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5.3 | ![]() |
EIGRP Components | |
5.3.1 | ![]() |
EIGRP packet types |
Like OSPF, EIGRP relies on several
different kinds of packets to maintain its various tables and
establish complex relationships with neighbor routers.
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The following sections describe these packet types in detail. Hello Packets EIGRP routers send hellos at a fixed
and configurable interval, called the hello interval. The default
hello interval depends on the bandwidth of the interface.
EIGRP hello packets are multicast. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10. An EIGRP router stores information about neighbors in the neighbor table, including the last time that each neighbor responded. The information will only be stored if any of its EIGRP packets, hello or otherwise, is received. If a neighbor is not heard from for the duration of the hold time, EIGRP considers that neighbor down, and DUAL must step in to reevaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired. Recall that OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each other's respective timers through the exchange of hello packets. They use that information to forge a stable relationship, unlike timers. Acknowledgment Packets Hello packets are always sent unreliably and therefore do not require acknowledgment. Update Packets Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors, alerting them about the change. All update packets are sent reliably. Query and Reply Packets If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the active state. At this point, the router multicasts a query to all neighbors, searching for a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no successor information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.
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