Multicast Features | 715Rate Limit IGMP Join RequestsIf you expect a burst of IGMP Joins, protect the IGMP process from overload by limiting that rate at whichnew groups can be joined using the command ip igmp group-join-limit from INTERFACE mode. Hosts whoseIGMP requests are denied will use the retry mechanism built-in to IGMP so that they’re membership isdelayed rather than permanently denied.View the enable status of this feature using the command show ip igmp interface from EXEC Privilege mode.Prevent a PIM Router from Forming an AdjacencyTo prevent a router from participating in Protocol Independent Multicast (PIM) (for example, to configurestub multicast routing), use the ip pim neighbor-filter command from INTERFACE mode.Prevent a Source from Registering with the RPUse the command ip pim register-filter from CONFIGURATION mode to prevent a source from transmittingto a particular group. This command prevents the PIM source DR from sending register packets to RP forthe specified multicast source and group; if the source DR never sends register packets to the RP, no hostscan ever discover the source and create an SPT to it.In Figure 33-3, Source 1 and Source 2 are both transmitting packets for groups 239.0.0.1 and 239.0.0.2. R3has a PIM register filter that only permits packets destined for group 239.0.0.2. An entry is created forgroup 239.0.0.1 in the routing table, but no outgoing interfaces are listed. R2 has no filter, so it is allowedto forward both groups. As a result, Receiver 1 receives only one transmission, while Receiver 2 receivesduplicate transmissions.
