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16 – IGMP
Multicast traffic on a network
nternet Group Management Protocol (IGMP) is defined in RFC 1112 as the standard for IP
multicasting in the Internet. It is used to establish host memberships in particular multicast
groups on a single network. The mechanisms of the protocol allows a host to inform its
local router, using Host Membership Reports that it wants to receive messages addressed to
a specific multicast group. All hosts conforming to level 2 of the IP multicasting
specification require IGMP.
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MP concepts
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he Magnum 6K family of switches supports IGMP L2 standards as
efined by RFC 1112. IGMP is disabled by default and needs to be
enabled on the Magnum 6K family of switches. IP multicasting is defined
as the transmission of an IP datagram to a "host group", a set of zero or more hosts identified by
a single IP destination address. A multicast datagram is delivered to all members of its destination
host group with the same "best-efforts" reliability as regular unicast IP datagram, i.e. the datagram
is not guaranteed to arrive at all members of the destination group or in the same order relative to
other datagram.
The membership of a host group is dynamic; that is, hosts may join and leave groups at any time.
There is no restriction on the location or number of members in a host group, but membership in
a group may be restricted to only those hosts possessing a private access key. A host may be a
member of more than one group at a time. A host need not be a member of a group to send
datagram to it.
A host group may be permanent or transient. A permanent group has a well-known,
administratively assigned IP address. It is the address and not the membership of the group that is
permanent; at any time a permanent group may have any number of members, even zero. A
transient group on the other hand is assigned an address dynamically when the group is created, at
the request of a host. A transient group ceases to exist, and its address becomes eligible for
reassignment, when its membership drops to zero.
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Most of the concepts are extracted from RFC 1112 and it is recommended that RFC 1112 be read and understood carefully if
IGMP is used or planned for the network.
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