Network & Infrastructure / OSPF & Underlay Convergence
OSPF ECMP reconvergence and transient microloops
During a topology change, traffic briefly loops between routers even though their final routing tables are correct. Independent recalculation and installation can create a temporary inconsistent forwarding graph.
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
Collect time-aligned hop observations, TTL-expired evidence, topology events and FIB installation timestamps. Distinguish a brief convergence loop from a persistent redistribution or next-hop recursion defect.
Explain the result
Routers do not necessarily update forwarding simultaneously. A transient mixture of old and new shortest paths can loop before all nodes reach the same final view.
Plan the fix
Evaluate supported ordered convergence or fast-reroute mechanisms and topology choices against the required loss budget. Avoid claiming ECMP alone guarantees loop-free recovery.
Check the result and rollback
Run controlled single-link and node failures with short-interval application probes. Revert if a mitigation adds excessive state, worsens steady-state paths or fails to reduce the measured outage interval.