3Com 4800G Series, Configuration Manual

Page: 615 / 1246

IGMP Overview 615
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 group, so the routers will delete the multicast forwarding entries
corresponding to that multicast group after a period of time.
Enhancements Provided
by IGMPv2
Compared with IGMPv1, IGMPv2 provides the querier election mechanism and
Leave Group mechanism.
Querier election mechanism
In IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM)
serves as the querier among multiple routers on the same subnet.
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
query messages (often referred to as general queries) to all hosts and routers on
the local subnet (the destination address is 224.0.0.1).
2 Upon hearing a general query, every IGMPv2 router compares the source IP
address of the query message with its own interface address. After comparison,
the router with the lowest IP address wins the querier election and all other
IGMPv2 routers become non-queriers.
3 All the non-queriers start a timer, known as “other querier present timer”. If a
router receives an IGMP query from the querier before the timer expires, it resets
this timer; otherwise, it assumes the querier to have timed out and initiates a new
querier election process.
Leave group” mechanism
In IGMPv1, when a host leaves a multicast group, it does not send any notification
to the multicast router. The multicast router relies on host response timeout to
know whether a group no longer has members. This adds to the leave latency.
In IGMPv2, on the other hand, when a host leaves a multicast group:
1 This host sends a Leave Group message (often referred to as leave message) to all
routers (the destination address is 224.0.0.2) on the local subnet.
2 Upon receiving the leave message, the querier sends a configurable number of
group-specific queries to the group being left. The destination address field and
group address field of the message are both filled with the address of the
multicast group being queried.
3 One of the remaining members, if any on the subnet, of the group being queried
should send a membership report within the maximum response time set in the
query messages.
4 If the querier receives a membership report for the group within the maximum
response time, it will maintain the memberships of the group; otherwise, the
querier will assume that no hosts on the subnet are still interested in multicast
traffic to that group and will stop maintaining the memberships of the group.