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DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch CLI Manual
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INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)
COMMANDS
IGMP or Internet Group Management Protocol is a protocol implemented by systems utilizing IPv4 to collect the
membership information needed by the multicast routing protocol through various query messages sent out from the
router or switch. Computers and network devices that want to receive multicast transmissions need to inform nearby
routers that they will become members of a multicast group. The Internet Group Management Protocol (IGMP) is used to
communicate this information. IGMP is also used to periodically check the multicast group for members that are no longer
active.
In the case where there is more than one multicast router on a subnetwork, one router is elected as the ‘querier’. This
router then keeps track of the membership of the multicast groups that have active members. The information received
from IGMP is then used to determine if multicast packets should be forwarded to a given subnetwork or not. The router
can check, using IGMP, to see if there is at least one member of a multicast group on a given subnetwork. If there are no
members on a subnetwork, packets will not be forwarded to that subnetwork.
The current release of the Switch now implements IGMPv3. Improvements of IGMPv3 over version 2 include:
• The introduction of the SSM or Source Specific Multicast. In previous versions of IGMP, the host would receive
all packets sent to the multicast group. Now, a host will receive packets only from a specific source or sources.
This is done through the implementation of include and exclude filters used to accept or deny traffic from these
specific sources.
• In IGMPv2, Membership reports could contain only one multicast group whereas in v3, these reports can contain
multiple multicast groups.
• Leaving a multicast group could only be accomplished using a specific leave message in v2. In v3, leaving a
multicast group is done through a Membership report which includes a block message in the group report packet.
• For version 2, the host could respond to either a group query but in version 3, the host is now capable to answer
queries specific to the group and the source.
IGMPv3 is backwards compatible with other versions of IGMP and all IGMP protocols must be used in conjunction with
PIM or DVMRP for optimal use.
The Internet Group Management Protocol (IGMP) is used by IPv4 systems (hosts and routers) to report their IP multicast
group memberships to any neighboring multicast routers.
Note that an IP multicast router can simultaneously be a member of one or more multicast groups, in which case it
performs dual functions as both a multicast router (the “multicast router part” of the protocol, namely to collect the
membership information needed by its multicast routing protocol); and as a group member (the “group member part” of
the protocol, that is to inform itself and other neighboring multicast routers of its memberships).