CCNA Foundations: OSPF Day 4 — Cost & Deterministic Path Selection

Archive lab

Build a 5-node triangle OSPF topology with a branch and a data center connected by two paths: a direct link and an indirect path via a core router. Implement OSPFv2 with clean hygiene (router-ids, passive on LANs), then influence path selection by adjusting interface cost so branch-to-DC traffic prefers the core path. Verify the chosen path from the end hosts using traceroute and confirm symmetric routing by validating the reverse path.

Learning objectives

• Configure OSPFv2 with proper hygiene: router-id, passive-interface default, and exact network statements.
• Create dual-path reachability between two sites and understand OSPF path selection.
• Manipulate OSPF interface cost to achieve deterministic, intended routing behavior.
• Verify forwarding from hosts with traceroute and correlate hops to the routing table.
• Troubleshoot common OSPF adjacency and path-selection issues (area mismatch, passive on transit, missing networks).

Troubleshooting focus

• Verify every link's interface status and IP addressing (`show ip interface brief`) and correct any interface that is down or misaddressed.
• Confirm each device's running configuration matches the lab objective, and bring any interface, route, or service that is not in its expected state back to health.
• Trace end-to-end reachability with ping/traceroute and isolate the first hop where forwarding fails.