EIGRP K-Value Mismatch Preventing Adjacency
Presenting Symptom
Two EIGRP-enabled routers fail to form an adjacency, and the neighbor state remains stuck in 'Init' or 'Loading'.
Network Context
Two Cisco routers (R1 and R2) are directly connected via GigabitEthernet0/0 in a small branch office. Both run IOS 15.x and are configured for EIGRP AS 100 on the same subnet. The network uses default EIGRP K-values (K1=1, K2=0, K3=1, K4=0, K5=0).
Diagnostic Steps
Check EIGRP neighbor status
show ip eigrp neighborsNo neighbors listed or neighbor in Init state.
If no neighbors appear or the neighbor is stuck in Init, the adjacency is not established. This indicates a mismatch in EIGRP parameters or a Layer 2 issue.
Verify EIGRP K-values on both routers
show ip protocols | include KEIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
Compare the K-values on both routers. If they differ (e.g., one router has K1=0, K3=1), the mismatch prevents adjacency. The output shows the current K-values.
Check EIGRP interface configuration
show ip eigrp interfacesInterface Peers Xmit Queue Mean Pacing Time Multicast Pending Gi0/0 0 0/0 0 0/10 0 0
If the interface is listed but has 0 peers, the router is not receiving EIGRP hellos from the neighbor. This could be due to K-value mismatch or other parameter mismatch.
Debug EIGRP packets to see rejection reason
debug eigrp packetsEIGRP: Received packet with K-values mismatch, rejecting
The debug output explicitly states a K-value mismatch. This confirms the root cause. Note: Use debug carefully in production; it can be CPU-intensive.
Root Cause
The EIGRP K-values are mismatched between the two routers. EIGRP requires that all K-values match exactly for adjacency to form. In this scenario, one router has default K-values (K1=1, K2=0, K3=1, K4=0, K5=0) while the other has been manually changed (e.g., K1=0, K2=0, K3=1, K4=0, K5=0).
Resolution
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 0 192.168.1.2 Gi0/0 13 00:01:23 1 200 0 5 R2# show ip eigrp neighbors EIGRP-IPv4 Neighbors for AS(100) H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 192.168.1.1 Gi0/0 13 00:01:23 1 200 0 5 Both routers show the neighbor in state 'Init' or 'Loading' no longer; adjacency is established.
Prevention
1. Use consistent EIGRP configurations across all routers, preferably default K-values unless advanced metric tuning is required. 2. Implement configuration templates or automation to ensure uniform EIGRP parameters. 3. Document any non-default K-value changes and communicate them to the team.
CCNA Exam Relevance
On the CCNA 200-301 exam, EIGRP K-value mismatch is a common troubleshooting topic. Questions may present a scenario where two routers fail to form an adjacency, and you must identify the cause from show command outputs or debug messages. The exam tests your ability to interpret 'show ip protocols' and 'debug eigrp packets' to spot K-value mismatches. Key fact: EIGRP requires all K-values to match exactly; even a single bit difference prevents adjacency.
Exam Tips
Memorize the default EIGRP K-values: K1=1, K2=0, K3=1, K4=0, K5=0.
In the exam, if you see 'K-value mismatch' in a debug output, that is the root cause — do not overthink.
Know that 'metric weights' command is used to change K-values; the syntax is 'metric weights tos k1 k2 k3 k4 k5'.
Commands Used in This Scenario
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
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions