ZyXEL Communications P-873HNU-51B, User Manual

Page: 173 / 339

P-873HNU(P)-51B User’s Guide
173
C
H A P T E R
1 3
IGMP
13.1 Overview
Traditionally, IP packets 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 packets to just a group
of hosts on the network.
IGMP (Internet Group Multicast Protocol) is a network-layer protocol used to establish membership
in a multicast group - it is not used to carry user data. See RFC 1112, RFC 2236, and RFC 3376 for
information on IGMP versions 1, 2, and 3 respectively.
13.1.1 What You Can Do in this Chapter
• Use the General screen to configure general IGMP proxy and IGMP packet processing settings
( Section 13.2 on page 175 ).
• Use the IGMP Filter screens to control IGMP access ( Section 13.3 on page 176 ).
• Use the IGMP ACL screens to block or allow access to specific multicast media channels ( Section
13.4 on page 180 ).
13.1.2 What You Need to Know
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).
IGMP Snooping
A layer-2 switch can passively snoop on IGMP Query, Report and Leave (IGMP version 2) packets
transferred between IP multicast routers/switches and IP multicast hosts to learn the IP multicast
group membership. It checks IGMP packets passing through it, picks out the group registration
information, and configures multicasting accordingly. IGMP snooping allows the VDSL Router to
learn multicast groups without you having to manually configure them.
The VDSL Router 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. The
VDSL Router discards multicast traffic destined for multicast groups that it does not know. IGMP
snooping generates no additional network traffic, allowing you to significantly reduce multicast
traffic passing through your device.