H3C Technologies H3C SecPath F1000-E - Manual (Page 37)

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multicast group. In this case, the router will delete the multicast forwarding entries for that multicast group
after a period of time.
Enhancements in IGMPv2
Compared with IGMPv1, IGMPv2 has introduced a querier election mechanism and a 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 its 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 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.
Enhancements in IGMPv3
Built upon and being compatible with IGMPv1 and IGMPv2, IGMPv3 provides hosts with enhanced
control capabilities and provides enhancements of query and report messages.
Enhancements in control capability of hosts
IGMPv3 has introduced source filtering modes (Include and Exclude), so that a host can specify a list of
sources it expect or does not expect multicast data from when it joins a multicast group: