EIGRP Convergence Metrics Successors and Feasible Successors

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary routing protocol that uses distance-vector and link-state routing technologies to exchange routing information between routers. EIGRP routers exchange routing information using hello packets, and once neighbors are discovered, they exchange EIGRP topology information.

EIGRP calculates the best path to a destination using a metric called the composite metric. The composite metric is a weighted sum of five different metrics:

Bandwidth: the amount of data that can be transmitted over a link in a given amount of time

Delay: the time it takes for a packet to traverse a link

Reliability: the probability of successful transmission over a link

Load: the current traffic load on a link

MTU (Maximum Transmission Unit): the largest packet size that can be transmitted over a link without fragmentation

EIGRP uses the feasible distance (FD) to determine the best path to a destination. The FD is the metric of the best path from the router to the destination. EIGRP routers also maintain a record of the metric to the destination advertised by each neighbor, which is called the reported distance (RD).

When a router receives information about a destination from a neighbor, it adds the neighbor’s reported distance to its own feasible distance to that destination. If this sum is less than the current FD for that destination, the router updates its routing table and sets the new FD to the sum of its own metric and the neighbor’s metric. The neighbor router becomes the successor for that destination, and its metric is used to calculate the FD. If the sum is greater than or equal to the current FD, the router ignores the update.

If a router receives information about a destination from multiple neighbors, it can use the reported distances to determine whether there are feasible successors to that destination. A feasible successor is a path that meets the feasibility condition, which states that the reported distance to a destination from a neighbor must be less than the current feasible distance for that destination. If there is a feasible successor, it is added to the router’s topology table, and its metric is used to calculate the feasible distance. If there are no feasible successors, the router must perform a query to find a new successor.

EIGRP uses Diffusing Update Algorithm (DUAL) to achieve rapid convergence, which helps minimize the convergence time and reduce the amount of unnecessary routing updates.