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Operation Manual – Multicast

H3C S3600 Series Ethernet Switches-Release 1510 Chapter 6 IGMP Configuration

6-3

router forwards the data to the local subnet so that the receivers on the subnet can

receive the data.

As IGMPv1 does not specifically define a Leave Group message, upon leaving a

multicast group, an IGMPv1 host stops sending reports with the destination address

being the address of that multicast group. If no member of a multicast group exists on

the subnet, the IGMP routers will not receive any report addressed to that multicast

report, so the routers will delete the forwarding entries corresponding to that multicast

group.

6.1.4 Enhancements Provided by IGMPv2

Compared with IGMPv1, IGMPv2 provides the querier election mechanism and Leave

Group mechanism.

I. Querier election mechanism

In IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM)

serves as the querier.

In IGMPv2, an independent querier election mechanism is introduced, The querier

election process is as follows:

1) Initially, every IGMPv2 router assumes itself as the querier and sends IGMP

general queries (with the destination address of 224.0.0.1) to all hosts and routers

on the local subnet.

2) Then, every IGMPv2 router compares the source IP address of the received

message with its own interface address. After comparison, the IGMPv2 router with

the lowest IP address wins the querier election and all other IGMPv2 routers are

non-queriers.

3) All the IGMP routers that have lost the querier election start a timer, namely the

“other querier present interval”. If a router receives an IGMP query from the

querier before the timer expires, it resets its timer; otherwise, it will assume the

querier to have timed out and initiate a new querier election process.

II. “Leave group” mechanism

In IGMPv1, when a host leaves a multicast group, it does not send any notification to

any multicast router. As a result, a multicast router relies on the response timeout to

know that a member has left a group.

In IGMPv2, on the other hand, when a host leaves a multicast group:

1) This host sends a leave message to the all-system group (224.0.0.2) on the local

subnet.

2) Upon receiving the leave message, the querier sends a group-specific query to the

group that the host announced to leave.

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