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IGMP is the mechanism used by hosts on a network to inform
directly-attached routers which multicast group(s) the host wants to
either join or leave. Multicast routers use IGMP to determine if any
members of the multicast groups are located on any of their attached
networks. If group members are present, multicast routers can then
join a particular multicast group and forward multicast traffic to
hosts that have joined the group(s). The original IGMP specification
is detailed in RFC 1112, “Host Extensions for IP Multicasting.” This
specification is typically referred to as IGMP version 1 and was
written by S. Deering of Stanford University in August 1989. A
subsequent RFC, written by W. Fenner of Xerox PARC, updated the
original IGMP version 1 RFC. The update is RFC 2236, “Internet Group
Management Protocol, Version 2.” Both RFCs will be examined because
a mix of IGMP version 1 and version 2 hosts and routers may be
present in the network, and you need to be aware of interoperability
issues between the versions. Following the discussion of IGMP
version 1 and version 2, we will examine configuring, monitoring,
and debugging IGMP on Cisco routers. |
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