CCNP Tutorial For The BSCI Exam:

IGMP Snooping

Chris Bryant, CCIE #12933

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Routers and Layer 3 switches have the capability to make intelligent decisions regarding multicast traffic, enabling them to create multicast trees and avoid unnecessary transmission of multicast streams.  

Layer 2 switches do not. 

One of the first things you learn about Layer 2 switches in your CCNA studies is that they handle multicast traffic in the exact same way they handle broadcasts - by flooding that traffic out every single port except the one the traffic came in on.

That's a very inefficient manner of handling multicasting, so two different methods of helping Layer 2 switches with multicasting have been developed:  IGMP Snooping and CGMP, the Cisco Group Membership Protocol.

So what is IGMP Snooping "snooping" on?  The IGMP reports being sent from a host to a multicast router.  The switch listens to these reports and records the multicast group's MAC address and the switch port upon which the IGMP report was received. 

This allows the switch to learn which ports actually need the multicast traffic, and will send it only to those particular ports instead of flooding the traffic.

IGMP Snooping is not supported by all Cisco switch hardware platforms, but is supported by the 2950 and 3550 families by default, as shown here on a 2950:

SW1#show ip igmp snooping
Global IGMP Snooping configuration:
-----------------------------------
IGMP snooping             : Enabled
IGMPv3 snooping (minimal) : Enabled
Report suppression        : Enabled
TCN solicit query         : Disabled
TCN flood query count     : 2

Vlan 1:
--------
IGMP snooping                       : Enabled
Immediate leave                     : Disabled
Multicast router learning mode      : pim-dvmrp
Source only learning age timer      : 10
CGMP interoperability mode          : IGMP_ONLY

To turn IGMP snooping off -- you guessed it -- run the command no ip igmp snooping .

SW1(config)#no ip igmp snooping

SW1#show ip igmp snooping
Global IGMP Snooping configuration:
-----------------------------------
IGMP snooping             : Disabled
IGMPv3 snooping (minimal) : Enabled
Report suppression        : Enabled
TCN solicit query         : Disabled
TCN flood query count     : 2

Vlan 1:
--------
IGMP snooping                       : Disabled
Immediate leave                     : Disabled
Multicast router learning mode      : pim-dvmrp
Source only learning age timer      : 10

From experience, I can tell you that one deciding factor between IGMP Snooping and CGMP is the switch's processor power.  IGMP Snooping is best suited for high-end switches with CPU to spare. If CPU is an issue, consider using CGMP.

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