OSPF Cost & Path Selection: Deterministic Control (3 Routers)

Bundle lab

Build a 3-router triangle with two end-to-end paths between Branch A and the Data Center. Stand up OSPF area 0, create an ECMP condition, then intentionally steer traffic using ip ospf cost. Verify from the end hosts and troubleshoot common OSPF control-plane mismatches.

Learning objectives

• Configure OSPF area 0 on addressed transit and LAN interfaces while keeping LANs passive
• Establish OSPF adjacencies and verify with neighbor and route tables
• Create ECMP and then prefer a specific path using interface costs
• Validate path selection and failover from end hosts using ping and traceroute
• Troubleshoot OSPF neighbor and route issues caused by passive-interface, area, or cost misconfiguration

Troubleshooting focus

• A neighbor sits in INIT or 2WAY: verify both sides are in area 0, not passive on that transit, and share hello/dead timers and MTU.
• No neighbor forms on a link at all: check the interface is un-passived, cabled, addressed in the same /30, and up/up.
• A remote LAN is missing from the routing table: it must be advertised with the correct wildcard mask, and its interface must be up.
• Only one path to 10.30.30.0/24 where you expect ECMP: the direct link's cost must equal the two-hop sum from each side. Cost is per egress interface — set it on both ends.
• traceroute from CLIENT-A01 still crosses the direct link after you raise its cost: confirm ip ospf cost is on Ethernet0/1 of both RTR-A-BR and RTR-C-DC, then re-read the route table.