220 CHAPTER 11: IP M ULTICAST FILTERING WITH IGMPOperating asthe QuerierFor a Layer 2 Switching Module to offer itself as a potential IGMP querierfor its subnetwork or broadcast domains (VLANs), you must:1 Enable the IGMP snooping option.The module cannot send queries if snooping is disabled.2 Enable the IGMP querying option.3 Configure an IP address that the module can insert as the source addressof query packets.Use the bridge multicast igmp queryIpAddress command.As the querier, the module sends general queries every 125 seconds andgroup-specific queries when prompted by host leave-group messages.Locating MulticastRoutersA switching module must be able to identify which of its ports areconnected to multicast routers so that it can forward appropriate IPmulticast traffic to them. For example:n If the module is operating as the querier, any upstream router (therouter that leads back toward an IP multicast source) also needs to seeIGMP reports so it can decide whether to begin, stop, or continueforwarding group traffic on the subnetwork that includes the switch.n Downstream routers need to receive all IP multicast traffic becauseother group members may be attached to them.A switching module records which of its ports lead to IP multicast routersby snooping on protocol advertisements that are sent by the routers. Themodule can recognize the advertisements of the following multicastrouting protocols:n Distance Vector Multicast Routing Protocol (DVMRP)n Multicast Open Shortest Path First (MOSPF)n Protocol Independent Multicast (PIM) version 1n Protocol Independent Multicast (PIM) version 2The module uses a 100-second interval to age the records it keeps formulticast router locations. Protocol advertisements are sent much morefrequently than this.
