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Routing / EIGRPCCNA 200-301

EIGRP Wildcard Mask Wrong: Neighbour Not Forming

Presenting Symptom

EIGRP is configured on a router but no neighbours appear in `show ip eigrp neighbors`. The router is not receiving or advertising any EIGRP routes. `show ip route` shows no D (EIGRP) routes from the expected neighbours.

Network Context

Two Cisco routers (R1 and R2) connected via a GigabitEthernet link (10.1.1.0/30 subnet). EIGRP AS 100 is configured on both. R1's Gi0/0 has IP 10.1.1.1/30. R2's Gi0/0 has IP 10.1.1.2/30. Both also have loopback interfaces in 192.168.0.0 space.

Diagnostic Steps

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Root Cause

EIGRP wildcard mask errors are the most common cause of neighbours not forming after initial configuration: **Wildcard mask too restrictive:** Example: Interface is 10.1.1.1/30 (subnet 10.1.1.0/30) Wrong: `network 10.1.1.1 0.0.0.0` — This matches only the exact IP 10.1.1.1. The network statement must match the interface IP, but the wildcard 0.0.0.0 is valid here — it works. Wrong: `network 10.1.1.0 0.0.0.0` — This matches only 10.1.1.0 exactly, which is the network address, not the interface IP 10.1.1.1. EIGRP will not activate on the interface. **Wildcard mask too broad causing wrong interface:** `network 10.0.0.0 0.255.255.255` — Activates EIGRP on ALL 10.x.x.x interfaces, which may include unintended interfaces. **AS number mismatch:** R1 configured with `router eigrp 100`, R2 with `router eigrp 101` — EIGRP uses multicast 224.0.0.10 to find neighbours with the same AS number. Mismatched AS numbers prevent any adjacency. **Network statement missing:** The network command not including the interface's subnet — EIGRP never activates on that interface.

Resolution

**Fix incorrect wildcard mask:** ``` R1(config)# router eigrp 100 R1(config-router)# no network 10.1.1.0 0.0.0.0 ! Remove wrong statement R1(config-router)# network 10.1.1.0 0.0.0.3 ! Correct: /30 = wildcard 0.0.0.3 ``` **Wildcard mask calculation:** Wildcard = 255.255.255.255 minus subnet mask - /30 = mask 255.255.255.252 → wildcard 0.0.0.3 - /24 = mask 255.255.255.0 → wildcard 0.0.0.255 - /16 = mask 255.255.0.0 → wildcard 0.0.255.255 **Alternative — use classful network statement (no wildcard needed):** ``` R1(config-router)# network 10.0.0.0 ``` This activates EIGRP on ALL interfaces with IPs in the 10.0.0.0/8 classful range. **Fix AS number mismatch:** ``` R2(config)# no router eigrp 101 R2(config)# router eigrp 100 R2(config-router)# network 10.1.1.0 0.0.0.3 ```

Verification

After applying the correct network statement: 1. `show ip eigrp neighbors` — R2 (10.1.1.2) appears with Hold time counting down, SRTT value, and Q Cnt of 0 2. `show ip route` — D routes appear for R2's networks 3. `show ip protocols` — 'Routing for Networks' section shows the correct network with correct wildcard 4. Ping test — R1 can ping R2's loopback addresses 5. `show ip eigrp interfaces` — Gi0/0 appears in the EIGRP interface table, confirming it is EIGRP-enabled

Prevention

Best practices to prevent EIGRP wildcard mask errors: 1. Always calculate wildcard masks from the subnet mask: wildcard = 255.255.255.255 - subnet_mask 2. For simple networks, use the classful `network 10.0.0.0` form (no wildcard needed) — EIGRP will activate on all matching interfaces 3. Verify with `show ip protocols` immediately after adding network statements — the 'Routing for Networks' section confirms what EIGRP matched 4. Use `show ip eigrp interfaces` to confirm which interfaces EIGRP is active on 5. Document the AS number in network diagrams — AS number mismatch is easy to miss in multi-router labs

CCNA Exam Relevance

EIGRP configuration errors are a high-frequency CCNA topic in the IP Connectivity domain. Wildcard mask mistakes are the most common EIGRP misconfiguration tested. Exam scenarios present a network diagram and router configs, then ask why EIGRP neighbours are not forming. Key indicators in show command output: `show ip protocols` showing wrong network in 'Routing for Networks', or `show ip eigrp neighbors` showing empty table.

Exam Tips

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CCNA exam EIGRP wildcards: memorise the four most common — /30=0.0.0.3, /29=0.0.0.7, /28=0.0.0.15, /27=0.0.0.31, /26=0.0.0.63, /25=0.0.0.127, /24=0.0.0.255

2.

EIGRP neighbours will NOT form if: (1) AS numbers differ, (2) K-values differ (default K1=K3=1, others=0), (3) authentication mismatch, (4) network statement doesn't match any interface

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Note: EIGRP hello timers do NOT need to match (unlike OSPF which does require matching dead timers)

Commands Used in This Scenario

show ip eigrp neighbors

Displays all EIGRP neighbours with their addresses, interfaces, hold time, uptime, SRTT (smooth round-trip time), RTO, queue count, and sequence numbers.

show ip interface brief

Displays a one-line summary of all interfaces showing the IP address assigned, operational status (up/down), and line protocol status. The fastest way to get a device health overview.

show ip route

Displays the IP routing table showing all known routes, their source protocols, administrative distances, metrics, next-hops, and outgoing interfaces.

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