Multicast; Igmp Snooping - Planet WGSW-2840 User Manual

4.9 Multicast

4.9.1 IGMP Snooping

Theory
Computers and network devices that want to receive multicast transmissions need to inform nearby
routers that they will become members of a multicast group. The Internet Group Management Protocol
(IGMP) is used to communicate this information. IGMP is also used to periodically check the multicast
group for members that are no longer active. In the case where there is more than one multicast router
on a sub network, one router is elected as the 'queried'. This router then keeps track of the membership
of the multicast groups that have active members. The information received from IGMP is then used to
determine if multicast packets should be forwarded to a given sub network or not. The router can check,
using IGMP, to see if there is at least one member of a multicast group on a given subnet work. If there
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 or leave at any time. IGMP provides the method for members and
multicast routers to communicate when joining or leaving a multicast group.
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optional data.
The format of an IGMP packet is shown below:
IGMP Message Format
Octets
0
Type
The IGMP Type codes are shown below:
Type Meaning
Membership Query (if Group Address is 0.0.0.0)
0x11
Specific Group Membership Query (if Group Address is Present)
0x11
Membership Report (version 2)
0x16
Leave a Group (version 2)
0x17
Membership Report (version 1)
0x12
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.
8 16
Response Time
Group Address (all zeros if this is a query)
Checksum
31