EIGRP (Enhanced Interior Gateway Routing Protocol): The Complete Guide

EIGRP (Enhanced Interior Gateway Routing Protocol, full form: Enhanced Interior Gateway Routing Protocol) is the routing protocol that occupies the awkward middle ground between OSPF and BGP. It is fast like OSPF but converges differently. It is policy-rich like BGP but limited to a single AS. It was Cisco-proprietary for 26 years, opened to standards in 2013, and is still found in 2026 Cisco-shop networks where someone built a deployment in 2008 and it still works.

This is the cluster overview for the full PingLabz EIGRP series: fundamentals, the DUAL algorithm that makes EIGRP convergence unique, the K values and composite metric, configuration on Cisco IOS XE, stub routing, and how EIGRP compares to OSPF and BGP. We will work through what EIGRP actually is, the DUAL state machine that drives loop-free convergence, the metric calculation that has confused generations of CCNP candidates, and the configuration patterns that work in production. If you are studying for CCNP/CCIE, inheriting a Cisco-proprietary EIGRP network, or trying to decide between OSPF and EIGRP for a new deployment, start here.

What EIGRP Is

EIGRP is an advanced distance-vector protocol (some call it a hybrid protocol because it has link-state-like features). Unlike pure distance-vector protocols (RIP, IGRP), EIGRP does not periodically broadcast its full routing table. Instead it sends incremental updates only when topology changes, uses the Diffusing Update Algorithm (DUAL) to compute loop-free paths, and maintains neighbor relationships via Hello packets.

The defining characteristics:

• Cisco-led, now open. EIGRP was Cisco-proprietary from 1992 until 2013 when Cisco published the basic protocol as RFC 7868. Adoption beyond Cisco remains rare; it is Cisco's IGP for Cisco shops.
• Fast convergence. Sub-second on direct failure when a feasible successor exists. The DUAL algorithm pre-computes alternates so failover does not require recomputation.
• Composite metric. Unlike OSPF (cost only) or RIP (hop count), EIGRP combines bandwidth, delay, and (configurably) load and reliability into a single 64-bit metric. The default uses bandwidth and delay only; the K values control the formula.
• Classless and hierarchical. Supports VLSM and route summarization at any boundary, manual or automatic.
• Reliable transport. RTP (Reliable Transport Protocol) provides ordered delivery for control-plane updates, similar to TCP but more efficient for small frequent messages.
• Multicast-based discovery. Hellos and updates use 224.0.0.10 (IPv4) or FF02::A (IPv6).

For the historical context and why EIGRP exists alongside OSPF and BGP, see BGP vs OSPF and the upcoming EIGRP vs OSPF comparison article.

How EIGRP Works (the 10,000-Foot View)

EIGRP forms neighbor relationships, exchanges full routing information once at session establishment, then sends incremental updates only when topology changes. Three protocol phases:

1. Neighbor discovery. Routers send Hello packets to multicast 224.0.0.10. If parameters match (AS number, K values, authentication), neighbors form. EIGRP validates parameters strictly; mismatches mean no adjacency.
2. Initial topology exchange. Once neighbors form, they exchange full Update packets containing all known routes. Updates use RTP for reliable delivery.
3. Steady state with DUAL. All routes are stored in the topology table. Best paths (and feasible successors) are installed. Topology changes trigger DUAL recomputation, which uses the topology table to find loop-free alternates without polling the network.

The genius of DUAL: when a primary path fails and a feasible successor exists in the topology table, the router fails over instantly using the cached alternate. No queries to neighbors needed. This is what makes EIGRP convergence so fast on healthy networks.

DUAL: The Algorithm That Makes EIGRP Different

DUAL (Diffusing Update Algorithm) is EIGRP's loop-prevention and convergence machinery. The four key concepts:

The Feasibility Condition: a route is a Feasible Successor if its Reported Distance is strictly less than the current Feasible Distance. RD < FD. This guarantees loop-freedom because the neighbor must already have a shorter path to the destination than ours.

When the successor fails:

• If a Feasible Successor exists: install it as the new successor immediately. Sub-second convergence. The route never enters Active state.
• If no Feasible Successor exists: the route enters Active state. The router queries neighbors for alternative paths. Slower convergence; can take seconds. If queries time out (Stuck-In-Active), the neighbor is declared dead.

For the deep-dive on DUAL with worked examples, see EIGRP DUAL Algorithm Deep Dive.

The EIGRP Composite Metric

EIGRP's metric is the part most engineers find confusing. It uses a 64-bit composite calculated from up to five inputs (bandwidth, delay, load, reliability, MTU), weighted by configurable K values.

The classic IGRP/EIGRP metric formula (default K1=1, K3=1, others=0):

metric = 256 * (10^7 / min_bandwidth + sum_delay)

Where:

• min_bandwidth is the minimum interface bandwidth along the path, in kbps
• sum_delay is the sum of all interface delays along the path, in tens of microseconds

The 64-bit "wide metric" introduced in IOS 15.x uses the same formula but with larger ranges and units, supporting modern interface speeds without saturation.

K values let you weight different inputs:

Critical: all routers in the same EIGRP AS must use the same K values or neighbor relationships will not form. Cisco strongly recommends never changing the defaults.

For the full math walkthrough including the wide-metric variants and how to verify metrics in show output, see EIGRP Metric and K Values Explained.

Neighbor States and Adjacency Requirements

Five things must match for an EIGRP neighbor relationship to form:

Verify with:

Router# show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H   Address      Interface  Hold  Uptime    SRTT   RTO  Q  Seq Num
0   10.0.12.2    Gi0/0/0    13    00:05:23  10     200  0  47
1   10.0.13.3    Gi0/0/1    14    00:05:21  8      200  0  52

The Q (Queue) column matters: a non-zero queue means EIGRP is buffering updates because the neighbor has not acknowledged. Persistent non-zero queue indicates RTP issues.

Configuration on Cisco IOS XE

The minimum EIGRP configuration:

R1(config)# router eigrp 100
R1(config-router)#  network 10.0.0.0 0.0.255.255
R1(config-router)#  passive-interface default
R1(config-router)#  no passive-interface GigabitEthernet0/0/0
R1(config-router)#  no auto-summary

Three things to notice. First, the AS number (100) must match across all neighbors in the same EIGRP domain. Second, the wildcard mask in network is inverted from a regular subnet mask. Third, no auto-summary is mandatory in modern networks - the legacy auto-summarization at classful boundaries causes problems with VLSM and is enabled by default in old code.

The named-mode configuration (preferred for new deployments since IOS 15.x):

R1(config)# router eigrp PROD
R1(config-router)#  address-family ipv4 unicast autonomous-system 100
R1(config-router-af)#   network 10.0.0.0 0.0.255.255
R1(config-router-af)#   af-interface default
R1(config-router-af-interface)#    passive-interface
R1(config-router-af-interface)#   exit-af-interface
R1(config-router-af)#   af-interface GigabitEthernet0/0/0
R1(config-router-af-interface)#    no passive-interface
R1(config-router-af-interface)#   exit-af-interface

Named mode separates IPv4 and IPv6 cleanly, scales better for multi-AS deployments, and is what new CCNP labs expect. For the full walkthrough, see EIGRP Configuration on Cisco IOS XE.

Stub Routing

EIGRP stub routing optimizes hub-and-spoke topologies by limiting what the spoke advertises and stopping the hub from querying the spoke during DUAL active states. The result: faster hub convergence and less control-plane churn.

! On a spoke router
router eigrp 100
 eigrp stub connected summary

Stub options control what the spoke can announce: connected, summary, static, redistributed, or receive-only. The dominant pattern for branch routers in hub-and-spoke designs is eigrp stub connected summary.

Detail in EIGRP Stub Routing.

EIGRP vs OSPF vs BGP

For the full comparison see EIGRP vs OSPF: When to Use Each and the cross-cluster BGP vs OSPF piece.

EIGRP Deep Dives in This Cluster

1. EIGRP DUAL Algorithm Deep Dive: Successors, Feasible Successors, FD, RD
2. EIGRP Metric and K Values Explained
3. EIGRP Configuration on Cisco IOS XE: Classic and Named Mode
4. EIGRP vs OSPF: When to Use Each
5. EIGRP Stub Routing for Hub-and-Spoke

Frequently Asked Questions

What does EIGRP stand for?

EIGRP stands for Enhanced Interior Gateway Routing Protocol. It is the successor to IGRP (Interior Gateway Routing Protocol), Cisco's earlier distance-vector protocol from the 1980s. EIGRP added classless support, VLSM, the DUAL algorithm, and route summarization at any boundary.

What is the administrative distance of EIGRP?

90 for internal EIGRP routes (learned from EIGRP neighbors in the same AS) and 170 for external EIGRP routes (redistributed from other protocols). Lower than OSPF (110) and IS-IS (115), higher than directly connected (0) and static (1).

What protocol number does EIGRP use?

EIGRP runs directly on top of IP using protocol number 88. It does not use TCP or UDP. It uses multicast 224.0.0.10 (IPv4) or FF02::A (IPv6) for Hellos and unsolicited updates.

EIGRP vs OSPF, which one should I use?

OSPF for vendor neutrality and CCNP/CCIE expectations. EIGRP for Cisco-only environments where convergence speed is paramount and the hub-and-spoke stub feature simplifies design. In practice, many enterprises run OSPF (the safer enterprise default in 2026) and EIGRP shows up where someone deployed it years ago and it still works.

Is EIGRP still Cisco-only?

The basic protocol was opened to RFC 7868 in 2013. Some non-Cisco implementations exist (Open EIGRP for Linux, partial support in some appliances). In production, EIGRP is essentially a Cisco-only protocol; deploying it for vendor-interop is not recommended.

What is an EIGRP AS number?

An AS number identifies the EIGRP routing domain. All routers in the same EIGRP AS exchange routes; routers in different ASes do not form neighbors. The AS number is locally significant in EIGRP (unlike BGP, which uses globally unique ASNs). Range is 1-65535.

Key Takeaways

EIGRP is the routing protocol that exists in the gap between OSPF (link-state, vendor-neutral, hierarchical) and BGP (path-vector, inter-AS, policy-rich). Its DUAL algorithm gives it sub-second convergence on healthy networks, its composite metric handles diverse interface speeds, and its stub feature simplifies hub-and-spoke designs. The cost is Cisco-affiliation: deploying EIGRP commits you to Cisco for IGP across that domain.

If you take one thing away from this guide, make it the DUAL feasibility condition: RD < FD guarantees loop-freedom and lets EIGRP fail over instantly without re-querying neighbors. Master DUAL and the rest of EIGRP is mechanics. Bookmark this page, work through the cluster articles in order, and lab every change.