EIGRP AS Number Mismatch Preventing Adjacency
Presenting Symptom
Two directly connected routers running EIGRP fail to form an adjacency; the neighbor relationship remains in the 'Init' state or never appears.
Network Context
Two Cisco routers (R1 and R2) are connected via a FastEthernet link in a small branch office. Both run IOS 15.x and are configured for EIGRP. The network uses a single area with no redistribution. The engineer observes that the EIGRP neighbor adjacency is not established, and routes are not being exchanged.
Diagnostic Steps
Check EIGRP neighbors
show ip eigrp neighborsR1# 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 Fa0/0 13 00:00:15 1 100 0 1If no neighbors are listed, the adjacency is not formed. If a neighbor appears but is stuck in 'Init' state, it indicates a problem with the initial handshake, often due to mismatched parameters.
Verify EIGRP AS number on each router
show running-config | section router eigrpR1# show running-config | section router eigrp router eigrp 100 network 192.168.1.0 0.0.0.255 R2# show running-config | section router eigrp router eigrp 200 network 192.168.1.0 0.0.0.255
Compare the AS numbers. If they differ (e.g., 100 vs 200), this is the root cause. EIGRP requires the same AS number on both routers to form an adjacency.
Check EIGRP interface parameters
show ip eigrp interfaces detailR1# show ip eigrp interfaces detail
EIGRP-IPv4 Interfaces for AS(100)
Interface Peers Xmit Queue Mean Pacing Time Multicast Pending
Un/Reliable SRTT Un/Reliable Flow Timer Routes
Fa0/0 0 0/0 0 0/0 50 0
Hello interval: 5 sec, Hold time: 15 secIf the AS number is correct, check that the interface is listed under the correct AS. If the interface does not appear, EIGRP may not be enabled on that interface. Also verify hello/hold timers match (defaults are fine).
Debug EIGRP packets to see rejection reason
debug eigrp packetsR1# debug eigrp packets EIGRP: Received packet on Fa0/0 with mismatched autonomous system, opcode = 1 (update) EIGRP: Packet from 192.168.1.2 ignored, AS mismatch
The debug output explicitly shows 'mismatched autonomous system', confirming the root cause. This is the definitive diagnostic step.
Root Cause
The EIGRP autonomous system (AS) number is configured differently on the two routers. R1 uses AS 100 while R2 uses AS 200. EIGRP requires the same AS number on all routers in the same routing domain to form neighbor adjacencies.
Resolution
Verification
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 Fa0/0 13 00:00:15 1 100 0 1 Both routers should now show each other as neighbors with state 'Init' quickly transitioning to 'Established'.
Prevention
1. Standardize the EIGRP AS number across all routers in the design documentation before configuration. 2. Use configuration templates or automation to ensure consistency. 3. Implement a change management process that includes peer review of routing protocol configurations.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where you must identify why EIGRP neighbors are not forming. The exam tests your ability to interpret 'show ip eigrp neighbors' and 'show running-config' output to spot an AS number mismatch. Key fact: EIGRP neighbors must share the same autonomous system number.
Exam Tips
Always check the AS number first when EIGRP neighbors fail to form; it's a common misconfiguration.
Remember that 'show ip eigrp neighbors' shows neighbors only if adjacency is established; if empty, check AS and network statements.
The debug command 'debug eigrp packets' is powerful but can be CPU-intensive; use it carefully in production.
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 running-config
Displays the current active configuration in DRAM, showing all non-default settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions