IGMP The Internet Group Management Protocol (IGMP) manages the multicast groups on a LAN. IP hosts use
IGMP to report their group membership to directly connected multicast switches. Switches executing a multicast
routing protocol maintain forwarding tables to forward multicast datagrams. Switches use the IGMP to learn whether
members of a group are pres ent on their directly attached sub-nets. Hosts join multicast groups by sending IGMP
report messages.
IGMP uses group addresses, which are Clas s D IP addresses. The high-order four bits of a Class D address are
1110. Therefore, host group addresses can be in the range 224.0.0.0 to 239.255.255.255.
The address 224.0.0.0 will no t be assigned to any group. The address 224.0.0.1 is assigned to all systems on a sub-
net. The address 224.0.0.2 is assig ned to all switches on a sub-net.
Multicast switches elect a designated switch for the LAN (subnet). The designated switch is the one with the highest
IP address. The switch is responsible for sending IGMP host-query messages to all hosts on the LAN. By default, the
designated switch sends IGMP host-query messages every 60 seconds in order to keep the IGMP overhead on hosts
and networks very low. IGMP snooping allows multi ca sts to be prune d to only the ports whose users have
requested the multicast. IGMP Querier should b e ena bled on one device on you local network. That
querier actively determines which port s of which devices reque st multicast se rvice.
Multicast switches send IGMP host-query messag es to discover which multicast groups are present on attached
networks. These messages are sent to the all-systems group address of 224.0.0.1 with a time-to-live (TTL) value
of 1.
Multicast switches continue to periodically send host-query messag es to refresh their kn owledge of memberships
present on their networks. If, after some number of queries, the switch software discovers that no local hosts are
members of a multicast group, the software stops forwarding onto the local network multicast packets from remote
origins for that group and sends a prune message upstream toward the source.
79 Asante IntraCore IC39240/480 User’s Manual