Cisco Cram or: How I Learned to Stop Lolling and Love the Book: CCIE 400-101: Layer 2 Technologies - IGMP snooping, IGMP querier, IGMP filter, IGMP proxy

IGMP snooping

IGMP snooping examines IGMP protocol messages within a VLAN to discover which interfaces are connected to devices interested in receiving multicast traffic.
By default, a switch would flood a multicast frame throughout the VLAN, just like it would do for a broadcast or unknown unicast frame. The reason is because a switch would never find a multicast MAC address in its CAM table because it is never used a source address.
The term snooping is used because Layer 3 control plane packets are intercepted and influence Layer 2 forwarding decisions.
When IGMP snooping is enabled, the switch monitors IGMP packets between receivers and multicast routers and uses the content of the packets to build a multicast cache table—a database of multicast groups and the interfaces that are connected to members of the groups.
When the switch receives multicast packets, it uses the cache table to selectively forward the traffic to only the interfaces that are connected to members of the appropriate multicast groups.

Multicast-router interfaces—These interfaces lead toward multicast routers or IGMP queriers.
Group-member interfaces—These interfaces lead toward hosts that are members of multicast groups.

The switch learns about these interfaces by monitoring IGMP traffic.
If an interface receives IGMP queries or PIM updates, the switch adds the interface to its multicast forwarding table as a multicast-router interface.
If an interface receives membership reports for a multicast group, the switch adds the interface to its multicast forwarding table as a group-member interface.
The multicast-router interface or a group-member interface can also be statically configured.
Note: For a switch to learn multicast-router interfaces and group-member interfaces, an IGMP querier must exist in the network. This is often a multicast router, but if there is no multicast router on the local network, the switch itself can be configured to be an IGMP querier.
Multicast traffic received on a switch interface in a VLAN on which IGMP snooping is enabled is forwarded according to specific rules.
IGMP traffic is forwarded as follows:

IGMP general queries received on a multicast-router interface are forwarded to all other interfaces in the VLAN.
IGMP group-specific queries received on a multicast-router interface are forwarded to only those interfaces in the VLAN that are members of the group.
IGMP reports received on a host interface are forwarded to multicast-router interfaces in the same VLAN, but not to the other host interfaces in the VLAN.

A multicast packet with a destination address of 224.0.0.0/24 is flooded to all other interfaces on the VLAN.
An unregistered multicast packet—that is, a packet for a group that has no current members—is forwarded to all multicast-router interfaces in the VLAN.
A registered multicast packet is forwarded only to those host interfaces in the VLAN that are members of the multicast group and to all multicast-router interfaces in the VLAN.

When there is no multicast router in the VLAN to originate the queries, you must configure an IGMP snooping querier to send membership queries.
The presence of at least one mrouter port is absolutely essential for the IGMP snooping operation to work across switches.
You must have the switches somehow learn or know of an mrouter port.
When a network/VLAN does not have a router that can take on the multicast router role and provide the mrouter discovery on the switches, you can turn on the IGMP querier feature.
The feature allows the Layer 2 switch to proxy for a multicast router and send out periodic IGMP queries in that network.
This action causes the switch to consider itself an mrouter port.
The remaining switches in the network simply define their respective mrouter ports as the interface on which they received this IGMP query.

IGMP filtering allows users to configure filters on a switch virtual interface (SVI), a per-port, or a per-port per-VLAN basis to control the propagation of IGMP traffic through the network.
IGMP filtering provides the capability to manage IGMP snooping, which in turn controls the forwarding of multicast traffic.
When an IGMP packet is received, IGMP filtering uses the filters configured by the user to determine whether the IGMP packet should be discarded or allowed to be processed by IGMP snooping.
With a IGMPv1 or IGMPv2 packet, the entire packet is discarded.
With a IGMPv3 packet, the packet is rewritten to remove message elements that were denied by the filters.
Access can be restricted based on the following:

Which multicast groups or channels can be joined on a port. Channels are joined by IGMPv3 hosts that specify both the group and the source of the multicast traffic (ip igmp snooping access-group acl [vlan vlan_id]).
Maximum number of groups or channels allowed on a specific port or interface (regardless of the number of hosts requesting service) (ip igmp snooping limit n [except acl] [vlan vlan_id]).
IGMP protocol versions (for example, disallow all IGMPv1 messages) (ip igmp snooping minimum-version 2 | 3).

In certain topologies, it is not necessary to run a multicast routing protocol (like PIM). It is sufficient for a device to learn and proxy group membership information and simply forward multicast packets based upon that information.
A proxy device performing IGMP forwarding has a single upstream interface and one or more downstream interfaces.
These designations are explicitly configured; there is no protocol to determine what type each interface is.
It performs the router portion of the IGMP on its downstream interfaces, and the host portion of IGMP on its upstream interface.