Troubleshooting EIGRP !!

1️⃣ Mismatched Autonomous System (AS) Numbers

Description: Both routers must be in the same EIGRP AS.

Fix:

router eigrp 100

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2️⃣ Subnet Mask Mismatch

Description: Interfaces must be in the same subnet. A different subnet mask makes them "logically" disconnected.

Fix:
Use matching subnet masks on both interfaces (e.g., /30 on WAN links).

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3️⃣ Passive Interface Enabled on EIGRP-Connected Interface

Description: Passive interface disables EIGRP hello packets.

Fix:

router eigrp 100
 no passive-interface e1/0

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4️⃣ MTU Mismatch

Description: EIGRP includes MTU in its hello packets. If received MTU is less than interface MTU, neighborship fails.

Fix: Ensure MTU matches on both sides of the WAN link.

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5️⃣ K-Values Mismatch

Description: Routers with different metric calculation settings (K-values) will not form neighbors.

Fix:

router eigrp 100
 metric weights 0 1 0 1 0 0  ! default

Or:

default metric weights

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6️⃣ Authentication Mismatch

Description: MD5 authentication must be enabled with same key-string on both routers.

Fix:

key chain AUTHKEY
 key 1
  key-string Cisco123
!
interface e1/0
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 AUTHKEY

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7️⃣ Interface Not in the network Command

Description: If the interface is not covered by the network command, EIGRP won’t operate on it.

Fix:

router eigrp 100
 network 12.1.1.0 0.0.0.3

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8️⃣ Interface is Down or Administratively Shut

Description: Basic, but often overlooked — if the interface is down, EIGRP won’t send or receive hello packets.

Fix:

interface e1/0
 no shutdown

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9️⃣ IP Address Misconfiguration

Description: Wrong IP address — routers are not in the same subnet.

Fix: Assign valid IPs in the same subnet on both sides.

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🔟 Access Control List (ACL) Blocking EIGRP

Description: ACLs on the interface or VTY lines may block EIGRP traffic (uses IP protocol number 88).

Fix: Allow EIGRP in ACL:

permit eigrp any any

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1️⃣1️⃣ Duplicate Router IDs

Description: Duplicate router IDs won’t stop neighbor formation, but can cause route instability or flapping.

Fix:

router eigrp 100
 eigrp router-id 1.1.1.1

Ensure unique router IDs.

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1️⃣2️⃣ Hello/Hold Timer Mismatch (older IOS versions)

Description: If timers differ significantly, routers may not recognize each other as valid neighbors.

Fix:

interface e1/0
 ip hello-interval eigrp 100 5
 ip hold-time eigrp 100 15

Set consistently on both routers.

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1️⃣3️⃣ Unidirectional Link (Physical/Cabling Issue)

Description: One router sends hellos, but the other can’t receive — often due to cable, NIC, or virtual lab misconfig.

Fix: Check cabling or rebuild link in virtual environments.

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1️⃣4️⃣ EIGRP Not Enabled for IPv6

Description: For IPv6, you must use ipv6 router eigrp, and EIGRP is configured per interface, not via network.

Fix:

interface e1/0
 ipv6 eigrp 100

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1️⃣5️⃣ Interface in the Wrong VRF

Description: If the interface is in a VRF and EIGRP isn’t running in that VRF, the neighbor won’t form.

Fix: Configure EIGRP under the correct VRF context.

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🧪 Verification Commands

Command	What It Shows
show ip eigrp neighbors	Neighbor status
show ip protocols	AS, timers, passive interfaces
debug eigrp packets	Hellos, updates, and errors
debug eigrp neighbors	Adjacency process
show running-config	Authentication, K-values, interfaces

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