IGMP Snooping

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Feb 11th, 2010
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Hi all,

I think I can understand IGMP snooping when there is a router on the LAN. But what about a group of hosts simply connected to a layer-2 switch (capable of doing IGMP snooping)? If there is no router connected, are IGMP messages sent? To who? If they are not, how can multicast traffic be limited to a subset of switch ports?

Thanks in advance for helping.


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Reza Sharifi Thu, 02/11/2010 - 10:30
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Hello Gianluca,

IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined. It provides switches with a mechanism to prune multicast traffic from links that do not contain a multicast listener (IGMP client)



Giuseppe Larosa Thu, 02/11/2010 - 11:22
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Hello Gianluca,

a good question that has two possible solutions:

a) to disable IGMP snooping on the vlan where these clients are connected

b) to configure one L2 LAN switch as the IGMP snooping querier to help IGMP snooping to work.

  this should be supported on C6500 and C4500 I'm not sure it can be used on lower end switches and has to be configured on a single device, no election of a querier can happen. the configured device will send out IGMP queries allowing all L2 switches to update their IGMP snooping tables with receiver answers.

The device will not act as full L3 multicast router (no PIM process) and would allow to use IGMP snooping and to confine multicast within a single vlan.



Hope to help



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