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A better name for a router query message could be a neighbor
inform message. When a neighbor receives a query message, the IP
address of the neighbor is recorded. No explicit mechanism
acknowledges that the query was received. Instead, the receiving
router will simply transmit its own query message that has the
effect of informing other PIM-DM routers on the network of its
existence. When a query message is received from a neighbor, the
interface is added to the outgoing interface list. The outgoing
interface list is used to determine which interfaces the PIM-DM
router should forward multicast traffic. Of course, if there are no
other PIM-DM routers on the network, the interface would be added to
the outgoing list if there are receivers requesting traffic for a
particular multicast group. For a multi-access network, such as an
ethernet, the query message is sent to the All-Routers multicast
address, 224.0.0.2, and serves as the Designated Router (DR) election mechanism. For dense
mode PIM, the designated router only has a function if IGMP version
1 is being used. In this case, the DR becomes the IGMP querier for
the network (see Chapter 3). The elected DR is the PIM-DM
enabled router with the highest IP address. The query process and DR
election is shown in Figure 6-8. For this scenario, router C is
elected DR since it has the highest IP address on the multi-access
network. |
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