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Management Guide
TigerAccess™ EE
9.1.2
SMC7824M/VSW
IGMP Version 2
In IGMP version 2, the new extensions such as the leave process, election of an IGMP
querier, and membership report suppression are added. New IGMP messages, the leave
group and group-specific query can be used by hosts to explicitly leave groups, resulting
in great reduction of the leave latency.
IGMPv2 Messages
There are three types of IGMPv2 messages of concern to the host-router interaction as
shown below:
Membership query
•
A multicast router determines if any hosts are listening to a group by sending mem-
bership queries. The membership queries have two subtypes.
– General query: This is used to determine if any hosts are listening to any group.
– Group-specific query: This is used to determine if any hosts are listening to a par-
ticular group.
Version 2 membership report
•
This is used by hosts to join a group (unsolicited) or to respond to membership que-
ries (solicited).
•
Leave group
This is used to explicitly leave a group.
IGMPv2 Operation
An IGMP querier is the only router that sends membership query messages for a network
segment. In IGMP version 2, the querier is a router with the lowest IP address on the
subnet. If the router hears no queries during the timeout period, it becomes the querier.
A host joins multicast groups by sending unsolicited membership report messages indi-
cating its wish to receive multicast traffic for those groups (indicating that the host wants
to become a member of the groups).
The querier sends general query messages periodically to discover which multicast
groups have members on the attached networks of the router. The messages are ad-
dressed to the all-hosts multicast group, which has the address of 224.0.0.1 with a time-
to-live (TTL) value of 1. If hosts do not respond to the received query messages for the
maximum response time advertised in the messages, a multicast router discovers that no
local hosts are members of a multicast group, and then stops forwarding multicast traffic
onto the local network from the source for the group.
When hosts respond to membership queries from an IGMP querier, membership reports
from the hosts other than the first one are suppressed to avoid increasing the unneces-
sary traffic. For an IGMP querier, it is sufficient to know that there is at least one inter-
ested member for a group on the network segment.
When a host is not interested in receiving the multicast traffic for a particular group any
more, it can explicitly leave the group by sending leave group messages. Upon receiving
a leave message, a querier then sends out a group-specific query message to determine
if there is still any host interested in receiving the traffic. If there is no reply, the querier
stops forwarding the multicast traffic.
CLI
351
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