Router configuration errors are unfortunately common and difficult to localize using current network verifiers. We introduce a novel configuration error localizer (CEL) that precisely identifies which configuration segments contribute to the violation of forwarding requirements. In particular, CEL generates a system of satisfiability modulo theories (SMT) constraints-which encode a network's configurations, control logic, and forwarding requirements-and uses a domain-specific minimal correction

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... set (MCS) enumeration algorithm to identify problematic configuration segments. CEL efficiently locates several configuration errors in real university networks and identifies all routing-related and at least half of all ACL-related errors we introduce.