H3C Technologies H3C S7500E Series Switches - Manual (Page 111)

5-4
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.
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:
z If it expects multicast data from specific sources like S1, S2, …, it sends a report with the
Filter-Mode denoted as “Include Sources (S1, S2, …).
z If it does not expect multicast data from specific sources like S1, S2, …, it sends a report
with the Filter-Mode denoted as “Exclude Sources (S1, S2, …).
As shown in Figure 5-2 , the network comprises two multicast sources, Source 1 (S1) and
Source 2 (S2), both of which can send multicast data to multicast group G. Host B is interested
only in the multicast data that Source 1 sends to G but not in the data from Source 2.