Installing and Administering Internet Services

308 Chapter 9
Configuring mrouted
Overview of Multicasting
IGMP to pass multicast routing information to other mrouted routers as
well as to poll the hosts to determine whether the host is still an active
group member.
IGMP uses IP datagrams to carry information and is a TCP/IP standard
that must be present on all systems that participate in IP multicast.
While IGMP defines a standard for communicating information, it does
not define a standard for how the multicast information is propagated
among multicast routers. Consequently, DVMRP enables multicast
routers to efficiently communicate group membership information
among themselves. DVMRP uses IGMP messages to carry routing and
group membership information. DVMRP also defines IGMP message
types that enable hosts to join and leave multicast groups and that allow
multicast routers to query one another for routing information.