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16.1 IGMP/MLD Commands
Use these commands to configure IGMP settings and display statistics.
Note: At the time of writing Multicast Listener Discovery (MLD) is for future expansion.
IGMP
• IGMP (Internet Group Management Protocol) - This is a protocol used to establish membership in
a multicast group.
• IP Multicast Addresses - In IPv4, a multicast address allows a device to send packets to a specific
group of hosts (multicast group) in a different sub-network. A multicast IP address represents a
traffic receiving group, not individual receiving devices. IP addresses in the Class D range
(224.0.0.0 to 239.255.255.255) are used for IP multicasting. Certain IP multicast numbers are
reserved by IANA for special purposes (see the IANA web site for more information).
In IPv6, multicast addresses provide the same functionality as IPv4 broadcast addresses.
Broadcasting is not supported in IPv6. A multicast address allows a host to send packets to all
hosts in a multicast group.
• IGMP join/leave reports (A) - An IGMP join report is sent
from a host when it wants to be a member of a multicast
group. When the host doesn't want to be a member of a
multicast group any more, it sends an IGMP leave report.
• IGMP query and report (B) - A router sends an IGMP query
to its downlink switch(es) to ask a multicast group member
list (also called multicast table). Then the switch(es) that
received the IGMP query send the list to the router.
• IGMP snooping - This feature groups multicast traffic (C)
and only forwards a group's traffic to ports that are
members of that group. Without IGMP snooping, a switch
does not understand multicast and will broadcast multicast
traffic to all the ports in a network. IGMP snooping
generates no additional network traffic, allowing you to
significantly reduce multicast traffic passing through your
switch.
• IGMP Proxy - To allow better network performance, you
can use IGMP proxy instead of a multicast routing protocol
in a simple tree network topology. In IGMP proxy, only
necessary IGMP join/leave reports are forwarded to its
uplink router. This can significantly reduce the upstream
multicast traffic to the aggregated device.
Note: The IES does not allow a subscriber port to send multicast traffic (except static
multicast traffic) to an uplink port. Only the uplink port can forward multicast traffic
to the subscriber port(s).
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