Enhancements In Igmpv2; Enhancements In Igmpv3 - HP A5500 SI Switch Series Configuration Manual

receive any report addressed to that 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.
IGMPv2 introduced an independent querier election mechanism. The querier election process is as
follows:
Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general query
1.
messages—often called "general queries"—to all hosts and routers on the local subnet. The
destination address is 224.0.0.1.
After receiving a general query, every IGMPv2 router compares the source IP address of the query
2.
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.
All the non-queriers start a timer known as "other querier present timer." If a router receives an
3.
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 the host response timeout timer to determine whether a group has
members. This adds to the leave latency.
In IGMPv2, when a host leaves a multicast group, the following steps occur:
This host sends a leave message to all routers on the local subnet. The destination address is
1.
224.0.0.2.
Upon receiving the leave message, the querier sends a configurable number of group-specific
2.
queries to the group that the host is leaving. The destination address field and group address field
of the message are both filled with the address of the multicast group that is being queried.
One of the remaining members—if any on the subnet—of the group being queried should send a
3.
membership report within the maximum response time set in the query messages.
If the querier receives a membership report for the group within the maximum response time, it will
4.
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

IGMPv3 is built on and is compatible with IGMPv1 and IGMPv2. It provides hosts with enhanced control
capabilities and provides enhancements of query and report messages.
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