440 CHAPTER 18: IP M ULTICAST R OUTINGConfiguring IGMPOptionsYou can enable or disable IGMP snooping and querying functions, set theinterface time-to-live (TTL) threshold, and obtain summary and detaildisplays of IGMP-related information.Querying andSnooping ModesYour Multilayer Switching Module divides IGMP functions into twomodes:n Querying — Allows an IP multicast routing interface to function asthe querier if so elected.n Snooping — Allows the module to forward multicast packets only tothe appropriate ports within its routing or bridging interfaces.ImportantConsiderationsn Both modes are enabled as the factory default. These settings apply tothe entire module. You cannot enable or disable snooping or queryingon specific interfaces.n 3Com recommends that you keep both modes enabled at all times.They add little processing overhead to the module.Configuring DVMRPInterfacesDVMRP is the protocol used to develop source-rooted spanning treesbetween routers in the network. You can enable or disable DVMRP onindividual routing interfaces.ImportantConsiderationsn The default setting for DVMRP on each new interface is disabled.n If DVMRP is disabled, the interface cannot participate in forming IPmulticast spanning trees. If the Internet Group Management Protocol(IGMP) is enabled, the module can still forward IP multicast traffic.n Enabling DVMRP causes the module to assign default values of 1 forboth the TTL threshold and metric on the interface. You can modifythese values at any time.n A TTL threshold value of 1 means the interface forwards all IPmulticast packets except those which have expired (packet TTL is 0).Before you change the TTL threshold value, consider the relativelocation of the module in the network and your networkingobjectives.
