ZyXEL Communications MES-2110, User Manual

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MES-2110 User’s Guide
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H A P T E R
16
IGMP
16.1 Overview
Traditionally, IP frames are transmitted in one of either two ways - Unicast (1
sender to 1 recipient) or Broadcast (1 sender to everybody on the network).
Multicast delivers IP frames to just a group of hosts on the network.
IGMP (Internet Group Management Protocol) is a network-layer protocol used to
establish membership in a multicast group - it is not used to carry user data. Refer
to RFC 1112, RFC 2236 and RFC 3376 for information on IGMP versions 1, 2 and 3
respectively.
16.1.1 IP Multicast Addresses
In IPv4, a multicast address allows a device to send frames to a specific group of
hosts (multicast group) in a different subnetwork. 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).
16.1.2 IGMP Snooping
A Switch can passively snoop on IGMP frames transferred between IP multicast
routers/switches and IP multicast hosts to learn the IP multicast group
membership. It checks IGMP frames passing through it, picks out the group
registration information, and configures multicasting accordingly. IGMP snooping
allows the Switch to learn multicast groups without you having to manually
configure them.
The Switch forwards multicast traffic destined for multicast groups (that it has
learned from IGMP snooping or that you have manually configured) to ports that
are members of that group. IGMP snooping generates no additional network
traffic, allowing you to significantly reduce multicast traffic passing through your
Switch.