EIGRP Named Mode vs Classic Mode Configuration Conflict
Presenting Symptom
EIGRP neighbors are not forming between two routers configured with different EIGRP modes (named mode vs classic mode).
Network Context
Two Cisco routers (R1 and R2) are connected via GigabitEthernet0/0 in a small branch office. R1 runs IOS 15.2 with EIGRP named mode configuration, while R2 runs IOS 12.4 with EIGRP classic mode configuration. Both are configured with the same autonomous system number (100) and network statements, but neighbor adjacency fails.
Diagnostic Steps
Check EIGRP neighbor status
show ip eigrp neighborsNo output or empty neighbor table
If no neighbors are listed, EIGRP adjacency is not established. This confirms the problem but does not identify the cause.
Verify EIGRP configuration on both routers
show running-config | section router eigrpR1: router eigrp TEST\n address-family ipv4 autonomous-system 100\n network 10.0.0.0\nR2: router eigrp 100\n network 10.0.0.0
R1 uses named mode (address-family under router eigrp with a name), while R2 uses classic mode (router eigrp AS). This mismatch prevents neighbor formation because the EIGRP modes are incompatible.
Check EIGRP packet debug
debug eigrp packetsEIGRP: Received packet with mismatched K-values or other errors
Debug output may show 'K-value mismatch' or 'Neighbor not on common subnet' but the root cause is mode mismatch. Named mode uses different K-value defaults (k1=1, k2=0, k3=1, k4=0, k5=0) while classic mode uses (k1=1, k2=0, k3=1, k4=0, k5=0) by default, but the mode itself is incompatible.
Verify EIGRP mode compatibility
show eigrp protocolsR1: EIGRP-IPv4 Protocol for AS(100) named mode\nR2: EIGRP-IPv4 Protocol for AS(100)
This command confirms the mode. Named mode explicitly states 'named mode' while classic mode does not. The two modes cannot interoperate.
Root Cause
EIGRP named mode and classic mode are incompatible and cannot form neighbor adjacencies. R1 is configured with named mode (router eigrp TEST with address-family), while R2 uses classic mode (router eigrp 100). Even though both use the same AS number and network statements, the mode mismatch prevents EIGRP hello packets from being processed correctly.
Resolution
Verification
After conversion, run 'show ip eigrp neighbors' on both routers. Expected output: R1 and R2 show each other in the neighbor table with state 'Init' transitioning to 'Established'. Also run 'show ip eigrp topology' to verify routes are exchanged.
Prevention
1. Standardize on one EIGRP mode across the entire network (preferably named mode for consistency and features). 2. Use network automation or configuration templates to ensure uniform EIGRP configuration. 3. Document the chosen EIGRP mode in network design documents.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why EIGRP neighbors are not forming. The exam tests understanding that named mode and classic mode are incompatible. A typical multiple-choice question might ask: 'Which configuration mismatch prevents EIGRP adjacency?' with options including K-values, AS numbers, network statements, and mode. The key fact is that EIGRP named mode and classic mode cannot interoperate.
Exam Tips
Remember that EIGRP named mode uses 'router eigrp <name>' with address-family, while classic mode uses 'router eigrp <AS>'. They are not compatible.
The exam may show partial configurations; look for the presence of 'address-family' to identify named mode.
Know that 'show eigrp protocols' displays the mode (named vs classic) and can quickly identify mismatches.
Commands Used in This Scenario
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