Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Multicast Layer 2 (Rev. 03)
IGMP Snooping
© 2008 Foundry Networks, Inc.
Page 4 of 43
IGMP version 1 does not have a Leave mechanism. When a host does not want to receive the
IGMP traffic any more, it just quits silently. IGMP multicast routers periodically send host
membership query messages (hereinafter called queries) to discover which host groups have
members on their attached local networks. If no reports are received for a particular group after a
certain number of queries, the routers assume that that group has no local members and that they
need not forward remotely-originated multicasts for that group onto the local network.
The host membership report messages are transmitted with the following datagram:
•
Layer 2 information:
o
Source MAC address: MAC address of the host
o
Destination MAC address MAC address for the GDA (01:00:5E:XX:XX:XX)
•
Layer 3 information:
o
Source IP address: IP address of the host
o
Destination IP address: GDA (from 224.0.0.0 to 239.255.255.255).
IGMP Version 2
The IGMP version 2 message fields as shown in
Figure 2
are as follows:
•
Type
(bits 0 to 7) - there are 3 types of IGMP messages. 0x11 = Membership Query, 0x16 =
Version 2 Membership Report, 0x17 = Leave Group. There is an additional type of message,
for backwards-compatibility with IGMPv1. 0x12 = Version 1 Membership Report.
•
Maximal Response Time (MRT)
(bits 8 to 15) - this field is meaningful only in membership
query messages, and specifies the maximum allowed time before sending a responding report
in units of 1/10 second. In all other messages, it is set to zero by the sender and ignored by
receivers.
•
GDA
(bits 32 to 63) - Group Destination Address
IGMP Version 2 Format
Type
MRT
Checksum
0
7
8
15
16
31
GDA
32
63
Figure 2: IGMP Version 2 Message Fields
Report group message is a membership report, which is issued by a host that wants to join a
specific Multicast Group (GDA). When the IGMP multicast router receives the membership report,
it adds the GDA to the multicast forwarding table and starts forwarding the IGMP traffic to this
group. 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 responds to a membership query from
IGMP multicast router.
If a host does not want to receive the IGMP traffic any more, it sends a Leave Group message.
When the IGMP multicast router receives this Leave Group message, it removes the GDA from the
multicast routing table. In addition, IGMP multicast routers periodically send host membership
query messages (hereafter called queries) to discover which host groups have members on their
attached local networks. If no reports are received for a particular group after a certain number of
queries, the routers assume that that group has no local members and that they need not forward
remotely-originated multicasts for that group onto the local network.