NetAtlas Workgroup User’s Guide
179
CHAPTER 17
Multicast Configuration
This chapter shows you how to configure multicast settings and MVR (Multicast VLAN
Registration) groups.
17.1 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 session-layer protocol used to establish
membership in a multicast group - it is not used to carry user data. Refer to RFC 1112 and
RFC 2236 for information on IGMP versions 1 and 2 respectively.
17.1.1 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 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).
17.1.2 IGMP Snooping
A 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
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.
For background information on IGMP filtering, refer to Section 6.3 on page 79.
