EIGRP

By Alec Gauldin, Principal Network Engineer · Updated July 2026

EIGRP is Cisco's advanced distance-vector interior gateway protocol, running the Diffusing Update Algorithm (DUAL) to guarantee loop-free paths and near-instant convergence. It was Cisco-proprietary for most of its life (a subset was later published as informational RFC 7868). For years EIGRP was a headline CCNA routing topic; the current 200-301 blueprint foregrounds OSPFv2 as its graded interior gateway protocol, so treat exact EIGRP exam weighting as light and subject to blueprint revisions. Even so, EIGRP is foundational Cisco routing knowledge — taught alongside OSPF and central to CCNP ENARSI — and it cements the distance-vector ideas (metrics, neighbors, backup paths) that recur across the exam.

Everything starts with neighbor formation. Routers discover each other with Hello packets multicast to 224.0.0.10, and an adjacency forms only when the autonomous-system (AS) number matches, the K-value metric weights match, and both ends share the same primary subnet (plus authentication, if configured). The AS number in 'router eigrp <asn>' is a domain identifier — not a BGP AS — and every router in the EIGRP domain must use the same number or neighbors never come up. Unlike OSPF, EIGRP does not require matching Hello/hold timers to form a neighbor.

The composite metric decides which path wins. By default EIGRP weights bandwidth and delay (K1 and K3 set to 1; load, reliability, and MTU off), deriving the value from the lowest bandwidth along the path and the cumulative interface delay. Because delay is additive and bandwidth is the bottleneck link, you can predict and tune path selection by adjusting interface 'bandwidth' and 'delay' — the EIGRP Commands Cheat Sheet lists the show commands that read the live metric.

Feasible successors are EIGRP's signature. The best metric to a destination is the feasible distance (FD); the successor is the lowest-FD next hop and is the route installed in the routing table. A feasible successor is a precomputed backup that satisfies the feasibility condition — the neighbor's reported distance is lower than your FD, which proves the backup cannot loop. All of this lives in the topology table (show ip eigrp topology). If a successor fails and a feasible successor exists, DUAL swaps to it instantly with no recomputation; with no feasible successor, the route goes 'active' and EIGRP queries its neighbors.

One classic gotcha ties the pieces together: EIGRP historically auto-summarizes routes at classful network boundaries, which breaks discontiguous subnets, so configurations add 'no auto-summary' to advertise specific prefixes (newer IOS releases default this off, but the exam expects you to set it explicitly). To master EIGRP, work in three passes: understand the map on this page, keep the cheat sheet open while you configure, and follow the 'How to Configure EIGRP on Cisco (Step by Step)' guide on a router — then prove it on the graded labs, where the grader checks real neighbor state, metric, and end-to-end reachability instead of just your typing.