IGMP (Internet Group Management Protocol)

IGMP

IGMP (Internet Group Management Protocol) is a networking protocol it is used with multicast by hosts and the routers. Hosts that want to receive traffic from a certain multicast group with a router using IGMP. There are three versions of IGMP; version 1, 2 and 3.

Enable communication between a router and connected hosts. 

The two most important goals of IGMP are as follows.

1. To inform a local multicast router that a host wants to receive multicast traffic for a specific group.

2. To inform local multicast routers that a host wants to leave a multicast group.

IGMP Versions

IGMP v1

IGMP v2 (by default)

IGMP V3

Internet Group Management Protocol version 1

IGMPv1 uses two specific message structures

Report message

                          Used by the client to join the group

Query messages

                          Used by router to see if the member of the group still exist

                          Always sent to 224.0.0.1 by multicast routers.

Internet Group Management Protocol (contend)

• R1 sends periodic IGMP queries to the 224.0.0.1 address.
• If only `one  member per group per subnet sends the IGMP report message to the router – in this case, H2
• While the others host H1 and H3 suppress theirs.
• IGMP query interval is 60 seconds and 180 delay timer.

 Internet Group Management Protocol version 2

 IGMPv2 brought a number of improvements.

1. leave group message from host to the router.( apart from report and query)
2. tunable timers
3. queries election
4. group specific queries

IGMPv2 leave group 

When a host want to leave the group they will send a IGMP  leave message to 224.0.0.2

Then router will query IGMP query message to 224.0.0.1 (all host multicast address)

As long as at least one client is in this group. Switch will forward IGMP membership report back to first hop router.

Group specific queries

• In version 1 query send to all hosts on 224.0.0.1
• In version 2 query generated to only devices to that specific group

Internet Group Management Protocol version 3

Allows us to do source specific multicast V1/v2 only support group specific multicast (*,g) join any source / specific group )\ V3 support source specific multicast (s,g) join.