Some EIGRP-related terms include the following:
■ Neighbor table: EIGRP routers use hello packets to discover neighbors. When a router discovers and forms an adjacency with a new neighbor, it includes the neighbor's address and the interface through which it can be reached in an entry in the neighbor table. This table is comparable to OSPF's neighbor table (adjacency database); it serves the same purpose, which is to ensure bidirectional communication between each of the directly connected neighbors. EIGRP keeps a neighbor table for each supported network protocol.
■ Topology table: When a router dynamically discovers a new neighbor, it sends an update about the routes it knows to its new neighbor and receives the same from the new neighbor. These updates populate the topology table. The topology table contains all destinations advertised by neighboring routers; in other words, each router stores its neighbors' routing tables in its EIGRP topology table. If a neighbor is advertising a destination, it must be using that route to forward packets; this rule must be strictly followed by all distance vector protocols. An EIGRP router maintains a topology table for each network protocol configured.
■ Advertised distance (AD) and feasible distance (FD): EIGRP uses the Diffusing Update Algorithm (DUAL). DUAL uses distance information, known as a metric or cost, to select efficient loop-free paths. The lowest-cost route is calculated by adding the cost between the next-hop router and the destination—referred to as the advertised distance—to the cost between the local router and the next-hop router. The sum of these costs is referred to as the feasible distance.
■ Successor: A successor, also called a current successor, is a neighboring router that has a least-cost path to a destination (the lowest FD) guaranteed not to be part of a routing loop. Successors are offered to the routing table to be used to forward packets. Multiple successors can exist if they have the same FD.
■ Routing table: The routing table holds the best routes to each destination and is used to forward packets. Successor routes are offered to the routing table. The router maintains one routing table for each network protocol.
■ Feasible successor: Along with keeping least-cost paths, DUAL keeps backup paths to each destination. The next-hop router for a backup path is called the feasible successor. To qualify as a feasible successor, a next-hop router must have an AD less than the FD of the current successor route. In other words, a feasible successor is a neighbor that is closer to the destination, but is not in the least-cost path and, therefore, is not used to forward data. Feasible successors are selected at the same time as successors but are kept only in the topology table. The topology table can maintain multiple feasible successors for a destination.
If the route via the successor becomes invalid because of a topology change or if a neighbor changes the metric, DUAL checks for feasible successors to the destination. If a feasible successor is found, DUAL uses it, thereby avoiding a recomputation of the route. If no suitable feasible successor exists, a recomputation must occur to determine the new successor. Although recomputation is not processor-intensive, it affects convergence time, so it is advantageous to avoid unnecessary recomputations.
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