Routers running EIGRP must become neighbors before exchanging routing information. To dynamically discover neighbors, EIGRP routers use the multicast address of 224.0.0.10. Each EIGRP router stores routing and topology information in three tables:
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Neighbor table – stores information about EIGRP neighbors
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Topology table – stores routing information learned from neighboring routers
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Routing table – stores the best routes
Administrative distance of EIGRP is 90, which is less than both the administrative distance of RIP and the administrative distance of OSPF, so EIGRP routes will be preferred over these routes. EIGRP uses Reliable Transport Protocol (RTP) for sending messages.
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EIGRP calculates its metric by using bandwidth, delay, reliability and load. By default, only bandwidth and delay are used when calulating metric, while reliability and load are set to zero.
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EIGPR uses the concept of autonomous systems. An autonomous system is a set of EIGRP enabled routers that should become EIGRP neighbors. Each router inside an autonomous system must have the same autonomous system number configured, otherwise routers will not become neighbors.
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EIGRP Neighbors
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EIGRP must establish neighbor relationships with other EIGRP neighboring routers before exchanging routing information. To establish a neighbor relationships, routers send hello packets every couple of seconds. Hello packets are sent to the multicast address of 224.0.0.10.
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NOTE
On LAN interfaces hellos are sent every 5 seconds. On WAN interfaces every 60 seconds.
The following fields in a hello packet must be the identical in order for routers to become neighbors:
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ASN (autonomous system number)
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subnet number
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K values (components of metric)
Routers send hello packets every couple of seconds to ensure that the neighbor relationship is still active. By default, routers considers the neighbor to be down after a hold-down timer has expired. Hold-down timer is, by default, three times the hello interval. On LAN network the hold-down timer is 15 seconds.
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Feasible and reported distance
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Two terms that you will often encounter when working with EIGRP are feasible and reported distance. Let’s clarify these terms:
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Feasible distance (FD) – the metric of the best route to reach destination network. That route will be listed in the routing table.
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Reported or Advertised distance (RD) – the metric advertised by a neighboring router for a specific route. It other words, it is the metric of the route used by the neighboring router to reach the network.
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Successor Path
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A successor is the best path to reach destination in the topology table
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Feasible Successor
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a feasible successor is the second best path to reach a destination after successor. It acts as a backup for the successor.