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IGMP
Communication on a LAN (between the end stations and the routers) is managed by the Internet Group
Management Protocol (IGMP). For complete information about IGMP, see RFC 1112, “Host Extensions...”
and RFC 2236, “Internet Group Management Protocol, Version 2” <ftp://ftp.isi.edu/in-notes/ rfc2236.txt>.
A router that supports multicast and IGMP sends periodic messages called “queries” on its LAN interfaces.
These queries inquire if any end stations want to join a multicast group. End stations signal their desire to
join the multicast group by responding with an IGMP “report.” By using a multicast routing protocol, such as
Protocol-Independent Multicast (PIM), routers maintain forwarding tables that they use to forward multicast
datagrams.
Packets delivered to members of the multicast group are identified by a single multicast group address. Any
host, regardless of whether it is a member of a group, can send to a group. However, only the members of a
group receive the message. Membership in an IP multicast group is dynamic; hosts can join and leave at
any time. There is no restriction on the location or number of members in a multicast group. A host can be a
member of more than one multicast group at a time.
IGMP Snooping
A traditional Layer-2 switch is unable to determine which end stations on the LAN are interested in which
multicast groups. To avoid unnecessary flooding, the switch may use IGMP Snooping. That means the
switch listens to IGMP messages to learn which ports want multicast traffic from which multicast groups. The
switch inserts the correct Ethernet multicast address into the forwarding table for the ports where an end
station has joined a multicast group.
4.4.1 Configuring IP Multicast Traffic Management
The Multicast Traffic Management Menu allows the manager to set up group transmission. To access the
Multicast Traffic Management Menu, type c in the Unicast Forwarding Database Configuration Menu. This
accesses a screen similar to the following:
IntraCore 35160-T IP Multicast Traffic Management Menu VID: [01]
IP Multicast Forwarding Database
--------------------------------
IP Multicast Address Count : 0
IGMP : [Disabled]
IGMP Query : [Disabled]
IGMP Proxy Report Forward : [Disabled]
Query Port Info: [+: Query Port -: Non-Query Port]
-------- --------
<Cmd> <Description>
i Toggle IGMP Enable/Disable
v Advanced IGMP Configuration
m IP Multicast Forwarding Database Configuration
r Reset IP Multicast Forwarding Database
d Display Group Addresses
a Display Group Addresses in All VLAN
q Return to previous menu
root>
S)elect VLAN N)ext VLAN P)rev VLAN
Enabling and Disabling IGMP Snooping
To enable or disable IGMP Snooping on the switch, from the Multicast Traffic Management Menu, type i to
toggle the status of IGMP Snooping.