Basic Operation
EIGRP stores data in three tables:
- Neighbor Table: Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.
- Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tables gathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.
- Routing table: Stores the actual routes to all destinations; the routing table is populated from the topology table with every destination network that has its successor and optionally feasible successor identified (if unequal-cost load-balancing is enabled using the variance command). The successors and feasible successors serve as the next hop routers for these destinations.
Unlike most other distance vector protocols, EIGRP does not rely on periodic route dumps in order to maintain its topology table. Routing information is exchanged only upon the establishment of new neighbor adjacencies, after which only changes are sent. Also, it uses route tagging.
Read more about this topic: Enhanced Interior Gateway Routing Protocol
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