ZyXEL Communications MSC1000G Series, User Manual

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Management Switch Card User’s Guide
185
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H A P T E R
8
Multicast Screens
This chapter describes the Multicast screens.
8.1 IGMP Introduction
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 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 RFCs 1112, 2236 and
3376 for information on IGMP versions 1, 2 and 3 respectively.
8.1.1 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.
8.1.2 IGMP Snooping
A layer-2 switch can passively snoop on IGMP Query, Report and Leave (IGMP versions 2 and 3)
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 system
to learn multicast groups without you having to manually configure them.
The system 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
system 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.
8.1.3 IGMP Proxy
To allow better network performance, you can use IGMP proxy instead of a multicast routing
protocol in a simple tree network topology.