Network & Infrastructure / OSPF & Underlay Convergence
OSPF reference bandwidth inconsistencies across faster links
Two routers assign different costs to equal-speed links after a partial bandwidth-policy update. Some high-speed links may also collapse to the same minimum cost under an old reference value.
Environment and inputs: OSPFv2 IPv4 underlays, with OSPFv3 explicitly identified where used; timers and diagnostics vary by platform. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Record reference bandwidth, explicit interface costs and computed path totals across the entire routing domain. Include slower backup circuits and logical interface bandwidth values.
Explain the result
OSPF sums advertised costs; each router originates its own link values. Inconsistent reference settings can create asymmetric preferences even when interface speeds appear identical.
Plan the fix
Choose a consistent cost model with deliberate exceptions and rollout ordering. Evaluate all intended primary and backup paths rather than changing a single core switch's reference value.
Check the result and rollback
Compare shortest paths from multiple routers and test a link failure. Restore the previous model if traffic shifts exceed capacity or a backup becomes preferred in only one direction.