EIGRP Network Statement with Wrong Wildcard Mask
Presenting Symptom
Routers in an EIGRP network are not forming neighbor adjacencies or are missing routes for certain subnets.
Network Context
A small enterprise campus network with three routers (R1, R2, R3) running EIGRP AS 100. All routers are Cisco IOS 15.x. The network uses a single area with point-to-point links. The problem occurs after a junior engineer added a new subnet to EIGRP on R1 using a network statement with an incorrect wildcard mask.
Diagnostic Steps
Check EIGRP neighbors
show ip eigrp neighborsNo neighbors listed or only partial neighbors.
If neighbors are missing, EIGRP is not forming adjacencies. Check network statements and interfaces.
Review EIGRP configuration
show running-config | section router eigrprouter eigrp 100 network 10.0.0.0 0.0.0.255 network 192.168.1.0 0.0.0.255 network 172.16.0.0 0.0.255.255
Look for network statements with wildcard masks. A common mistake is using a wildcard mask that is too broad or too narrow, causing interfaces to be included or excluded incorrectly.
Verify which interfaces are participating in EIGRP
show ip eigrp interfacesInterface Peers Xmit Queue Un/Reliable Mean SRTT Pacing Time Multicast Flow Timer Pending Routes Gi0/0 0 0/0 0 0 0 0 0 Gi0/1 1 0/0 10 100 0 0 0
If an interface that should be running EIGRP is missing, the network statement may not cover it. Compare with the actual interface IP addresses.
Check the wildcard mask calculation
show ip interface briefInterface IP-Address OK? Method Status Protocol Gi0/0 10.0.0.1 YES manual up up Gi0/1 192.168.1.1 YES manual up up Gi0/2 172.16.1.1 YES manual up up
For each interface, verify if its IP address falls within the network statement's range. For example, network 172.16.0.0 0.0.255.255 includes 172.16.0.0-172.16.255.255, so 172.16.1.1 is included. But if the mask was 0.0.0.255, it would only include 172.16.0.0/24, missing 172.16.1.1.
Root Cause
The network statement for subnet 172.16.0.0 was configured with wildcard mask 0.0.255.255 instead of 0.0.0.255. This caused EIGRP to advertise the entire 172.16.0.0/16 range, but more critically, it may have caused mismatched neighbor conditions or route filtering issues. In this case, the wrong wildcard mask led to the interface being included but with an incorrect network advertisement, preventing proper neighbor formation due to mismatched K-values or authentication? Actually, the typical issue is that the wildcard mask is too broad, causing EIGRP to advertise a supernet, which may not be accepted by neighbors if they have more specific routes. However, the most common problem is that the wildcard mask does not exactly match the interface subnet, causing the interface to be excluded from EIGRP. For example, if the interface IP is 172.16.1.1/24, the correct network statement should be 'network 172.16.1.0 0.0.0.255'. Using 'network 172.16.0.0 0.0.255.255' would include the interface, but the advertised route would be 172.16.0.0/16, which might cause routing loops or suboptimal routing. In this scenario, the root cause is that the wildcard mask does not match the actual subnet mask, leading to incorrect route advertisement and potential neighbor issues.
Resolution
Verification
Run 'show ip eigrp neighbors' to confirm neighbor adjacency is established. Expected output shows neighbors in the 'Init' or 'Up' state. Also run 'show ip route eigrp' to verify the correct routes are learned. Expected output includes the specific subnet (e.g., 172.16.1.0/24) rather than a supernet.
Prevention
1. Always use the exact subnet and wildcard mask that matches the interface IP and subnet mask. 2. Use the 'network' command with the interface's network address and the wildcard mask derived from the subnet mask (e.g., for /24, use 0.0.0.255). 3. Consider using the 'passive-interface' command to control which interfaces participate in EIGRP.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where you must identify why EIGRP is not forming neighbors or why routes are missing. The exam tests your ability to interpret 'show ip eigrp neighbors', 'show ip eigrp interfaces', and 'show running-config' to spot incorrect wildcard masks. A key fact is that the wildcard mask in the network statement must match the subnet mask of the interface; otherwise, the interface may be excluded or the wrong route advertised.
Exam Tips
Remember that the wildcard mask is the inverse of the subnet mask. For a /24, the wildcard mask is 0.0.0.255.
If you see a network statement with a wildcard mask that is too broad (e.g., 0.0.255.255 for a /24), it will advertise a supernet, which may cause routing issues.
Use 'show ip eigrp interfaces' to quickly see which interfaces are participating; if an interface is missing, check the network statement.
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 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.
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