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  • What is EIGRP (Enhanced Interior Gateway Routing Protocol)?

Security glossary

What is EIGRP (Enhanced Interior Gateway Routing Protocol)?

EIGRP (Enhanced Interior Gateway Routing Protocol) is a network routing protocol that lets routers within a single autonomous system exchange routes and select loop-free paths automatically. It uses the Diffusing Update Algorithm and a composite metric, and its basic functionality is published as RFC 7868.

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EIGRP (Enhanced Interior Gateway Routing Protocol) is a distance-vector routing protocol that enables routers within an autonomous system to automatically discover neighbors, exchange routes, and select loop-free paths using the Diffusing Update Algorithm (DUAL) and a composite metric based on bandwidth and delay. The protocol operates at Layer 3, uses IP protocol number 88, and is documented in RFC 7868, offering faster convergence and event-driven partial updates that reduce overhead compared to periodic full table dumps on large networks. Operational challenges typically stem from configuration issues such as mismatched autonomous system numbers, inconsistent metric settings, or summarization errors, while security requires neighbor authentication and careful documentation to prevent rogue devices from injecting false routes and disrupting traffic.

  • EIGRP uses the Diffusing Update Algorithm (DUAL) to compute loop-free paths and maintains both a successor route and a feasible successor backup route, enabling immediate failover without querying neighbors when the primary path fails.
  • The protocol supports unequal-cost load-balancing and route summarization across large networks, capabilities that most other interior routing protocols do not provide by default.
  • Common operational problems include neighbors failing to form due to mismatched autonomous system numbers or metric K values, routes becoming stuck in active state when DUAL queries time out, and flapping adjacencies caused by packet loss or mismatched MTU settings.

EIGRP at a glance

AttributeValue
Full nameEnhanced Interior Gateway Routing Protocol
CategoryNetwork routing protocol
Defined inRFC 7868 (informational publication of the basic protocol, originally developed by Cisco)
Protocol and portIP protocol number 88, not TCP or UDP, so no port number
OSI layerLayer 3 (network)
Primary useAutomating route exchange and path selection within an autonomous system
Related termsOSPF, BGP, Autonomous system, Routing table, IP address

How EIGRP works

EIGRP routers discover neighbors, exchange routing information, and compute best paths with the Diffusing Update Algorithm (DUAL). Each router keeps three tables: neighbors, topology, and routing.

  1. Discover neighbors: Routers send hello packets on enabled interfaces. Routers that share matching parameters, such as the autonomous system number and metric settings, form a neighbor relationship.
  2. Exchange routes: New neighbors send each other their full set of known routes. After that, only changes are sent, and updates go only to the routers affected.
  3. Build the topology table: Each router stores every route learned from its neighbors, along with the metric each neighbor advertises for it.
  4. Compute paths with DUAL: DUAL picks the lowest-metric path as the successor. It keeps a backup, called a feasible successor, only if that path is guaranteed to be loop-free.
  5. Install and react: The successor route goes into the routing table. If it fails and a feasible successor exists, the router switches to it immediately. Otherwise it queries its neighbors for a new path.

The default composite metric is based on bandwidth and delay, and load and reliability can be added through configuration. EIGRP can also summarize routes and load-balance across paths of unequal cost, which most other interior routing protocols do not do by default.

Why EIGRP matters in enterprise networks

EIGRP matters because routing stability affects every service that depends on the network, including DNS, DHCP, and application traffic. Because it sends partial, event-driven updates instead of periodic full table dumps, it keeps overhead low on large networks, and DUAL lets it converge quickly when a feasible successor already exists.

Operational problems usually come from configuration rather than the protocol. Mismatched autonomous system numbers or metric settings prevent neighbors from forming. Inconsistent summarization can hide failures or create suboptimal paths. In large networks, a route that loses its successor with no feasible successor can enter an active state, and slow replies to queries can leave routes stuck in that state and disrupt traffic.

Security also needs attention. A rogue device that joins the routing domain can inject false routes and redirect or drop traffic. EIGRP supports neighbor authentication, and enabling it, together with passive interfaces on links that do not need routing, reduces that exposure. Clear documentation of which devices and subnets participate in routing is essential for troubleshooting and audits.

EIGRP vs OSPF

EIGRP is a distance-vector-based protocol that uses DUAL and a composite metric, while OSPF is a link-state protocol that builds a full topology map of an area and runs a shortest-path-first calculation. What is OSPF?

AttributeEIGRPOSPF
Protocol typeAdvanced distance vectorLink state
Path computationDiffusing Update AlgorithmShortest path first algorithm
Topology knowledgeRoutes advertised by neighborsFull link-state database per area
StandardizationBasic functionality published as RFC 7868Open standard, with OSPFv2 defined in RFC 2328
HierarchyFlat design with optional summarizationAreas, with a required backbone area

EIGRP vs RIP

EIGRP converges faster and scales to larger networks than RIP, which uses hop count as its metric and is limited to a small maximum network diameter. What is Routing Information Protocol?

AttributeEIGRPRIP
MetricComposite metric based on bandwidth and delay by defaultHop count
UpdatesPartial, event-driven after initial exchangePeriodic advertisements of the routing table
Loop preventionDUAL with feasible successorsSplit horizon, route poisoning, and hold-down timers
Network sizeSuited to large networksLimited to a maximum of 15 hops

Common EIGRP issues

Adjacencies do not come up when the autonomous system number or metric K values differ between routers. Check for a mismatch first, then confirm the interfaces are enabled under the process and are not set as passive.

A route stays in the active state when a neighbor does not answer a DUAL query in time, and the router eventually resets the neighbor. Common causes are congested or unreliable links, overloaded routers, and very large query domains with no summarization.

Summary routes that cover address space not reachable behind the summarizing router can pull traffic to the wrong place or hide failures downstream. Review summary boundaries whenever subnets are added or moved.

Traffic may take a path that seems worse than another when interface bandwidth or delay values are set incorrectly, since the composite metric depends on them. Configured values that do not match real link speeds produce misleading metrics.

Neighbors that repeatedly drop and re-form usually point to packet loss, mismatched MTU, or a hold timer that is too short for the link. Logs showing hold time expiry indicate that hello packets are not arriving reliably.

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Related terms

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Open Shortest Path First (OSPF)

A routing protocol for IP networks that uses a link state routing algorithm and falls into the group of interior gateway protocols, operating within a single autonomous system.

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Routing Information Protocol

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BGP (Border Gateway Protocol)

A standardized exterior gateway protocol to exchange routing and reachability information among autonomous systems on the internet.

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Internet Protocol Address (IP Address)

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