A thing that has bugging me for some time is why the above command is not on by default on Cisco 3750 switches when IP IGMP Snooping is
surely it is best to only have the ports that have joined the Multicast group to receive the traffic.
anybody knows why?
Cisco develop their equipment to suit millions of customers world wide. Those customers do a multitude of different things with that equipment, and very rarely will you find two networks that operate in exactly the same way.
Because of this it would be silly of Cisco to turn on bunches of features by default and expect the customer to turn them off. It is much better (and smarter) to give the customer a blank canvas, and the customer can then turn the features they require on to suit their environment.
Using the blank canvas analogy, the 3750 by definition is an Ethernet switch. Yes it has layer 3 capabilities, yes it has advanced multicast capabilities, so if you require these then turn them on. Much easier than shipping with a bunch of defaults that need to be turned off to get it back to a basic Ethernet switch.
The same applies to your multicast question - you paint whatever picture you want on your canvas. You turn IGMP snooping on but find it is not quite right, so you turn igmp snooping querier on. Other peoples canvas might only warrant IGMP snooping.
Just my personal opinion of course
I see your point, but I can't see a situation where you would want snooping without the querier on also, so if one is default why not the other. If L3 multicast routing is on it overrides the command, but does not cause any problems.
lets see if I get any other responses
Afaik The deal is that IGMP snooping and snooping querier add to the CPU processing. Without those switch just consider multicast as L2 broadcast and send to all ports in HW. With those features on - CPU processing is added and it is up to customer to decide if they still want to keep CPU lower with some unnecessary L2 broadcast in HW (multicast sent to host which didn't expect it) or they want CPU to drive mcast, going up, but limit the scope of outgoing ports.
That is quite serious question to consider - thus feature is disabled by default.
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It's off, by default, because a multicast enabled router should be querying for IGMP membership. The purpose of this command it to allow IGMP snooping to work when there isn't such a router, which would imply multicast being used just at L2.
See http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008059a9df.shtml for more of an explanation.