EIGRP Lab 7: Securing Adjacencies with MD5

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Harden EIGRP adjacencies with MD5 authentication between two iol-xe routers over a /30 transit, then validate that only trusted peers form neighbors. You will configure a key chain, bind it to the transit interface, and enable EIGRP (AS 100) to exchange two user LANs. Verification includes neighbor state, key chain presence, and end-to-end host reachability.

Learning objectives

• Configure classic EIGRP with a consistent AS (100) and no auto-summary
• Define and apply an MD5 key chain to protect an EIGRP adjacency
• Verify authenticated adjacencies with IOS show commands
• Diagnose adjacency failure from AS, key, or interface participation mismatches

Troubleshooting focus

• If the neighbor never forms, confirm both routers share the same EIGRP AS (100) and matching K-values (defaults).
• Verify the transit interface participates in EIGRP via a precise network statement and that the interface is up/up.
• Check MD5 authentication on both sides: the key chain name, key-id, and key-string must match; ensure the interface has both 'mode md5' and the key-chain applied.
• Inspect for mismatched wildcard masks that accidentally omit the transit from EIGRP.
• Validate reachability with host pings; if routing looks right but pings fail, recheck host default gateways and masks.