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IntermediatePublished 2026-07-07

CCNA Foundations: OSPF Day 3 — DR/BDR Election on a Broadcast Segment

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Three IOS routers share a true broadcast multi-access segment through a Layer-2 switch. You will deploy OSPFv2, influence the DR/BDR election using interface priorities, and verify full adjacencies to the DR. An alpine client behind RTR-HQ-R3 must reach loopbacks on RTR-HQ-R1 and RTR-HQ-R2 via OSPF-learned routes. The lab emphasizes proper multi-access deployment hygiene (router-ids, passive default, interface selection) and realistic verification from the end host.

Track
Est. time
45 minutes
Difficulty
Intermediate
Topics

Learning objectives

  • Configure OSPFv2 on a broadcast multi-access segment and understand DR/BDR roles
  • Influence DR/BDR election using ip ospf priority on router interfaces
  • Apply OSPF hygiene: stable router-ids, passive-interface default, selective no passive on transit links
  • Verify adjacencies and designated router selection; validate end-to-end reachability from a host

Troubleshooting focus

  • Two DROTHERs stay 2-WAY with each other by design.
  • show ip ospf interface Ethernet0/0 names the wrong DR or BDR: elections are non-preemptive. Change the priorities, then clear ip ospf process or bounce the transit interface to force a new election.
  • CLIENT10 cannot ping 10.255.0.1 or 10.255.0.2: verify CLIENT10's address and default gateway 10.30.30.1, and that RTR-HQ-R3 advertises 10.30.30.0/24 into OSPF while the user LAN interface stays passive.

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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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