IGMP-in English

IGMP --Internet Group Management Protocol

It is a layer 4 protocol. It is used to manage IP groups and apply in LAN. Three types of datagram were defined including IGMP Report, IGMP Leave, IGMP Query.

IGMP Report----If a host wants to join a group  it will send IGMP report to the router and then the host will become the member of the group.

I want to know if the report packet include how long the host will be the member of group.

After some time(Query interval) the router send IGMP General Query packets to all groups.  If the member wants to stay in the group it will reply the IGMP report packets to the router. On the other hand if the router doesn't receive the IGMP report packets from a member,  the host will bedeleted in the group.

If the host wants to leave the group which it has joined, it will send IGMP leave packets to the router.  Then the router will send IGMP Group-specific Query packets to check if there is any other member in the group.

IGMP Snooping

If the switch works in the IGMP Snooping mode,  it will just transmit the IGMP packets transparently. The port which receives the IGMP Report packets will be added in the group.  After that only the ports added in the group can receive the IGMP packets.   In this way the packets will not flood in other ports.

IGMP Proxy

If the swich works in this mode, it will replace the source MAC of IGMP Reports from UNI ports by the MAC of switch.  And then if the switch receive IGMP leave packets,  the switch will decide if send leave packets to the upper router according to the fact that there is any other member in the group.

 

RFC 2236

Routers periodically [Query Interval] send a General Query on each attached network for which this router is the Querier, to solicit membership information.

 

What is the use of IGMP protocol?

Internet Group Management Protocol: - It allows internet hosts to participate in multicasting. The IGMP messages are used to learn which hosts is part of which multicast groups. The mechanism also allow a host to inform its local router, that it wants to receive messages. 

Internet Group Management Protocol, allows internet hosts to multicast. i.e. to send messages to a group of computers. There may be a group of internet hosts interested to multicast. IGMP allows router to determine which host groups have members on a given network segment. It helps to establish group memberships. It is commonly used for streamlining videos and gaming. The protocol can be implemented both as a host side and router side. The host side is responsible to notify its membership in a group. The notification is made to a local router. This local router (router side) in turn sends out queries.

 

IGMP snooping

By default, L2 devices treat IP multicast traffic in the same manner as broadcast traffic-namely, by forwarding frames received on one interface to all other interfaces. This may create excessive traffic on the network.

L3 devices have less rampant broadcast and multicast traffic because they are able to segment networks and forward traffic only to actual destination interfaces.

With IGMP Snooping, L2 devices also can make intelligent multicast forwarding decisions by examining the contents of each frame's Layer 3 IP header.