Planet WGSW-2840 [75/104] Multicast

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 8 16 31

Type Response Time

Checksum

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)

0x11 Specific Group Membership Query (if Group Address is Present)

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.

A host sends an IGMP “report” to join a group