Back to EIGRP Guide Routing fundamentals

EIGRP Overview: Neighbors, DUAL and Fast Convergence

Learn how EIGRP discovers routers, chooses loop-free paths, keeps backup routes, and reacts when a link fails. Follow one small lab from configuration to troubleshooting.

Protocol 88DUALSuccessorMetricsVerification

EIGRP at a Glance

EIGRP is a dynamic interior routing protocol. It was created by Cisco and uses the Diffusing Update Algorithm (DUAL) to choose paths that are free from routing loops.

IP protocol88
IPv4 multicast224.0.0.10
Administrative distance90 internal · 170 external
Default metric inputsBandwidth + delay
Simple mental model: EIGRP learns neighbors, stores every useful path in its topology table, and installs the best loop-free path in the routing table.

In This Lesson

  1. Read the three-router lab
  2. Understand the core behavior
  3. Form neighbor relationships
  4. Follow the three EIGRP tables
  5. Learn successors and feasible successors
  6. Understand metric and K-value basics
  7. Explain and validate one learned route
  8. Troubleshoot in layers
  9. Continue with one focused topic
  10. Complete the guided practice
  11. Review the frequently asked questions

1. Read the Three-Router Lab

All three routers use EIGRP autonomous system 100. R1 reaches R3 through R2. The loopbacks give us stable networks to advertise and test.

R1RID 1.1.1.1Loopback: 10.1.1.1/24
10.0.12.0/30R1 .1 · R2 .2NEIGHBORS
R2RID 2.2.2.2Transit router
10.0.23.0/30R2 .1 · R3 .2NEIGHBORS
R3RID 3.3.3.3Loopback: 10.3.3.3/24
Healthy baseline: R1 and R2 are neighbors, R2 and R3 are neighbors, and R1 learns 10.3.3.0/24 through R2.
What to predict: R1 does not need a direct neighbor relationship with R3. R2 advertises R3's loopback to R1 and R1 installs it with route code D.

2. Understand EIGRP's Core Behavior

EIGRP does not send its entire routing table at a fixed interval. After neighbors synchronize, it normally sends an update only when information changes.

  • Partial updates: An update carries changed route information, not the complete table.
  • Bounded delivery: Information is sent to the neighbors that need it, rather than flooded through every unrelated segment.
  • DUAL: The algorithm tests alternate paths before using them, which protects the network from loops.
  • Flexible path use: EIGRP supports equal-cost and controlled unequal-cost load balancing.
  • Classless routing: Subnet masks travel with routes, so VLSM and CIDR are supported.

3. Form Neighbor Relationships

Two connected routers first discover each other and agree on key settings. They use Hello packets to start and maintain the relationship.

1

Send Hello

The enabled interface sends an EIGRP Hello.

2

Check settings

AS number, K-values, subnet, and authentication must be compatible.

3

Become neighbors

Each router adds the other to its neighbor table.

4

Exchange routes

They synchronize useful routes and calculate loop-free paths.

SettingRequirementIf wrong
Autonomous systemMust match in classic modeNo neighbor relationship
K-valuesMust matchHello is rejected
Primary IP subnetMust be compatible on the linkNo direct neighbor
AuthenticationMethod and key must match when enabledPackets are rejected
Hello and hold timersDo not have to matchEach router advertises the hold time it expects

4. Follow the Three EIGRP Tables

Use the tables in this order when learning or troubleshooting. Each one answers a different question.

Neighbor table

Who can I exchange routes with? It stores directly connected EIGRP peers and transport health.

Topology table

What paths did I learn? It stores route candidates, metrics, and DUAL state.

Routing table

Which path forwards traffic? It receives the selected successor route.

Inspect each table

show ip eigrp neighbors
show ip eigrp topology
show ip route eigrp

Read the result

Neighbor present  → exchange is possible
Prefix in topology → EIGRP learned a path
D route installed  → path won selection

5. Learn Successors and Feasible Successors

DUAL compares route metrics and keeps the best loop-free result. Tie each word to one destination, such as R3's 10.3.3.0/24 loopback.

TermPlain meaningPractical use
SuccessorThe best loop-free pathInstalled in the routing table
Feasible successorA proven loop-free backup pathCan replace the successor quickly
Feasible distance (FD)The local router's best total metricUsed as the comparison point
Reported distance (RD)The neighbor's advertised metricShows how far the neighbor says it is
Feasibility conditionAlternate RD is lower than the current successor FDProves the alternate cannot loop through this router
When no feasible successor exists: The route becomes Active while EIGRP asks neighbors for another path. Active is a search state; it is not the same as an active route.

6. Understand Metric and K-Value Basics

By default, EIGRP chooses a path using the lowest configured bandwidth along the path and the total configured delay. It does not measure current link utilization to create the default metric.

K11 · bandwidth enabled
K20 · load disabled
K31 · delay enabled
K4 and K50 · reliability disabled
  • Bandwidth: EIGRP uses the configured interface bandwidth. A wrong value can cause a surprising path choice.
  • Delay: Every outbound interface delay along the path contributes to the total.
  • K-values: Neighbors must use the same metric weights. Keep defaults unless the design has been carefully validated.

7. Explain and Validate One Learned Route

Use one destination from start to finish: R3's 10.3.3.0/24 loopback. R1 must first recognize R2 as a neighbor, learn the prefix through R2, select it as the successor, and finally install it for forwarding.

Step 1: Neighbor

R1# show ip eigrp neighbors
H  Address      Interface   Hold  Uptime  Q
0  10.0.12.2   Gi0/0         12  00:18:42 0

Step 2: Topology

R1# show ip eigrp topology 10.3.3.0/24
State is Passive, Query origin flag is 1
1 Successor, FD is 3072
via 10.0.12.2 (3072/2048), Gi0/0

Step 3: Routing

R1# show ip route eigrp
D 10.3.3.0/24 [90/3072]
  via 10.0.12.2, GigabitEthernet0/0
Say the result in plain English: “R1 trusts R2 as an EIGRP neighbor. R2 reports a loop-free path to R3's loopback. DUAL selects that path, the routing table installs it through 10.0.12.2, and a sourced ping proves forward and return traffic.”
FieldHealthy meaningWarning sign
HoldCounts down and refreshes when Hellos arriveReaches zero repeatedly
Q countNormally stays at zeroRemains above zero
Passive topology stateDUAL calculation is completeRoute remains Active for too long
Route code DInternal EIGRP route installedExpected prefix missing
Route code D EXExternal route imported into EIGRPUnexpected redistribution

8. Troubleshoot in Layers

Do not clear the EIGRP process as your first step. Find the first failed layer, record the evidence, and change one setting at a time.

SymptomCheck firstUseful commands
No neighborInterface, subnet, AS number, K-values, passive state, authentication, protocol 88 filteringshow ip eigrp interfaces
show ip protocols
Neighbor resetsHold timer expiry, link loss, authentication, duplicate addressesshow ip eigrp neighbors detail
show logging
Neighbor up, route missingNetwork selection, topology entry, filtering, summarization, better route sourceshow ip eigrp topology
show ip route
Route stays ActiveQuery scope, unreachable neighbor, packet loss, overloaded routershow ip eigrp topology active
show processes cpu
Unexpected pathBandwidth, delay, K-values, variance, interface settingsshow interfaces
show ip eigrp topology all-links
  1. Confirm every required interface is up/up with correct primary addressing.
  2. Confirm EIGRP is enabled on the interface and is not passive.
  3. Compare AS number, K-values, and authentication on both ends.
  4. Trace the prefix from topology table to routing table, then test the return path.
Safety: Use debug commands only in a lab or approved maintenance window. Prefer show commands because heavy debugging can affect a busy router.

9. Continue with One Focused Topic

This overview stops after the core learning path. Choose one lesson below when you are ready to study an advanced behavior without mixing several concepts together.

Packet behavior

Learn Hello, Update, Query, Reply, and acknowledgement behavior as one complete exchange.

Open packet types

Metrics and path choice

Study bandwidth, delay, K-values, successors, and feasible successors with calculated examples.

Open metrics

Configuration

Build the three-router lab step by step, then verify the neighbor, topology, and routing tables.

Open configuration

10. Guided Practice Lab

Complete one task at a time. Restore the healthy baseline before starting the next fault so the evidence remains clear.

Task 1: Build the Baseline

  1. Address the links and loopbacks shown in the topology.
  2. Enable EIGRP AS 100 on R1, R2, and R3.
  3. Make the loopbacks passive.
  4. Verify two neighbor pairs and both remote loopback routes.

Expected result: R1 reaches 10.3.3.3 and R3 reaches 10.1.1.1.

Task 2: Follow a Route

  1. Choose R3's loopback prefix.
  2. Find it in R1's topology table.
  3. Record FD, RD, successor, and next hop.
  4. Confirm the same next hop in the routing table.

Expected result: You can explain how a learned path becomes a forwarding route.

Task 3: Repair an AS Mismatch

  1. In the isolated lab, change R3 to AS 200.
  2. Confirm the R2–R3 interface remains up/up.
  3. Compare show ip protocols on both routers.
  4. Restore AS 100 and prove routes return.

Expected result: You identify the routing-process mismatch without changing the IP address.

11. Frequently Asked Questions

Is EIGRP a distance-vector or link-state protocol?

EIGRP is commonly described as an advanced distance-vector protocol. It learns routes from neighbors but also keeps a topology table and uses DUAL to prove loop-free paths.

What must match for EIGRP neighbors to form?

In classic mode, the AS number and K-values must match. The connected interfaces also need compatible primary IP addressing, and any configured authentication must agree.

Do EIGRP Hello and hold timers have to match?

No. Each router tells its neighbor how long to keep the relationship without another Hello. Different timer values can work, although a consistent design is easier to operate.

Why is a feasible successor useful?

It is a backup path that DUAL has already proved loop-free. If the successor fails, EIGRP can use that backup without first asking every neighbor for a new route.

What does Passive mean in the topology table?

Passive is healthy. It means DUAL has finished calculating the route. Active means EIGRP is currently asking neighbors for another path.

What is the difference between D and D EX?

D marks an internal EIGRP route. D EX marks a route that entered EIGRP through redistribution from another routing source.

Can EIGRP use unequal-cost paths?

Yes. The variance command can allow qualified unequal-cost paths. The paths must still satisfy EIGRP's loop-free rules, so test the design before using it in production.