D-Link DES-3326S manual Igmp Type Codes, 114

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

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Table 5-14. IGMP Type Codes

Multicast routers use IGMP to manage multicast group memberships:

•An IGMP “report” is sent by a user’s computer to join a group

•IGMP version 1 does not have an explicit ‘leave’ message. Group members have an expiration timer, and if this timer expires before a query response is returned, the member is dropped from the group.

•IGMP version 2 introduces an explicit “leave” report. When a user wants to leave a group, this report is sent to the multicast router (for IGMP version 2).

•Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group members exist on their subnetworks. If there is no response from a particular group, the router assumes that there are no group members on the network, and multicast packets are not forwarded.

The TTL field of query messages is set to 1 so that the queries do not get forwarded to other subnetworks.

IGMP version 2 introduces a few extensions to IGMP version 1 such as, the election of a single multicast querier for each network, explicit ‘leave’ reports, and queries that are specific to a particular multicast group.

The router with the lowest IP address is elected as the querier. The explicit group leave message is added to decrease latency, and routers can ask for membership reports from a particular multicast group ID.

114 Switch Management and Operating Concepts