Back to EIGRP Guide Configuration and verification

EIGRP Configuration: Build, Verify and Repair a Lab

Configure a three-router EIGRP network from empty interfaces to working routes. Every step includes the reason, the command, the expected result, and the first check when it fails.

AS 100Classic ModeStep by StepValidationRepair

Configuration at a Glance

A reliable EIGRP build follows one order: make the links work, enable the correct interfaces, form neighbors, learn topology information, install routes, and finally prove traffic in both directions.

Lab domainEIGRP AS 100
Transit linksActive interfaces
LoopbacksAdvertised but passive
Success proofNeighbor + route + ping
Golden rule: Configure and verify one layer at a time. A large pasted configuration makes it harder to locate the first wrong setting.

In This Lesson

  1. Plan the three-router lab
  2. Configure and verify interface addressing
  3. Start classic EIGRP
  4. Use precise network statements
  5. Set a stable router ID
  6. Control neighbor formation with passive interfaces
  7. Validate neighbor formation
  8. Explain and validate the learned route
  9. Troubleshoot configuration failures
  10. Continue with one focused topic
  11. Complete the guided configuration lab
  12. Review the frequently asked questions

1. Plan the Three-Router Lab

R2 is the transit router. R1 and R3 advertise loopbacks that represent user or server networks. All router-to-router links use EIGRP AS 100.

R1RID 1.1.1.1Lo0: 10.1.1.1/24
10.0.12.0/30R1 .1 · R2 .2Gi0/0 ↔ Gi0/0
R2RID 2.2.2.2Transit router
10.0.23.0/30R2 .1 · R3 .2Gi0/1 ↔ Gi0/0
R3RID 3.3.3.3Lo0: 10.3.3.3/24
Expected result: R1 and R2 become neighbors, R2 and R3 become neighbors, and the two edge loopbacks are reachable in both directions.
RouterInterfaceAddressEIGRP role
R1Gi0/010.0.12.1/30Active transit
R1Lo010.1.1.1/24Advertised, passive
R2Gi0/010.0.12.2/30Active transit
R2Gi0/110.0.23.1/30Active transit
R3Gi0/010.0.23.2/30Active transit
R3Lo010.3.3.3/24Advertised, passive

2. Configure and Verify Interface Addressing

EIGRP cannot repair an incorrect Layer 3 link. Configure addresses first and prove that directly connected peers can ping each other.

R1 interfaces

interface GigabitEthernet0/0
 ip address 10.0.12.1 255.255.255.252
 no shutdown
interface Loopback0
 ip address 10.1.1.1 255.255.255.0

R2 interfaces

interface GigabitEthernet0/0
 ip address 10.0.12.2 255.255.255.252
 no shutdown
interface GigabitEthernet0/1
 ip address 10.0.23.1 255.255.255.252
 no shutdown

R3 interfaces

interface GigabitEthernet0/0
 ip address 10.0.23.2 255.255.255.252
 no shutdown
interface Loopback0
 ip address 10.3.3.3 255.255.255.0

Verify state

show ip interface brief
show interfaces description
show ip route connected

Verify each link

R1# ping 10.0.12.2
R2# ping 10.0.23.2

Both should succeed before
EIGRP is configured.

3. Start Classic EIGRP

Classic mode uses router eigrp followed by the autonomous-system number. Direct neighbors must use the same AS.

R1

router eigrp 100
 eigrp router-id 1.1.1.1
 network 10.0.12.0 0.0.0.3
 network 10.1.1.0 0.0.0.255
 passive-interface Loopback0
 no auto-summary

R2

router eigrp 100
 eigrp router-id 2.2.2.2
 network 10.0.12.0 0.0.0.3
 network 10.0.23.0 0.0.0.3
 no auto-summary

R3

router eigrp 100
 eigrp router-id 3.3.3.3
 network 10.0.23.0 0.0.0.3
 network 10.3.3.0 0.0.0.255
 passive-interface Loopback0
 no auto-summary
Modern note: Current software normally behaves classlessly, but no auto-summary remains common in classic-mode labs and older configurations. Confirm your platform defaults.

4. Use Precise Network Statements

An EIGRP network statement selects local interfaces whose primary address matches the statement. It does not create a network, assign an address, or directly point at a remote prefix.

StatementLocal matchOperational effect
network 10.0.12.0 0.0.0.3Addresses in 10.0.12.0/30Enables EIGRP on the R1–R2 link
network 10.1.1.0 0.0.0.255Addresses in 10.1.1.0/24Includes R1 Loopback0
network 10.0.0.0Potentially a broad classful rangeMay enable unintended interfaces
  • Prefer precise matches: They are easier to review and reduce accidental neighbor formation.
  • Verify the result: Use show ip eigrp interfaces rather than assuming the statement matched correctly.
  • Connected prefix: Once the interface participates, its connected network can be advertised to EIGRP neighbors.

5. Set a Stable Router ID

A router ID is a 32-bit identifier written like an IPv4 address. Configure it explicitly so the value remains predictable when interfaces change.

Configure and verify

router eigrp 100
 eigrp router-id 1.1.1.1

show ip protocols
show ip eigrp topology

Keep it unique

R1  1.1.1.1
R2  2.2.2.2
R3  3.3.3.3

Duplicate router IDs can
damage route identification.
Change caution: A running process may not adopt a new router ID immediately. Follow the platform procedure during an approved maintenance window and expect adjacency or topology impact.

6. Control Neighbor Formation with Passive Interfaces

A passive interface advertises its connected network but does not send EIGRP Hellos or form neighbors. User VLANs and loopbacks are usually passive.

Safer default pattern

router eigrp 100
 passive-interface default
 no passive-interface GigabitEthernet0/0
 no passive-interface GigabitEthernet0/1

Confirm the list

show ip protocols
show ip eigrp interfaces

Only router-to-router links
should send Hellos.
Interface typeTypical settingReason
Router transit linkNot passiveMust discover and maintain the peer
LoopbackPassiveAdvertise the prefix without useless Hellos
User or server VLANPassiveAvoid unintended adjacency attempts

7. Validate Neighbor Formation

A neighbor entry proves Hello exchange and parameter compatibility. Read the health fields instead of checking only whether an address appears.

Expected R2 neighbors

R2# show ip eigrp neighbors
H  Address      Interface  Hold  Uptime   Q
1  10.0.23.2   Gi0/1        13  00:12:10 0
0  10.0.12.1   Gi0/0        11  00:12:18 0

Interpret health

Hold   Counts down and refreshes
Uptime Increases without resets
SRTT   Reliable-packet round trip
RTO    Retransmission timeout
Q      Normally remains zero
No neighbor? Check interface state, primary subnet, AS number, K-values, passive status, authentication, and IP protocol 88 filtering on both ends.

8. Explain and Validate the Learned Route

After the neighbors are stable, follow one prefix through the router. R1 should learn R3's 10.3.3.0/24 network from R2, keep the path in the topology table, install the successor in the routing table, and forward traffic in both directions.

Neighbor table

Confirms which directly connected peers can exchange routes.

Topology table

Shows successors, alternates, FD, RD, and Passive or Active state.

Routing table

Shows the selected D or D EX route and next hop.

R1 route evidence

show ip eigrp topology 10.3.3.0/24
show ip route 10.3.3.0
show ip cef 10.3.3.3 detail

End-to-end proof

R1# ping 10.3.3.3 source 10.1.1.1
R3# ping 10.1.1.1 source 10.3.3.3

Test both directions.
Validation statement: “The prefix is present in EIGRP's topology table, installed as a D route through R2, resolved by CEF, and reachable with sourced pings in both directions.” If one part is missing, stop at that layer and fix it before continuing.

9. Troubleshoot Configuration Failures

Find the first failed layer. Do not clear the process or paste a new configuration until the evidence identifies one cause.

SymptomLikely causeCheck first
No EIGRP interfaceNetwork statement does not match or address is missingshow ip eigrp interfaces and interface config
Interface present, no neighborAS, K-values, subnet, passive setting, authentication, or ACLshow ip protocols on both ends
Neighbor resetsHold expiry, link loss, key lifetime, or transport troubleNeighbor detail, logs, Q count, and interface errors
Neighbor stable, route missingSource prefix absent, policy, summary, or better routeTopology entry and full route lookup
Wrong path selectedBandwidth, delay, AD, variance, or route sourceAll-links topology plus interface values
Route remains ActiveNo feasible successor, large query domain, loss, or slow peerActive topology output and named peer
  1. Confirm interface state, address, mask, and direct ping.
  2. Confirm EIGRP activation and passive-interface state.
  3. Compare neighbor parameters on both ends.
  4. Trace the prefix through topology, routing, CEF, and return path.

10. Continue with One Focused Topic

The base configuration is now complete. Study advanced features separately so each lesson has one clear goal and its own validation method.

Named mode and VRF

Learn the address-family hierarchy and verify IPv4, IPv6, and VRF routing contexts without mixing tables.

Open address families

Neighbor security

Apply authentication to the correct interfaces and diagnose a key or algorithm mismatch.

Open authentication

Summaries and defaults

Build a routing boundary, verify component reachability, and recognize possible black-hole behavior.

Open summarization

11. Guided Configuration Lab

Begin with empty EIGRP configuration. Complete one task, capture the result, and restore the healthy baseline before the next fault.

Task 1: Build the Baseline

  1. Configure the six interfaces in the addressing plan.
  2. Prove both transit links with direct pings.
  3. Configure classic EIGRP AS 100.
  4. Make loopbacks passive.
  5. Verify neighbors and end-to-end loopback traffic.

Expected result: R1 and R3 learn each other's loopbacks as internal D routes.

Task 2: Repair a Passive Transit Link

  1. Configure passive-interface default on R2.
  2. Observe both neighbors disappear.
  3. Use show commands to identify the passive links.
  4. Remove passive mode only from Gi0/0 and Gi0/1.
  5. Confirm neighbors and routes recover.

Expected result: You preserve the safer default while restoring only intended adjacencies.

Task 3: Diagnose an AS Mismatch

  1. Move R3 to EIGRP AS 200.
  2. Confirm Gi0/0 stays up/up.
  3. Compare show ip protocols on R2 and R3.
  4. Restore AS 100.
  5. Verify uptime begins again and routes return.

Expected result: You separate physical reachability from EIGRP parameter compatibility.

12. Frequently Asked Questions

What does an EIGRP network statement actually do?

It matches local interface addresses, enables EIGRP on those interfaces, and allows their connected prefixes to be advertised. It does not configure a remote network.

Must EIGRP neighbors use the same AS number?

Yes for the same classic EIGRP domain. In named mode, neighbors must use the same autonomous-system value inside the address family.

Do Hello and hold timers have to match?

No. Each router advertises the hold time it expects its peer to use. Consistent timers are easier to operate, but equality is not a neighbor requirement.

Does a passive interface advertise its network?

Yes, when the interface is selected by EIGRP. Passive mode stops Hellos and neighbor formation; it does not automatically remove the connected prefix from advertisements.

Why is the neighbor up but the route missing?

The source router may not advertise the prefix, policy or summarization may hide it, or another route source may win. Follow the prefix from topology table to routing table.

Should I use classic mode or named mode?

Use the mode supported by your design and platform standard. Classic mode is simple for basic IPv4 labs; named mode organizes multiple address-family and interface features more clearly.

Does EIGRP authentication encrypt route information?

No. It authenticates routing packets so unexpected senders are rejected, but it does not provide confidentiality for the route contents.