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BeginnerPublished 2026-06-23

OSPFv2: Router IDs, Neighbor States, and Loopback Adjacency

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Deploy OSPFv2 across three routers with unique, loopback-derived router-ids. Observe OSPF neighbor state transitions, advertise loopbacks and LANs, and troubleshoot an adjacency failure caused by a duplicate router-id. Validate full end-to-end reachability from client hosts after fixing the issue.

Track
Est. time
55 minutes
Difficulty
Beginner
Topics

Learning objectives

  • Configure explicit OSPFv2 router-ids and understand how IOS selects the router-id.
  • Advertise loopbacks and client LANs in OSPFv2 while keeping host-facing interfaces passive.
  • Describe and verify the OSPFv2 neighbor states: Down, Init, 2-Way, ExStart, Exchange, Full.
  • Diagnose an OSPF adjacency failure due to a duplicate router-id and restore to Full.
  • Validate the routing table and OSPF LSDB contents after correcting the fault.

Troubleshooting focus

  • Identify OSPF neighbor state stalls (Init, 2-Way, ExStart) and causes.
  • Detect and correct duplicate/misconfigured router-ids that block adjacency.
  • Use targeted show and debug commands to confirm LSDB sync and route installation.

Topology

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How this lab is graded
  • Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with access-class, are stated in the guide and do have to match.
  • Addresses, modes and protocol keywords are exact. An IP address, a subnet mask, switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide.
  • Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the running-config — a change that only exists in a terminal session never reaches the grader.
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