EIGRP – Enhanced Interior Gateway Routing Protocol: A Technical Guide

Routing Protocol

EIGRP
Enhanced Interior Gateway

One of Cisco’s most powerful and efficient dynamic routing protocols

📅 April 2026
🕒 15 min read

🌐What is EIGRP?

EIGRP – Enhanced Interior Gateway Routing Protocol – is a Cisco-developed advanced distance-vector routing protocol designed for use within a single autonomous system (AS). Originally proprietary to Cisco, EIGRP was partially open-standardised under RFC 7868 in 2016, making it implementable on non-Cisco platforms, though it remains most closely associated with Cisco IOS environments. EIGRP sits firmly in the interior gateway protocol (IGP) category, meaning it manages routing within an organisation’s internal network rather than between autonomous systems—that function belongs to exterior gateway protocols such as BGP.

EIGRP’s origins lie in Cisco’s older IGRP (Interior Gateway Routing Protocol), which debuted in the 1980s. EIGRP emerged in the early 1990s as a major evolution, inheriting IGRP’s composite metric system but adding a sophisticated convergence algorithm, partial updates, and support for VLSM (Variable Length Subnet Masking) and CIDR (Classless Inter-Domain Routing). These enhancements transformed a simple distance-vector protocol into what Cisco describes as a “hybrid” or “advanced distance-vector” protocol, blending the simplicity of distance-vector operation with some of the scalability benefits typically associated with link-state protocols like OSPF.

At its core, EIGRP maintains three separate tables—a neighbour table, a topology table, and a routing table – and uses the Diffusing Update Algorithm (DUAL) to guarantee loop-free, rapid convergence. Unlike protocols that recalculate the entire topology on every change, EIGRP routers communicate only the differences in topology, dramatically reducing bandwidth consumption and CPU load on large networks. This efficiency makes EIGRP a practical and popular choice for enterprise campus networks, WAN edge deployments, and multi-site corporate infrastructures where fast failover and minimal overhead are critical operational requirements.

ℹ️

Protocol Classification EIGRP is categorised as an advanced distance-vector protocol (sometimes called a hybrid protocol). It uses distance-vector behaviour for route advertisement but incorporates DUAL—a diffusing computation algorithm—which provides link-state-like loop-free guarantees without requiring a full topology database synchronisation on every update.

EIGRP supports IPv4 and IPv6 natively, and its Named EIGRP mode (introduced in IOS 15.0(1)M) consolidates IPv4 and IPv6 configuration under a single process, simplifying management significantly. It also supports multiple network-layer protocols through a protocol-dependent module (PDM) architecture, although in modern deployments this is predominantly IP. Administrative distance for EIGRP internal routes is 90, while EIGRP external routes (redistributed from other protocols) carry an administrative distance of 170—important numbers to memorise for both real-world troubleshooting and certification exams.

🧩Key Components

EIGRP’s architecture is built around several tightly integrated components that work together to enable fast, loop-free routing decisions. Understanding each component individually is essential for configuring, troubleshooting, and optimising EIGRP in production environments. The following cards detail the primary building blocks of the protocol.

📋

Neighbour Table

Maintained per interface, this table records every directly adjacent EIGRP router (neighbour). Each entry contains the neighbour’s IP address, the local interface used to reach it, hold time, smooth round-trip time (SRTT), and RTO. EIGRP discovers neighbours by exchanging Hello packets; a neighbour relationship is established only when both routers agree on AS number, K-values, and authentication parameters.

🗺️

Topology Table

The topology table stores every destination network advertised by all EIGRP neighbours. For each prefix, EIGRP records the feasible distance (FD), reported distance (RD) from each neighbour, and the route state. This table is the source from which the DUAL algorithm selects the successor route and any feasible successors, enabling instant failover without re-querying the network.

📡

Routing Table

The routing table contains only the best (successor) routes selected by DUAL from the topology table. These are the paths installed in the Forwarding Information Base (FIB) and used for actual packet forwarding. EIGRP routes appear in the routing table with the code “D” for internal routes and “D EX” for redistributed external routes.

⚙️

DUAL Algorithm

The Diffusing Update Algorithm is the mathematical heart of EIGRP. DUAL guarantees loop-free operation at every instant during topology changes, and it can activate a pre-computed feasible successor immediately without any further network queries—achieving sub-second convergence. If no feasible successor exists, DUAL initiates a diffusing computation (active state) to find an alternative path.

📊

Composite Metric

EIGRP calculates path cost using a composite metric based on five K-values: bandwidth (K1), load (K2), delay (K3), reliability (K4), and MTU (K5). By default only K1 (bandwidth) and K3 (delay) are enabled. The metric formula is: Metric = [K1 × (10⁷ / min-bandwidth) + K3 × (sum-delay / 10)] × 256. All routers in an AS must use identical K-values.

📨

Hello & Hold Timers

EIGRP uses Hello packets (multicast to 224.0.0.10) to maintain neighbour adjacencies. On LAN interfaces the default Hello timer is 5 seconds with a hold time of 15 seconds. On WAN interfaces (T1 and slower) the defaults are 60/180 seconds. If a router does not receive a Hello from a neighbour before the hold timer expires, the neighbour is declared down and DUAL is invoked.

🔄

Reliable Transport Protocol (RTP)

EIGRP uses its own transport mechanism—RTP—to guarantee ordered, reliable delivery of EIGRP packets (updates, queries, replies) without relying on TCP or UDP. Hello and acknowledgement packets are sent unreliably via multicast or unicast. RTP uses sequence numbers and per-neighbour acknowledgement tracking to ensure no critical routing information is ever silently dropped.

🛡️

Protocol-Dependent Modules (PDM)

EIGRP’s modular architecture separates the DUAL logic from network-layer specifics through PDMs. Each PDM handles the nuances of a particular routed protocol—originally IP, IPX, and AppleTalk, but in contemporary deployments, IPv4 and IPv6. This design allows EIGRP’s core algorithm to remain unchanged while supporting multiple network-layer protocols simultaneously on the same router.

⚠️

K-Value Mismatch – A Common Adjacency Failure One of the most frequent causes of EIGRP neighbour adjacencies failing to form is a K-value mismatch. If two routers have different K-values configured, they will log a “K-value mismatch” message and refuse to establish a neighbour relationship. Always verify K-values are consistent across all routers within the same EIGRP autonomous system using show ip protocols.

⚙️ How It Works

EIGRP operation can be conceptually divided into three phases: neighbour discovery and adjacency formation, topology exchange and DUAL computation, and ongoing maintenance with incremental updates. Understanding the sequence of events from router startup to stable convergence gives network engineers the insight needed to predict EIGRP behaviour during topology changes and to interpret debugs and show outputs accurately.

Step-by-Step: EIGRP Convergence Process

Hello Packet Transmission
When EIGRP is enabled on an interface, the router immediately begins multicasting Hello packets to the well-known multicast address 224.0.0.10 using IP protocol number 88. Hello packets contain the router’s AS number, K-values, hold time, and authentication information. These are sent unreliably (no acknowledgement required) at the configured Hello interval.

Neighbour Adjacency Formation
When a router receives a Hello from another EIGRP router, it checks that the AS number and K-values match. If they do, it adds the sender to its neighbour table and sends a unicast Hello in return. The two routers are now neighbours. The hold timer for each neighbour begins counting down and is reset every time a Hello is received.

Full Routing Table Exchange
Immediately after adjacency formation, both routers exchange their full routing tables via Update packets. These are sent reliably using RTP and must be acknowledged by the receiving neighbour. Once the exchange is complete, both routers send an “end-of-table” marker via an Update packet with the Init bit clear and the End-of-Table flag set, signalling that the initial synchronisation is finished.

DUAL Successor Selection
For each destination prefix received, EIGRP calculates the Feasible Distance (FD)—the total composite metric from the local router to the destination. For each neighbour advertising that prefix, the Reported Distance (RD) is recorded. The neighbour with the lowest FD becomes the Successor (best path). Any neighbour whose RD is strictly less than the local FD qualifies as a Feasible Successor (FS)—a backup path guaranteed to be loop-free.

Routes Installed in Routing Table
The successor route for each prefix is installed into the routing table with an administrative distance of 90 (internal) or 170 (external). The feasible successors remain in the topology table as pre-computed backups but are not placed in the routing table unless the successor fails. This is key to EIGRP’s rapid convergence capability—no recalculation is needed when a FS is available.

Incremental Updates & Triggered Convergence
Once initial convergence is complete, EIGRP sends only partial, bounded updates when topology changes occur—not full routing table dumps. If the successor for a route fails and a feasible successor exists, EIGRP promotes the FS to successor status instantly. If no FS exists, the router transitions the route to “Active” state and multicasts a Query packet to all neighbours, seeking an alternative path. Routers that receive the Query respond with a Reply. When all replies are received, the router selects a new successor and returns to “Passive” state.

💡

Feasibility Condition The feasibility condition for EIGRP is: a neighbour qualifies as a feasible successor if its Reported Distance (RD) is strictly less than the local router’s Feasible Distance (FD) to the same destination. This guarantees the neighbour cannot have a path that loops back through the local router. If the feasibility condition is not met, the neighbour is still stored in the topology table—it just cannot be used as an instant-failover backup without first running a DUAL query.

📊Usage and Functions

EIGRP is a versatile protocol deployed across a wide range of enterprise and service-provider scenarios. The table below maps common use cases to their corresponding EIGRP functions and provides the relevant Cisco IOS command context for each. These represent the practical deployment patterns engineers encounter most frequently in real-world network design and support roles.

Use Case	EIGRP Function	Cisco IOS Context
Enterprise Campus Routing	Fast convergence with DUAL; partial updates reduce bandwidth overhead across access, distribution, and core layers	`router eigrp 100` / `network 10.0.0.0`
WAN Edge Failover	Pre-computed feasible successors provide sub-second failover when primary WAN link fails; no query propagation required if FS available	`variance 2` for unequal-cost load balancing
Unequal-Cost Load Balancing	Unique to EIGRP among IGPs; the `variance` command allows traffic distribution across paths with different metrics up to a configurable multiplier	`variance <multiplier>` under EIGRP process
Route Summarisation	Manual summary routes reduce topology table size and limit query scope; reduces convergence time in large networks	`ip summary-address eigrp 100 <prefix> <mask>` on interface
Route Redistribution	Importing routes from OSPF, RIP, static, or connected; redistributed routes appear as EIGRP external (D EX) with AD 170	`redistribute ospf 1 metric 10000 100 255 1 1500`
IPv6 Routing (EIGRPv6)	Native IPv6 support; EIGRPv6 operates independently from IPv4 EIGRP but uses the same DUAL algorithm and table structure	`ipv6 router eigrp 100` / `ipv6 eigrp 100` per interface
Named EIGRP (Multi-AF)	Single EIGRP process managing both IPv4 and IPv6 address families; simplifies configuration and centralises policy	`router eigrp CORP` / `address-family ipv4 autonomous-system 100`
Stub Routing	Stub routers (typically spoke sites) advertise only connected/summary routes and suppress Query propagation; reduces convergence overhead in hub-and-spoke topologies	`eigrp stub connected summary`
Authentication	MD5 or SHA-256 (Named mode) authentication prevents rogue routers from forming EIGRP adjacencies and injecting false routing information	`ip authentication mode eigrp 100 md5` / key-chain configuration
Bandwidth Control	Limits the percentage of interface bandwidth EIGRP can consume for routing updates; critical on low-bandwidth WAN links	`ip bandwidth-percent eigrp 100 50` on interface

Cisco IOS CLI Configuration Examples

The following examples demonstrate classic EIGRP configuration scenarios that every network engineer should be comfortable executing. The first example covers a standard EIGRP deployment with authentication and summarisation; the second demonstrates verification and troubleshooting commands.

Example 1 – Classic EIGRP Configuration with MD5 Authentication and Route Summarisation



! ── Step 1: Configure key chain for MD5 authentication ──────────────

key chain EIGRP-KEYS

key 1

key-string Str0ngP@ssword
! ── Step 2: Enable EIGRP process in AS 100 ───────────────────────────

router eigrp 100

eigrp router-id 1.1.1.1

network 10.0.0.0 0.0.255.255   ! Wildcard mask covers 10.0.x.x subnets

no auto-summary               ! Disable classful auto-summary (always recommended)

passive-interface default     ! Suppress Hellos on all interfaces by default

no passive-interface GigabitEthernet0/0   ! Re-enable on uplink interfaces

no passive-interface GigabitEthernet0/1

! ── Step 3: Apply authentication and summarisation per-interface ────── interface GigabitEthernet0/0 ip authentication mode eigrp 100 md5 ip authentication key-chain eigrp 100 EIGRP-KEYS ip summary-address eigrp 100 10.0.0.0 255.255.0.0 ! Advertise /16 summary ip bandwidth-percent eigrp 100 50 ! Limit EIGRP to 50% of interface BW

Example 2 – EIGRP Verification and Troubleshooting Commands



! ── Verify EIGRP neighbour adjacencies ───────────────────────────────

show ip eigrp neighbors

! Output columns: H (handle), Address, Interface, Hold, SRTT, RTO, Q Cnt, Seq Num
! ── Inspect the EIGRP topology table ─────────────────────────────────

show ip eigrp topology

! Shows all prefixes, FD, RD, successors (P = passive/stable, A = active/querying)
show ip eigrp topology all-links

! Includes non-feasible successors – useful for troubleshooting stuck-active routes
! ── Review installed EIGRP routes in the routing table ───────────────

show ip route eigrp

! D = EIGRP internal (AD 90),  D EX = EIGRP external (AD 170)
! ── Check EIGRP process parameters (K-values, AS, networks) ──────────

show ip protocols
! ── Verify interface-level EIGRP statistics ───────────────────────────

show ip eigrp interfaces detail

! Displays Hello/hold timers, bandwidth percent, authentication status per interface

! ── Real-time neighbour adjacency changes (use with caution in prod) ── debug eigrp neighbors no debug all ! Always disable debug promptly after use

✅Best Practices

Deploying EIGRP effectively requires more than enabling the protocol and advertising networks. The following best practices represent lessons distilled from real-world deployments. Implementing these recommendations reduces the risk of instability, security vulnerabilities, and difficult-to-diagnose performance issues.

Always disable auto-summary. In IOS versions prior to 15.x, EIGRP auto-summary is enabled by default, which causes discontiguous networks to be summarised at classful boundaries. This produces black holes and routing loops when VLSM addressing is in use. Issue no auto-summary under the EIGRP process on every router, regardless of IOS version—it costs nothing and prevents a hard-to-trace failure mode.
Use passive-interface default and selectively enable active interfaces. Enabling EIGRP on all interfaces indiscriminately exposes the protocol to untrusted segments (e.g., access ports, internet-facing links). The best practice is to set all interfaces to passive by default—suppressing Hello transmission and neighbour formation—and then explicitly enable EIGRP only on interfaces that connect to legitimate EIGRP peers.
Implement MD5 or SHA-256 neighbour authentication. Without authentication, any router connected to the network and configured with the correct AS number and K-values can form an EIGRP adjacency and inject arbitrary routes. Always deploy key-chain-based authentication, and prefer SHA-256 (available in Named EIGRP mode) over MD5 for new deployments, as MD5 is considered cryptographically weak.
Configure manual route summarisation at distribution and core boundaries. Large, flat EIGRP domains suffer from slow convergence and wide Query propagation during failures. Summarising at logical boundaries (e.g., per-site or per-building) limits the Query scope, reduces topology table size on remote routers, and confines the impact of local failures to the local summarisation boundary.
Deploy EIGRP stub on all spoke/branch routers. In hub-and-spoke topologies, spoke routers should be configured as EIGRP stubs. Stub routers do not receive Query packets from the hub, preventing the spoke from delaying convergence by participating in a diffusing computation it has no relevant information about. Use eigrp stub connected summary as the standard starting configuration for branch sites.
Set explicit EIGRP router IDs. By default, Cisco IOS selects the EIGRP router ID from the highest loopback IP or the highest physical interface IP if no loopback exists. This auto-selected value can change when interfaces are added or removed, potentially causing EIGRP instability. Always assign a stable, unique loopback address and configure eigrp router-id <loopback-ip> explicitly.
Use the Named EIGRP mode for new deployments. Named EIGRP (Multi-AF mode) consolidates IPv4 and IPv6 configuration under a single process, supports SHA-256 authentication, and provides more granular per-AF policy control. It is the preferred configuration style for all modern Cisco IOS and IOS-XE platforms and aligns with Cisco’s current best practice guidance.
Limit EIGRP bandwidth consumption on low-speed WAN interfaces. On serial or MPLS WAN links, EIGRP can consume a disproportionate share of available bandwidth during a convergence event—particularly if large numbers of Query and Reply packets are exchanged. Use ip bandwidth-percent eigrp <AS> <percent> to cap EIGRP’s bandwidth usage. A value between 25–50% is commonly recommended for constrained WAN circuits.
Monitor and avoid SIA (Stuck-In-Active) routes. An EIGRP route enters the Active state when no feasible successor exists and a Query is sent. If a neighbour does not reply within the SIA timer (default 3 minutes per query cycle), the non-responsive neighbour is removed from the neighbour table. SIA events indicate either a network reliability problem (packet loss causing dropped queries/replies), a design issue (insufficient summarisation allowing Queries to propagate too widely), or an overloaded router. Use show ip eigrp topology active to monitor active routes in production.
Document and audit K-values before any topology changes. Because K-value mismatches silently prevent adjacency formation, it is important to maintain a documented baseline of K-values for each EIGRP AS in the network. Before introducing a new router or upgrading IOS, verify that the K-values on the new device match those of the existing network using show ip protocols. Changes to K-values require a coordinated rollout to avoid widespread adjacency drops.

⚖️Pros and Cons

No routing protocol is universally optimal—each carries trade-offs relative to the network environment, vendor ecosystem, and operational priorities. Understanding EIGRP’s strengths and limitations allows network architects to make informed decisions about when to deploy it, when to prefer OSPF, and where hybrid or multi-protocol designs might be warranted.

✔ Advantages

Extremely fast convergence due to pre-computed feasible successors; sub-second failover achievable when an FS is available
Partial, bounded updates conserve bandwidth—only changes are sent, and only to affected routers
Supports unequal-cost load balancing via the variance command, a feature unique among common IGPs
Simple configuration relative to OSPF—no area design, no DR/BDR election, no LSA flooding control required
EIGRP stub feature significantly simplifies hub-and-spoke WAN design and reduces Query propagation
Native support for both IPv4 and IPv6 within a single Named EIGRP process
Composite metric provides fine-grained traffic engineering by weighting bandwidth, delay, load, and reliability
No complex SPF computation; DUAL is computationally lightweight compared to Dijkstra’s algorithm used in OSPF
Route summarisation reduces topology table size and limits the blast radius of topology changes
Widely supported on Cisco IOS, IOS-XE, NX-OS, and now open-standard (RFC 7868) for third-party implementations

✘ Disadvantages

Historically Cisco-proprietary; RFC 7868 standardisation is partial and real-world multi-vendor EIGRP interoperability remains limited
Composite metric complexity can make traffic engineering unintuitive; K-value mismatches silently prevent adjacencies
DUAL query process (SIA) can cause extended convergence delays in poorly designed, flat, or large networks
Requires all routers to be in the same autonomous system; lacks the hierarchical design options of OSPF’s multi-area model
Limited visibility into the full network topology compared to link-state protocols—routers only know what neighbours advertise, not the complete graph
Administrative distance of 170 for external routes makes EIGRP redistributed routes vulnerable to being overridden by other protocol routes with lower AD
Auto-summary behaviour in older IOS versions is a well-known source of routing black holes if not explicitly disabled
Less commonly tested and deployed in modern SDN and cloud-native environments compared to BGP or OSPF

⚠️

SIA – Stuck in Active: Know the Warning Signs A Stuck-In-Active (SIA) condition occurs when EIGRP sends a Query for a destination and does not receive a Reply within the SIA timer window (90 seconds per default, checked every 90 seconds up to the full 3-minute default). The router then resets the non-responsive neighbour’s adjacency. SIA events often indicate network design problems—specifically, insufficient summarisation allowing Queries to traverse too many routers—rather than simple hardware failures. If you see recurring adjacency resets in your EIGRP logs, investigate Query propagation depth before assuming link instability.

🎯 Conclusion

EIGRP – Enhanced Interior Gateway Routing Protocol – remains one of the most capable and operationally practical dynamic routing protocols available to network engineers working in Cisco-centric environments. Its combination of rapid convergence, bandwidth efficiency through partial updates, and unique features like unequal-cost load balancing and EIGRP stub make it a strong choice for enterprise campus and WAN edge deployments where simplicity of configuration and predictable failover behaviour are paramount.

The DUAL algorithm at EIGRP’s core is a genuinely elegant solution to the loop-prevention problem. By pre-computing feasible successor paths and mathematically guaranteeing their loop-free nature, EIGRP achieves something that pure distance-vector protocols cannot: instant, query-free failover when backup paths exist. This capability, paired with the protocol’s minimal computational overhead compared to SPF-based link-state protocols, makes EIGRP particularly well-suited to networks where routers operate under CPU or memory constraints.

That said, EIGRP is not without limitations. Its historical Cisco-proprietary nature restricts its use in multi-vendor environments, and poorly designed EIGRP networks—particularly those lacking summarisation and stub configuration—can suffer from Query storms and SIA events that erode the protocol’s convergence advantages. The best EIGRP deployments are those built on sound hierarchical design principles: meaningful route summarisation at topology boundaries, stub deployment at leaf sites, and authentication throughout.

Mastering EIGRP means understanding not just the configuration commands but the underlying mechanics—the feasibility condition, the three tables, the RTP delivery mechanism, and the lifecycle of a DUAL query. That depth of understanding translates directly into faster troubleshooting and more confident network design decisions in the field. Whether you are preparing for a certification exam or optimising a production network, EIGRP rewards the engineer who takes the time to understand it properly.

📖Glossary

The following glossary defines the core terminology associated with EIGRP – Enhanced Interior Gateway Routing Protocol. Familiarity with these terms is essential for reading show command output, interpreting protocol behaviour, and communicating accurately with colleagues and certification exam question writers.

EIGRP

Enhanced Interior Gateway Routing Protocol. An advanced distance-vector IGP developed by Cisco, using DUAL for loop-free, fast convergence within a single autonomous system.

DUAL

Diffusing Update Algorithm. The mathematical core of EIGRP that guarantees loop-free routing at every instant and enables pre-computed backup paths (feasible successors).

Feasible
Distance

(FD) The total composite metric from the local router to a destination network, including both the metric reported by the next-hop neighbour and the local cost to reach that neighbour.

Reported
Distance

(RD) Also called Advertised Distance. The composite metric that a neighbouring router reports for a given destination—i.e., the neighbour’s own cost to reach that network.

Successor

The neighbour providing the best (lowest FD) path to a destination. The successor’s route is installed in the routing table and used for active packet forwarding.

Feasible
Successor

(FS) A backup neighbour whose Reported Distance is strictly less than the local router’s Feasible Distance. An FS can be promoted to successor instantly without triggering a DUAL query.

Neighbour
Table

A per-interface table recording all adjacent EIGRP routers, their IP addresses, hold times, SRTT, and RTO values. Maintained by Hello packet exchange.

Topology
Table

A database of all EIGRP-learned prefixes, including FD, RD, successor, and feasible successor information. The source from which DUAL selects routes for the routing table.

RTP

Reliable Transport Protocol. EIGRP’s own delivery mechanism that guarantees ordered, acknowledged delivery of Update, Query, and Reply packets without relying on TCP or UDP.

SIA

Stuck-In-Active. A condition where an EIGRP router sends a Query and does not receive a Reply within the SIA timer, resulting in the non-responsive neighbour being removed from the neighbour table.

K-Values

Five weighting constants (K1–K5) used in EIGRP’s composite metric formula. By default K1=1 (bandwidth) and K3=1 (delay) are active; K2, K4, K5 are zero. All routers in an AS must share identical K-values.

Passive
Interface

An interface configured to suppress EIGRP Hello transmission and neighbour formation. Networks on passive interfaces are still advertised, but no adjacency is formed on that segment.

EIGRP Stub

A router configured to advertise only a limited subset of routes (e.g., connected, summary) and to signal to hub routers that it should not receive Query packets. Commonly deployed on branch/spoke routers.

Named EIGRP

A configuration mode (introduced IOS 15.0(1)M) that consolidates IPv4 and IPv6 EIGRP address families under a single named process, supporting SHA-256 authentication and per-AF policy control.

Variance

An EIGRP command that enables unequal-cost load balancing. Routes with a metric up to variance × FD of the successor’s metric are eligible for inclusion in the routing table as equal-cost load-balanced paths.

Auto-Summary

A legacy EIGRP behaviour (default in IOS < 15.x) that automatically summarises networks to their classful boundary at major network borders. Must be disabled (no auto-summary) when using VLSM or discontiguous addressing.

EIGRP – Enhanced Interior Gateway