IGMP

The Internet Group Management Protocol (IGMP) is a layer-2 protocol that runs between hosts and their immediately neighboring multicast routers.

Routers implement IGMP to allow hosts to signal to the network their desire to receive multicast traffic for a specific group. This enables the routers to learn about the presence of group members on their directly attached subnetworks.

This receiver-initiated join process has excellent scaling properties since, as the multicast group increases in size, it becomes ever more likely that a new group member is able to locate a nearby branch of the multicast distribution tree.

IGMP Query-Response Process

IGMP uses a query-response process to allow a multicast router to determine which multicast groups are active on their local subnetworks.

IGMP query messages are addressed to the all-hosts group address (224.0.0.1) and have a Time to Live (TTL) value of 1.

The router periodically multicasts an IGMP membership query to the “all hosts” multicast group, on the local subnetwork.

All hosts that support IGMP are automatically members the all hosts group and accept packets address to the all hosts group.

All hosts receive the membership query and one or more hosts, host 2 in our example, respond by multicasting an IGMP Membership Report to the multicast group, of which the host is a member.

(225.1.1.1) This report tells the router on the subnetwork that a host is interested in receiving multicast traffic for group 225.1.1.1. The host responds within the configured Host Response Interval.

The multicast router promiscuously accept all possible multicast addresses, updating its IGMP multicast group table with each new update.

After a multicast router knows what multicast groups that its leaf subnetworks require, it then use a multicast  routing protocol to communicate with other routers to ensure that the correct multicast group traffic is delivered from the source.

Routers maintain an IGMP multicast group table for each interface.

IGMP Host Membership Reports and Host Membership Queries, are sent with a TTL of 1. The default query interval is 125 seconds. The default response interval is 10 seconds.

Querier Election

In a multi-access network there may be more than one router that is IGMP enabled. Only one

multicast querier (router) can exist for each LAN at a time. So, there needs to be an election to determine which router becomes the IGMP querier.

IGMP v1 does not have an election mechanism and relies on the routing protocol to select a designated router.

IGMP v2 uses a General Query message on start-up. When routers receive the General Query messages they compare the source IP address with their own. The router with the lowest IP address is elected the IGMP querier. General query messages are sent to the all-routers multicast group using address 224.0.0.2.