EIGRP Fundamentals

Why EIGRP matters

• Supports unequal-cost load balancing using the variance feature, unlike most IGPs, allowing efficient use of diverse links.
• Converges rapidly by leveraging the Diffusing Update Algorithm (DUAL) and precomputed loop‑free backups called feasible successors.

Core algorithm: DUAL

• DUAL tracks route states, calculates best paths, and coordinates queries/replies to ensure loop-free convergence during changes.
• If a successor fails and a feasible successor exists, the route switches instantly without running a full diffusion, minimizing downtime.

Key tables

• Neighbor table holds directly connected EIGRP peers discovered via Hellos; topology table stores all learned routes with their metrics and candidates; routing table installs only chosen successors (and additional paths per variance).
• The topology table is DUAL’s workspace, tracking each destination’s successors, feasible successors, and metrics for rapid failover.

Distances and metrics

• Reported/Advertised Distance (RD/AD): the metric a neighbor claims to the destination.
• Feasible Distance (FD): the local total metric to the destination via a given neighbor; the lowest FD determines the successor and is used for variance calculations.

Fast convergence behavior

• With an FS present, convergence is instantaneous upon successor loss; without an FS, DUAL diffuses queries to find a new loop‑free path.
• Triggers for DUAL include successor loss without an FS, topology changes, or received queries that necessitate recomputation.

Blog-ready definitions

• DUAL: Loop-free route computation and rapid reconvergence engine for EIGRP.
• Variance: Multiplier to allow unequal-cost load balancing by admitting additional FS paths whose FD is within the threshold.
• Successor route: The path A → B → X with the lowest cost.
• Successor: Router B (next-hop from A).
• Feasible Distance: Total cost from A to X via B.
• Reported Distance: Cost from B to X (reported by B).
• Feasibility condition: Is the cost from B to X (RD) less than A’s own best total cost to X (FD)?
• Feasible successor: Say Router C reports a path to X with RD < A’s FD—A can keep C as a backup route to X.

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Multiple processes

• A single router can run several EIGRP processes simultaneously; each process is scoped to an AS number and maintains its own neighbor, topology, and routing tables.
• Processes in different ASNs do not share routes by default; they behave as separate routing domains unless redistribution is configured.

Autonomous systems

• An EIGRP autonomous system is a common routing domain for routers that share metrics and exchange updates; do not confuse it with an Internet BGP AS.
• Routers must use the same EIGRP ASN to form adjacencies; the ASN is configured with router eigrp ASN in classic mode or under address-family in named mode.

Route transfer between ASNs

• EIGRP does not automatically move prefixes across different EIGRP ASNs; implement redistribution between processes to leak or share routes intentionally.
• When redistributing, plan summarization and filtering to avoid accidental leaks or suboptimal paths, similar in spirit to controlling leaks in other protocols.

Protocol-dependent modules (PDMs)

• PDMs give EIGRP multiprotocol capability; each PDM handles send/receive, DUAL notifications, and installing routes for its specific protocol.
• Current implementations focus on IPv4 and IPv6 PDMs, while historic PDMs included AppleTalk and IPX.

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EIGRP maintains a dedicated topology table per AS/address family that DUAL uses to compute loop-free successors and feasible successors for rapid convergence.

What it stores

• The table lists every known destination prefix within the EIGRP domain along with next-hop candidates learned from neighbors.
• For each prefix, it records the neighbors that advertised it, their reported/advertised distance, and hop count metadata.
• It also retains per-path metric components used in EIGRP’s composite metric: minimum bandwidth, cumulative delay, and optionally load and reliability.

Why it’s different

• Unlike pure distance-vector protocols, EIGRP’s topology table lets DUAL precompute loop-free alternates, enabling immediate failover when a successor fails.
• Only the best path (successor) is installed in the routing table by default; backups remain in the topology table unless features like FRR preinstall them.

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EIGRP uses its own Reliable Transport Protocol (RTP) over IP protocol number 88 to deliver routing messages, multicasting to 224.0.0.10 (MAC 01:00:5E:00:00:0A) when possible and unicasting when reliability demands it.

Neighbor adjacencies

• Neighbors are discovered with periodic Hello packets and tracked in the adjacency table; full topology exchange occurs only when the adjacency forms, after which only incremental updates are sent.
• Loss of acknowledgments or missed Hellos triggers recovery: EIGRP may shift from multicast to unicast per-neighbor and can reset the adjacency after retry limits.

Packet types

• Hello: used for discovery and liveliness detection; sent unreliably via multicast, no ACK required.
• Update: carries routing and reachability; sent reliably via multicast or unicast depending on scope.
• Query: asks neighbors for alternate paths during convergence; sent reliably, typically multicast.
• Reply: response to a Query; sent reliably via unicast.
• ACK: acknowledgment of reliable packets; always unicast and itself does not require acknowledgment.

Multicast details

• Default IPv4 multicast group for EIGRP control traffic is 224.0.0.10, which maps on Ethernet to MAC 01-00-5E-00-00-0A; only hosts joined to this group process the frames.
• Routers fall back to unicast when a neighbor fails to acknowledge reliable multicasts, preventing unnecessary retransmissions to other neighbors

RTP reliability mechanics

• Every reliable EIGRP packet carries a nonzero sequence number; the receiver must return an ACK echoing that sequence for ordered delivery.
• If ACKs are not received before the retransmission timeout (RTO) derived from SRTT, EIGRP retransmits; after up to 16 retries, the neighbor is reset.

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EIGRP forms a neighbor adjacency before installing routes in the RIB; on receiving Hellos, routers attempt to peer but only succeed when key parameters match.

Must-match parameters

• Metric formula K values must be identical on both routers; any mismatch prevents adjacency and should be changed consistently AS-wide.
• The interfaces’ primary IP subnets must match; if one side uses a different primary network (even with a matching secondary), neighbors do not form.
• The Autonomous System Number (ASN) for the EIGRP process/address-family must be the same on both peers.
• Authentication settings (mode and keys) must match across neighbors if enabled.

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