TP-Link T3700G-28TQ T3700G-28TQ V1 UG - Page 225

IGMPv2 Work Process, Querier-Election Mechanism

Page 225 highlights

(3) After receiving the IGMP query message, the host that is interested in multicast group G1, either Host B or Host C (depending on whose latency timer runs out first) - for example Host B, will firstly multicast IGMP membership report message to G1 to declare it belongs to G1. As all the hosts and routers can receive this membership report message and the IGMP routers (Router A and Router B) already know there is a host interested in G1, Host C will not send its report message for G1 after it receives the report message of Host B. This is called the membership report preventing mechanism and it helps to reduce the traffic in the local network. (4) At the same time, as Host A is interested in G2, it will multicast report message to G2 to declare it belongs to G2. (5) Through the above query-and-response process, the IGMP router learns that there are group members of G1 and G2 in the local network. It will generate the multicast forwarding entries (*, G1) and (*, G2) via the multicast routing protocol, such as PIM, as the basis of the multicast traffic forwarding. The symbol * represents any multicast source. (6) When multicast packets sending to G1 or G2 from the multicast source arrive at the IGMP router via multicast routing, the multicast forwarding entries (*, G1) and (*, G2) in the IGMP router will guide the multicast packets to the local network and the receiver hosts can receive them. IGMPv1 doesn't specially define the leave group message. When a host running IGMPv1 leaves one multicast group, it wouldn't send the report message to this multicast group. If no member exists in the multicast group, the IGMP router will not receive any report message to this multicast group, thus it will delete this multicast group's corresponding multicast forwarding entries after a period of time.  IGMPv2 Work Process IGMPv2 adds the querier-election mechanism and leave-group mechanism based on IGMPv1. 1. Querier-Election Mechanism The querier-election mechanism in IGMPv2 is illustrated as below: (1) Every IGMP router will assume itself as the querier at its initialization, and send IGMP general query message to all the hosts and routers with the multicast address 224.0.0.1 in the local network. (2) After the other IGMPv2 routers in the local network receive this IGMP general query message, it will compare the message's source IP address with its interface address. Through the comparison, the router with the smallest IP address will be elected as the querier and the other routers as the non-querier. (3) All the non-queriers will start up a timer, known as the Other Querier Present Timer. This timer will be reset if the non-querier receives the IGMP query message before the timer runs out; otherwise the former querier will be assumed as invalid and a new querier-election will be initiated. 214

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(3) After receiving the IGMP query message, the host that is interested in multicast group G1,
either Host B or Host C (depending on whose latency timer runs out first) — for example Host
B, will firstly multicast IGMP membership report message to G1 to declare it belongs to G1.
As all the hosts and routers can receive this membership report message and the IGMP
routers (Router A and Router B) already know there is a host interested in G1, Host C will not
send its report message for G1 after it receives the report message of Host B. This is called
the membership report preventing mechanism and it helps to reduce the traffic in the local
network.
(4)
At the same time, as Host A is interested in G2, it will multicast report message to G2 to
declare it belongs to G2.
(5)
Through the above query-and-response process, the IGMP router learns that there are group
members of G1 and G2 in the local network. It will generate the multicast forwarding entries (*,
G1) and (*, G2) via the multicast routing protocol, such as PIM, as the basis of the multicast
traffic forwarding. The symbol * represents any multicast source.
(6) When multicast packets sending to G1 or G2 from the multicast source arrive at the IGMP
router via multicast routing, the multicast forwarding entries (*, G1) and (*, G2) in the IGMP
router will guide the multicast packets to the local network and the receiver hosts can receive
them.
IGMPv1 doesn’t specially define the leave group message. When a host running IGMPv1 leaves
one multicast group, it wouldn’t send the report message to this multicast group. If no member
exists in the multicast group, the IGMP router will not receive any report message to this multicast
group, thus it will delete this multicast group’s corresponding multicast forwarding entries after a
period of time.
IGMPv2 Work Process
IGMPv2 adds the querier-election mechanism and leave-group mechanism based on IGMPv1.
1. Querier-Election Mechanism
The querier-election mechanism in IGMPv2 is illustrated as below:
(1)
Every IGMP router will assume itself as the querier at its initialization, and send IGMP general
query message to all the hosts and routers with the multicast address 224.0.0.1 in the local
network.
(2)
After the other IGMPv2 routers in the local network receive this IGMP general query message,
it will compare the message’s source IP address with its interface address. Through the
comparison, the router with the smallest IP address will be elected as the querier and the
other routers as the non-querier.
(3)
All the non-queriers will start up a timer, known as the Other Querier Present Timer. This timer
will be reset if the non-querier receives the IGMP query message before the timer runs out;
otherwise the former querier will be assumed as invalid and a new querier-election will be
initiated.
214