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EIGRPCCNA 200-301

EIGRP Neighbor Adjacency Not Forming

Presenting Symptom

EIGRP neighbor adjacency is not forming between two directly connected routers.

Network Context

Two Cisco routers (R1 and R2) are directly connected via GigabitEthernet0/0 interfaces in a small branch office. Both run IOS 15.x. EIGRP is configured on both routers with the same autonomous system number (100). The link is up/up, and IP connectivity exists (ping works).

Diagnostic Steps

1

Verify EIGRP neighbor status

show ip eigrp neighbors
No output or empty table

If no neighbors are listed, adjacency is not formed. This confirms the symptom.

2

Check EIGRP configuration on both routers

show running-config | section router eigrp
router eigrp 100
 network 10.0.0.0
 no auto-summary

Verify that both routers have the same autonomous system number and that the network statement includes the interface subnet. If network statement is missing or incorrect, EIGRP will not advertise on that interface.

3

Verify interface is not passive

show ip eigrp interfaces
Interface             Peers  Xmit Queue   Mean   Pacing Time   Multicast    Pending
                       Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Gi0/0                  0       0/0           0       0/0           0           0

If the interface is not listed, it may be passive. Check 'show ip protocols' to see if the interface is listed under Passive Interfaces. A passive interface does not send or receive EIGRP hellos.

4

Check for mismatched EIGRP parameters

show ip eigrp interfaces detail Gi0/0
Hello interval: 5 secs, Hold time: 15 secs
 Authentication mode: none

Compare hello/hold timers and authentication settings between neighbors. Mismatched timers or authentication (e.g., one side configured with MD5 key, other without) will prevent adjacency.

5

Verify K-values match

show ip protocols | include K
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

Both routers must have identical K-values. If one router has non-default K-values (e.g., K1=0), adjacency will not form.

Root Cause

The interface Gi0/0 on R2 is configured as a passive interface under EIGRP, preventing the exchange of hello packets.

Resolution

Remove the passive-interface configuration on R2: R2(config)# router eigrp 100 R2(config-router)# no passive-interface GigabitEthernet0/0 Alternatively, if passive-interface default is configured, use 'no passive-interface Gi0/0' to allow EIGRP on that interface.

Verification

Run 'show ip eigrp neighbors' on both routers. Expected output: R1# show ip eigrp neighbors EIGRP-IPv4 Neighbors for AS(100) H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 10.0.0.2 Gi0/0 13 00:00:15 1 100 0 1 R2# show ip eigrp neighbors EIGRP-IPv4 Neighbors for AS(100) H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 10.0.0.1 Gi0/0 13 00:00:15 1 100 0 1

Prevention

["Avoid using passive-interface default unless necessary; explicitly configure passive interfaces only on LAN-facing interfaces where EIGRP neighbors are not expected.","Use 'show ip eigrp interfaces' during initial configuration to verify which interfaces are active for EIGRP.","Standardize EIGRP parameters (AS number, K-values, authentication) across all routers in the domain."]

CCNA Exam Relevance

On the CCNA 200-301 exam, EIGRP neighbor adjacency issues appear in troubleshooting scenarios, often as drag-and-drop or multiple-choice questions. The exam tests understanding of EIGRP operation, including the role of hello packets, passive interfaces, and mismatched parameters. A candidate must know that passive interfaces prevent adjacency by suppressing hello packets.

Exam Tips

1.

Memorize that a passive interface under EIGRP stops sending and receiving hello packets, thus preventing neighbor adjacency.

2.

Remember that 'show ip eigrp neighbors' is the first command to check when adjacency fails; an empty output indicates no neighbors.

3.

Be aware that mismatched K-values or authentication will also prevent adjacency, but passive interface is a common misconfiguration.

Commands Used in This Scenario

show ip eigrp interfaces

Displays detailed information about interfaces on which EIGRP is enabled, including neighbor status, pending routes, and interface statistics, used to verify EIGRP adjacency and interface participation.

show ip eigrp neighbors

Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.

show ip protocols

Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.

Test Your CCNA Knowledge

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