OSPF Inter-Area Summarization with ABR Area Range

Bundle lab

Build a 5-node CML lab to design OSPFv2 across two areas with an ABR that summarizes non-backbone routes using area range. Validate adjacencies, end-to-end reachability, and that the backbone learns a single summary in place of specific inter-area routes. Intentionally introduce an area mismatch, diagnose why the adjacency does not form, and repair it.

Learning objectives

• Design and deploy a multi-area OSPFv2 topology with stable router-ids and passive-interface hygiene.
• Implement ABR summarization using the area range command to aggregate non-backbone prefixes into a single summary.
• Verify that specific inter-area routes are replaced by a single summary in the backbone while reachability to component subnets remains intact.
• Perform end-to-end testing from hosts across multiple areas and confirm symmetric forwarding.
• Troubleshoot an intentional OSPF area mismatch and validate adjacency recovery.

Troubleshooting focus

• If adjacencies fail: verify shared area IDs, identical MTUs, and that the transit ports are not passive. Area mismatch is the most common cause—fix the area assignment on the offending interface.
• If hosts can’t reach each other: confirm routing tables contain inter-area routes (or the 10.10.0.0/16 summary) and that no interface is down. Use traceroute to find the first hop where forwarding fails.
• If the backbone still shows specific /24s instead of a /16: confirm the ABR area range is applied under the correct area (Area 10) and that the component routes actually exist in Area 10.