EIGRP

EIGRP, or Enhanced Interior Gateway Routing Protocol, is a Cisco-developed interior gateway routing protocol designed to provide fast convergence and loop-free routing within an autonomous system. It originated as an improvement to IGRP and blends features of distance-vector and link-state approaches to reduce routing updates while maintaining quick response to topology changes. EIGRP is used for IPv4 networks and, in modern implementations, also supports IPv6 through separate processes.

EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate routes and maintains three sets of routes in its topology: successors (the best routes), feasible successors (backup routes meeting a feasibility condition), and the reported distance to destinations. Neighbors establish adjacencies by exchanging Hello messages and share routing information via updates transmitted reliably to these neighbors. The routing metric is a composite based on bandwidth and delay by default and can include reliability, load, and MTU through adjustable K-values.

Key features include fast convergence and loop-free routing, support for unequal-cost load balancing via the variance option, and route summarization capabilities (automatic at classful boundaries by default, with manual summarization available). EIGRP also supports route filtering, authentication (such as MD5), and various network management features to control route redistribution and filtering.

Administrative distance and deployment details: internal EIGRP routes have an administrative distance of 90, with external routes at 170, influencing preference in mixed environments. EIGRP operates for both IPv4 and IPv6 via separate implementations, and remains a Cisco-proprietary protocol, widely deployed in Cisco networks for enterprise and service-provider routing.