IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic. The feature allows a network switch to listen in on the IGMP conversation between hosts and routers.By listening to these conversations the switch maintains a map of which links need which IP multicast streams.
A switch will, by default, flood multicast traffic to all the ports in a broadcast domain (or the VLAN equivalent). Multicast can cause unnecessary load on host devices by requiring them to process packets they have not solicited. When purposefully exploited this is known as one variation of a denial-of-service attack. IGMP querier [edit]. In order for IGMP, and thus IGMP snooping, to function, a multicast router must exist on the network and generate IGMP queries. The tables created for snooping (holding the member ports for each a multicast group) are associated with the querier. The Internet Group Management Protocol (IGMP) is a communications protocol used by hosts and adjacent routers on IPv4 networks to establish multicast group memberships. IGMP is an integral part of IP multicast.  A Layer 2 switch supporting IGMP Snooping can passively snoop on IGMP Query, Report, and Leave (IGMP version 2) packets transferred between IP Multicast routers/switches and IP Multicast hosts to determine the IP Multicast group membership. Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports on the network. Usually, there are 2 things you need to do: 1. enable IGMP snooping. 2. add a router and enable PIM. or if the switch supports it, enable the switch as a IGMP querier. Note that Windows Server can also act as an IGMP querier. Although its best if the switch does it. Cant say I've seen it on a real cisco switch (doesnt mean its not there), but my linksys by cisco managed switch can do it, even though its undocumented. Oh, i forgot, also watch the multicast address you use.  With Internet Group Management Protocol (IGMP) snooping, Layer 2 devices also can make intelligent multicast forwarding decisions by examining the contents of each frame’s Layer 3 IP header. Consider the example of a heterogeneous Layer 2 and Layer 3 network that does not use IGMP snooping.  While the IGMP protocol provides some limited ability to create or extend multicast-enabled IP internetworks, it is not the equivalent of a multicast routing protocol, such as DVMRP or PIM. Do not use the Windows 2000 IGMP routing protocol to create a multicast-enabled IP internetwork of an arbitrary size or topology. Because there are no multicast routing protocols provided with Windows 2000, the maintenance of entries in the IP multicast forwarding table is a function of IGMP, a component that is added as an IP routing protocol. IGMP snooping allows a switch to only forward multicast traffic to the links that have solicited them. Essentially, IGMP snooping is a layer 2 optimization for the layer 3 IGMP. IGMP snooping takes place internally on switches and is not a protocol feature. IGMP querier [edit]. In order for IGMP, and thus IGMP snooping, to function, a multicast router must exist on the network and generate IGMP queries. The tables created for snooping (holding the member ports for each a multicast group) are associated with the querier. IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic. The feature allows a network switch to listen in on the IGMP conversation between hosts and routers. By listening to these conversations the switch maintains a map of which links need which IP multicast streams. IGMP querier [edit]. In order for IGMP, and thus IGMP snooping, to function, a multicast router must exist on the network and generate IGMP queries. The tables created for snooping (holding the member ports for each a multicast group) are associated with the querier. Note: In order to maintain group membership, the multicast router sends a IGMP query every 60 seconds. This query is intercepted by the switch, and forwarded to all ports on the switch. All hosts that are members of the group answer that query. The second solution is to use CGMP, which is a Cisco proprietary protocol that runs between the multicast router and the switch. CGMP enables the Cisco multicast router to understand IGMP messages sent by hosts, and informs the switch about the information contained in the IGMP packet. IGMP is a network layer (Layer 3) protocol used to establish membership in a Multicast group and can register a router to receive specific Multicast traffic. (Refer to RFC 1112 and RFC 2236 for information on IGMP versions 1 and 2.). Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports on the network.
https://en.wikipedia.org/wiki/IGMP_snooping https://en.wikipedia.org/wiki/IGMP http://www.vsicam.com/_faq/what-is-igmp-querying-and-igmp-snooping-and-why-would-i-need-it-on-my-network/ https://learningnetwork.cisco.com/thread/31364 http://www.dell.com/downloads/global/products/pwcnt/en/app_note_18.pdf https://technet.microsoft.com/en-us/library/cc957920.aspx https://en.wikipedia.org/wiki/IGMP_snooping https://en.wikipedia.org/wiki/IGMP_snooping http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/10559-22.html http://www.vsicam.com/_faq/what-is-igmp-querying-and-igmp-snooping-and-why-would-i-need-it-on-my-network/
what does multicast querier mean