D-Link DES-3326SRM Product Manual - Page 214
Internet Group Management Protocol (IGMP), IGMP Versions 1 and 2, Meaning, IGMP Type Codes
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D-Link DES-3326S Layer 3 Switch Internet Group Management Protocol (IGMP) End users that want to receive multicast packets must be able to inform nearby routers that they want to become a multicast group member of the group these packets are being sent to. The Internet Group Management Protocol (IGMP) is used by multicast routers to maintain multicast group membership. IGMP is also used to coordinate between multiple multicast routers that may be present on a network by electing one of the multicast routers as the 'querier'. This router then keep track of the membership of multicast groups that have active members on the network. IGMP is used to determine whether the router should forward multicast packets it receives to the subnetworks it is attached to or not. A multicast router that has received a multicast packet will check to determine if there is at least one member of a multicast group that has requested to receive multicast packets from this source. If there is one member, the packet is forwarded. If there are no members, the packet is dropped. IGMP Versions 1 and 2 Users that want to receive multicast packets need to be able to join and leave multicast groups. This is accomplished using IGMP. The IGMP Type codes are shown below: IGMP Type Codes Type 0x11 0x11 0x16 0x17 0x12 Meaning Membership Query (if Group Address is 0.0.0.0) Specific Group Membership Query (if Group Address is Present) Membership Report (version 2) Leave a Group (version 2) Membership Report (version 1) 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. The transition states a host will go through to join or leave a multicast group are shown in the diagram below. 204