need to be looking at IGMP snooping.  L3, start looking into PIM.

Reza Sharifi Tue, 08/17/2010 - 18:55
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Hi Matt,

Yes, all you need is IGMP snooping enabled on that vlan and no other configuration is required.



Lei Tian Tue, 08/17/2010 - 19:24
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Hi Mathew,

 We plan to implement ipTV. I have a seperate vlan just for that and I am planning to keep the source & hosts in the same vlan. In this case do we need to have IGMP snooping, anyway even in broadcast it goes only to the desired hosts. Could you comment.

If it is in a layer 2 domain, same VLAN across multiple switches, then you need to configure IGMP querier as well. Another option is to disable IGMP snooping; that can make your multicast application works as well, switches will flood the multicast to all ports in that VLAN.

Please check the following link of configuring multicast in same VLAN.


Lei Tian

Chad Peterson Wed, 08/18/2010 - 09:54
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Technically you don't need IGMP snooping.  If you don't use IGMP snooping then all mcast packets will be flooded by your switches to ports in the same vlan.  The packets will be dropped by the end hosts that do not want to receive it.

I would recommend that you do use IGMP snooping to prevent this flooding.  As Lei stated above, you will just need to configure an IGMP querier for the vlan.  This will prevent the multicast from being flooded, and instead it will only be forwarded to hosts that join the multicast group (IPTV stream).


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