Technical data

www.westermo.com Theoretical and general applications 135
IGMP/IGMP snooping
Internet Group Management Protocol (IGMP) is a protocol used by IP hosts to report
membership in Multicast groups to the closest multicast routers. Multicast routers
periodically send out a “Host Membership Query message” to remain updated about
group membership for the local network. The hosts on the local network then answer
with a Report-datagram. The hosts only respond to the request for the groups they
belong to. When nothing is reported for a specific group after a certain amount of
requests the router presupposes that no group members still remain on the local net-
work. Subsequently, no more datagrams are forwarded for this group from other net-
works to the local network.
Generally layer 2 switches support IP multicast traffic in the same way as a broad-
cast, i.e. by distributing data to all ports. This can result a large load and reduce net-
work performance. Using IGMP Snooping, a switch can filter traffic and in this way
reduce unwanted traffic. This takes place through the switch listening to the IGMP
conversation between the host and router. The switch identifies whether a host
becomes a member of a group or ends its membership and by that knows which
devices are included in a multicast group. At the present time there are three levels of
IGMP defined:
… IGMPv1 (REF 1112) the original version of IGMP, this includes how a host
requests membership in a group. On the other hand, in v1 there is no method
to terminate membership, thus a router must use a timer to terminate
membership.
… IGMPv2 (REF2236), this version includes membership termination.
… IGMPv3 (REF3376), general revision of IGMP.
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