Routing & Switching

EIGRP

Enhanced Interior Gateway Routing Protocol — Cisco's advanced distance-vector protocol, now an open standard. Four chapters cover the neighbour machinery, the composite metric, the DUAL algorithm that makes EIGRP loop-free by design, and the summarisation and stub features that make it scale.

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4 chapters

read in order, or dip in

updated 31 Aug 2026
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updated 31 Aug 2026

Contents

  1. 01EIGRP Fundamentals: Neighbors, Tables and Packet TypesEIGRP is an advanced distance-vector routing protocol (RFC 7868) that forms explicit neighbour adjacencies over multicast 224.0.0.10, exchanges routes once, and afterwards sends only partial updates when something changes. It maintains three tables — neighbour, topology and routing — and uses five packet types: hello, update, query, reply and acknowledgement.
  2. 02EIGRP Metric Calculation ExplainedBy default EIGRP computes its metric from just two of five possible components: the minimum bandwidth along the path and the cumulative delay, combined as 256 × (10⁷ / min-bandwidth-kbps + total-delay-in-tens-of-µs). K-values enable or weight the components and must match between neighbours, or no adjacency forms.
  3. 03DUAL: Successors, Feasible Successors and the Query ProcessDUAL, the Diffusing Update Algorithm, is how EIGRP guarantees loop-free paths: a neighbour may be used as a backup (feasible successor) only if its own distance to the destination is strictly less than the current best path's total metric — the feasibility condition. Routes with a feasible successor fail over in milliseconds; routes without one go active and the router queries its neighbours.
  4. 04EIGRP Summarization, Stub Routing and AuthenticationEIGRP scales through manual route summarisation at any interface, stub routing that shields spoke routers from the query process, and per-interface authentication. Summaries bound query scope and shrink tables; stub routers advertise only connected and summary routes and are never queried — the standard design for hub-and-spoke WANs.

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