Future communication networks are expected to be highly automated, disburdening human operators of their most complex tasks. While the first powerful and automated network analysis tools are emerging, existing tools provide only limited and inefficient support of reasoning about failure scenarios. We present P-REX, a fast what-if analysis tool, that allows us to test important reachability and policy-compliance properties even under an arbitrary number of failures and in polynomial-time, i.e., without enumerating all failure scenarios (the usual approach today, if supported at all). P-REX targets networks based on Multiprotocol Label Switching (MPLS) and its Segment Routing (SR) extension which feature fast rerouting mechanisms with label stacks. In particular, P-REX allows to reason about recursive backup tunnels, by supporting potentially infinite state spaces. As P-REX directly operates on the actual dataplane configuration, i.e., forwarding tables, it is well-suited for debugging. Our tool comes with an expressive query language based on regular expressions. We also report on an industrial case study and demonstrate that our tool can perform whatif reachability analyses on average in about 5 seconds for a 24-router network with over 250,000 MPLS forwarding rules. This is a significant improvement to an earlier prototype of our tool presented in the conference version of our paper where the verification took on average about 1 hour.

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