Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Multicast Layer 2 (Rev. 03)
IGMP Snooping
© 2008 Foundry Networks, Inc.
Page 3 of 43
IGMP Snooping
Traditionally, IP packets are transmitted in either of two ways - Unicast (one sender to one
recipient) or Broadcast (one 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 can register a device to receive
specific multicast traffic.
A device that supports IGMP Snooping 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 that pass through it,
picks out the group registration information, and configures multicasting accordingly.
Without IGMP Snooping, multicast traffic is forwarded to all ports, the same as broadcast traffic.
With IGMP Snooping, multicast traffic is only forwarded to ports that are members of the specific
multicast group. IGMP Snooping generates no additional network traffic, allowing the user to
significantly reduce multicast traffic passing through the device.
Overview
Multicast Address
Multicast IP addresses range from 224.0.0.0 to 239.255.255.255. They are also referred to as
Group Destination Address (GDA). A MAC address is associated to each GDA. This GDA MAC
address is formed by 01:00:5E:XX:XX:XX, followed by the latest 23 bits of the GDA multicast IP
address in hex.
IGMP Version 1
The IGMP version 1 message is 8 bytes long and contains the following fields (see
Figure 1
):
•
Version
(bits 0 to 3) - should be 1
•
Type
(bits 4 to 7) - there are 2 types of IGMP messages:
1 = Host Membership Query
2 = Host Membership Report.
•
GDA
(bits 32 to 63) - Group Destination Address.
IGMP Version 1 Format
Version
Type
Unused
Checksum
0
3
4
7
8
15
16
31
GDA
32
63
Figure 1: IGMP Version 1 Message Fields
A host membership report is issued by a host that wants to join a specific multicast group (GDA).
When the IGMP multicast router receives the host membership report, it adds the GDA to the
multicast forwarding table and starts forwarding the IGMP traffic to this group. Host membership
queries are issued by the IGMP multicast router at regular intervals to check whether there is still a
host interested in the GDA in that segment. Host membership reports are sent either when the host
wants to receive GDA traffic or in response to a host membership query from the IGMP multicast
router.