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IntermediatePublished 2026-07-08

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

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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.

Track
Est. time
55 minutes
Difficulty
Intermediate
Topics

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

  • show ip ospf neighbor is missing a transit link: confirm both ends are in area 0, the interface is not passive, and subnets match. EXSTART/EXCHANGE points at MTU or timers.
  • show ip route 10.20.20.0 on RTR-BRANCH still points at 10.0.13.2: the direct Branch-DC cost is not above the sum via the Core. Raise both ends; clear ip ospf process drops stale routes.
  • HOST-BRANCH cannot reach 10.10.10.1: verify the host IP, mask and default route, and the LAN interface is up/up.

Topology

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Grade your work

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.
  • You can submit as many times as you like. Your best score stands, and each attempt tells you which checks passed so you can work the gaps.
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