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

EIGRP (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 up 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. Every capture on this page comes from a real four-router IOS XE lab (R1-R2-R3-R4 in a partial mesh, with an R2-R4 backup link so DUAL can be observed end-to-end).

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 (an Informational RFC documenting Cisco's protocol, not an IETF standards-track spec). 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 - the mechanics and the design fixes are in the query process and SIA deep dive.

The differentiator capture: a real DUAL state transition. R3 in the lab uses R2 as the successor for several prefixes (including 10.255.0.2/32, R2's loopback). Killing R3's Ethernet0/0 (the link to R2) makes EIGRP mark every R2-reached destination Active, query R4, receive the reply, and re-install the routes via R4 - the whole thing in roughly 23 milliseconds:

R3(config)#interface Ethernet0/0
R3(config-if)#shutdown          ! kill the successor

! debug eigrp fsm output (captured to logging buffer):
*May 11 06:18:02.245: DUAL: AS(100) Dest 10.255.0.2/32 entering active state for tid 0.
*May 11 06:18:02.245: EIGRP-IPv4(100): Set reply-status table. Count is 1.
*May 11 06:18:02.245: EIGRP-IPv4(100): Not doing split horizon
*May 11 06:18:02.268: EIGRP-IPv4(100): dest(10.30.31.0/30) active
*May 11 06:18:02.268: EIGRP-IPv4(100): rcvreply: 10.30.31.0/30 via 10.30.32.2 metric 262144000/196608000
*May 11 06:18:02.268: EIGRP-IPv4(100): reply count is 1
*May 11 06:18:02.268: DUAL: AS(100) Clearing handle 1, count now 0
*May 11 06:18:02.268: EIGRP-IPv4(100): Find FS for dest 10.30.31.0/30 ... found
*May 11 06:18:02.268: DUAL: AS(100) RT installed 10.30.31.0/30 via 10.30.32.2
*May 11 06:18:02.269: EIGRP-IPv4(100): rcvreply: 10.255.0.1/32 via 10.30.32.2 metric 262225920/196689920
*May 11 06:18:02.269: DUAL: AS(100) RT installed 10.255.0.1/32 via 10.30.32.2
*May 11 06:18:02.269: EIGRP-IPv4(100): rcvreply: 10.255.0.2/32 via 10.30.32.2 metric 196689920/131153920
*May 11 06:18:02.269: DUAL: AS(100) RT installed 10.255.0.2/32 via 10.30.32.2

Five things are happening in that trace. The route enters Active state. EIGRP sets a reply-status table for the remaining neighbor (R4 via 10.30.32.2) and sends a Query. R4 replies with its own known path. EIGRP runs Find FS to confirm the new path satisfies the feasibility condition. The new successor is installed and the prefix returns to Passive. No other IGP makes the state transition this visible - OSPF and IS-IS just flood an LSA / LSP and every router re-runs Dijkstra in its own head.

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 four inputs (bandwidth, delay, load, reliability), weighted by configurable K values. MTU and hop count are carried in EIGRP updates but never enter the metric calculation.

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. The full 64-bit format, picosecond delay units, and rib-scale are decoded in the EIGRP wide metrics deep dive.

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.

The K values are visible in show ip protocols on any EIGRP router:

R3#show ip protocols
Routing Protocol is "eigrp 100"
  EIGRP-IPv4 VR(PINGLABZ) Address-Family Protocol for AS(100)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0 K6=0
    Metric rib-scale 128
    Metric version 64bit
    Soft SIA disabled
    NSF-aware route hold timer is 240
    Router-ID: 10.255.0.3
    Topology : 0 (base)
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Maximum hopcount 100
      Maximum metric variance 1
      Total Prefix Count: 8
      Total Redist Count: 0
  Automatic Summarization: disabled

Two things are worth noticing in that output: Metric version 64bit confirms named-mode EIGRP is using the wide metric, and the Distance: internal 90 external 170 line is the EIGRP administrative distance you will spot in any RIB tie-break against another protocol. Maximum metric variance 1 is the unequal-cost load-balancing knob - leaving it at 1 means EIGRP installs equal-cost paths only; raising it to 2 would install any path whose metric is up to 2x the successor.

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 (and when one does not, the adjacency troubleshooting guide shows the exact failure output for each):

Verify with show ip eigrp neighbors. From the lab, R3 has two neighbors (R2 via Et0/0 and R4 via Et0/1):

R3#show ip eigrp neighbors
EIGRP-IPv4 VR(PINGLABZ) Address-Family Neighbors for AS(100)
H   Address        Interface     Hold Uptime   SRTT   RTO  Q  Seq
                                 (sec)         (ms)       Cnt Num
1   10.30.32.2     Et0/1           12 00:01:01    2   100  0  6
0   10.30.31.1     Et0/0           11 00:01:07 1263  5000  0  13

The H column is a DUAL-internal handle. Hold is the seconds remaining before the neighbor is declared dead (default 15 on broadcast media, refreshed by every Hello). SRTT is the smoothed round-trip time used by EIGRP's reliable transport (RTP). 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 - link congestion, ACL filtering, or a dying neighbor.

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.

Once the process is up, the usual next steps are route summarization and default-route origination and route filtering with distribute lists; routes that later go missing are usually one of those two doing its job, per the missing-routes troubleshooting guide.

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

R1(config)# router eigrp PINGLABZ
R1(config-router)#  address-family ipv4 unicast autonomous-system 100
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 Ethernet0/0
R1(config-router-af-interface)#    no passive-interface
R1(config-router-af-interface)#   exit-af-interface
R1(config-router-af)#   network 10.30.30.0 0.0.0.3
R1(config-router-af)#   network 10.255.0.1 0.0.0.0
R1(config-router-af)#   eigrp router-id 10.255.0.1
R1(config-router-af)#  exit-address-family

Named mode separates IPv4 and IPv6 cleanly, scales better for multi-AS deployments, and is what new CCNP labs expect. After convergence, the topology table shows the successors for every learned prefix:

R3#show ip eigrp topology
EIGRP-IPv4 VR(PINGLABZ) Topology Table for AS(100)/ID(10.255.0.3)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.255.0.4/32, 1 successors, FD is 131153920
        via 10.30.32.2 (131153920/163840), Ethernet0/1
P 10.255.0.1/32, 1 successors, FD is 196689920
        via 10.30.31.1 (196689920/131153920), Ethernet0/0
P 10.30.33.0/30, 2 successors, FD is 196608000
        via 10.30.31.1 (196608000/131072000), Ethernet0/0
        via 10.30.32.2 (196608000/131072000), Ethernet0/1
P 10.30.30.0/30, 1 successors, FD is 196608000
        via 10.30.31.1 (196608000/131072000), Ethernet0/0
P 10.30.32.0/30, 1 successors, FD is 131072000
        via Connected, Ethernet0/1
P 10.30.31.0/30, 1 successors, FD is 131072000
        via Connected, Ethernet0/0
P 10.255.0.2/32, 1 successors, FD is 131153920
        via 10.30.31.1 (131153920/163840), Ethernet0/0
P 10.255.0.3/32, 1 successors, FD is 163840
        via Connected, Loopback0

Every line is P (Passive) - the steady state for a destination, meaning DUAL has finished all calculations and the route is installed. The FD (Feasible Distance) and the metric tuple (total/RD) are exactly what DUAL feeds into the feasibility condition. The show ip eigrp topology all-links variant shows every path EIGRP has heard about, not just the successors - that is the table DUAL consults when picking feasible successors.

For the full walkthrough, see EIGRP Configuration on Cisco IOS XE.

Named mode also carries EIGRP into IPv6 through address families - the IPv6 complete guide covers the addressing side.

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 (queries and their failure mode, stuck-in-active, are covered separately). 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.

Stub configuration is standard practice on DMVPN spokes, where EIGRP runs over multipoint GRE tunnels and an un-stubbed spoke can accidentally become transit for the whole WAN.

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.

Both comparisons run deeper than one section: see the OSPF complete guide and the BGP complete guide for the full treatment of each protocol. In MPLS L3VPN environments, EIGRP also survives at the WAN edge as a PE-CE protocol - context in the MPLS complete guide.

EIGRP Deep Dives in This Cluster

Sixteen articles, in reading order. Foundations first, then configuration and policy, then the operational material.

Foundations

1. The DUAL algorithm: successors, feasible successors, FD and RD
2. The EIGRP composite metric and K values
3. 64-bit wide metrics in named mode
4. EIGRP administrative distance: internal 90, external 170

Configuration and Policy

5. EIGRP configuration on IOS XE: classic and named mode
6. Route summarization, the Null0 discard route, and default routes
7. Unequal-cost load balancing with variance
8. Authentication with MD5 key chains and HMAC-SHA-256
9. Route filtering with distribute lists, prefix lists, and route maps
10. Stub routing for hub-and-spoke WANs

Convergence and Scale

11. The 5 neighbor requirements that must match
12. The query process and stuck-in-active (SIA)

Troubleshooting

13. Troubleshooting neighbor adjacencies that will not form
14. Troubleshooting route advertisement and missing routes

Design and Comparisons

15. EIGRP vs OSPF: when to use each
16. Running EIGRP, OSPF, and BGP over GRE tunnels
17. Routing over DMVPN: split horizon, next-hop-self, and OSPF network types on an mGRE overlay

Expert EIGRP: SoO, Leak Maps, DMVPN and Integration (CCIE level)

EIGRP at professional depth is DUAL, feasible successors and a clean metric. EIGRP at expert depth is what happens when you put it over a multipoint tunnel, hand it a dual-homed site with no loop-prevention marker, and ask it to share a network with two other protocols. These articles are built on real Cisco IOS XE output from a dual-hub DMVPN in CML, with an OSPF domain and a BGP peer hanging off the hub so the redistribution scenarios are genuine.

Studying for the CCIE?

This cluster is part of the full CCNA to CCNP to CCIE Enterprise ladder on PingLabz, every rung built on real Cisco output. For expert-level depth across every EI v1.1 blueprint domain - and the four integration Super Labs - see the CCIE Enterprise Infrastructure study hub.

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?

Cisco opened the basic protocol in 2013; it was later published as RFC 7868, an Informational RFC that documents Cisco's protocol rather than defining an IETF standard. 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.

References

• RFC 7868 - Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP)
• Cisco EIGRP technology documentation