.9 Multicast
4.9.1 IGMP Snooping
y
Computers and network devices that want to receive multicast transmissions need to inform nearby
a multicast group. The Internet Group Management Protocol
ommunicate this information. IGMP is also used to periodically check the multicast
here there is more than one multicast router
eeps track of the membership
embers. The information received from IGMP is then used to
warded to a given sub network or not. The router can check,
member of a multicast group on a given subnet work. If there
or leave at any time. IGMP provides the method for members and
r leaving a multicast group.
nal data.
The format of an IGMP packet is shown below:
hecksum
4
Theor
routers that they will become members of
(IGMP) is used to c
group for members that are no longer active. In the case w
on a sub network, one router is elected as the ‘queried’. This router then k
of the multicast groups that have active m
determine if multicast packets should be for
using IGMP, to see if there is at least one
are no members on a sub network, packets will not be forwarded to that sub network.
IGMP Versions 1 and 2
Multicast groups allow members to join
multicast routers to communicate when joining o
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optio
IGMP Message Format
Octets
0
8 16
31
Type Response
Time
C
Group Address (all zeros if this is a query)
The IGMP Type codes are shown below:
Type Meaning
0x11
Membership Query (if Group Address is 0.0.0.0)
0x16
Membership Report (version 2)
0x17
Leave a Group (version 2)
0x12
Membership Report (version 1)
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their
respective sub networks. The following outlines what is communicated between a multicast router and a
multicast group member using IGMP.
0x11
Specific Group Membership Query (if Group Address is Present)
