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One improvement that IGMP version 2 has over version 1
concerns multi-access networks, such as ethernet, that have more
than one attached multicast router (see Figure 3-12). Only one
router needs to send Membership Query messages because all attached
routers running IGMP hear the Membership Report messages from the
hosts. IGMP version 2 adds a feature that enables routers to
determine which router is responsible for sending Membership Query
messages with the other routers becoming Non-Querier routers. In
Figure 3-12, assume that router A sends the first Membership Query
message onto the multiaccess network. Router B receives this message
and, because router A has a lower IP address than router B, router A
remains the Querier for the network and router B the Non-Querier. If
router B had sent the first Membership Query message (all routers
start in the role of Querier), this would not suppress Membership
Query messages from router A because router A has the lower IP
address. Router A would send a Membership Query message and router
B, upon receiving this message, would become a Non-Querier for the
network. |
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