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IGMP snooping

                   what is use of this command.. What happened if enabled or disabled?

It will support in switches or routers?

Higher end switch only or all L2 switches this command will support ?

Some more deep explanation required about this command with eg... Because i am not used this command any where? but recently seeing this command configured in switches for Vlan( No igmp snooping)


Re: IGMP snooping

Hi Dinesh,

it is designed to be used on switches. This feature is not restricted to high-end switches only and I believe that most of L2 switches have this feature.

Internet Group Management Protocol (IGMP) snooping streamlines multicast traffic handling for VLANs. By examining  (snooping) IGMP membership report messages from interested hosts, multicast traffic is limited to the subset of VLAN interfaces on which the hosts reside.

And that is its main purpose - to limit multicast traffic to be sent only to those hosts which want that specific multicast. Otherwise (if igmp snooping is disabled), it would be flooded everywhere! Now, you know that multicast traffic can be bandwidth consuming so it is really a good thing to have igmp snooping enabled.

I just highlited some facts from this document, that could help you

Internet Group Management  Protocol (IGMP) snooping constrains the flooding of multicast traffic by  dynamically configuring the interfaces so that multicast traffic is  forwarded only to those interfaces associated with IP multicast devices.  The LAN switch snoops on the IGMP traffic between the host and the  router and keeps track of multicast groups and member ports. When the  switch receives an IGMP join report from a host for a particular  multicast group, the switch adds the host port number to the associated  multicast forwarding table entry. When it receives an IGMP Leave Group  message from a host, it removes the host port from the table entry.  After it relays the IGMP queries from the multicast router, it deletes  entries periodically if it does not receive any IGMP membership reports  from the multicast clients.

When  IGMP snooping is enabled, the multicast router sends out periodic IGMP  general queries to all VLANs. The switch responds to the router queries  with only one join request per MAC multicast group, and the switch  creates one entry per VLAN in the Layer 2 forwarding table for each MAC  group from which it receives an IGMP join request. All hosts interested  in this multicast traffic send join requests and are added to the  forwarding table entry.

Layer 2 multicast groups learned through IGMP snooping are dynamic

By default, IGMP snooping is globally enabled on  the switch. When globally enabled or disabled, it is also enabled or  disabled in all existing VLAN interfaces. By default, IGMP snooping is  enabled on all VLANs, but it can be enabled and disabled on a per-VLAN  basis.

Best regards,


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