The STP (Spanning Tree Protocol) discarding state on port1 of Core-2, after Core-1 and Access-1 are managed and authorized by FortiGate-1, is likely due to the lack of an MCLAG (Multi-Chassis Link Aggregation Group) configuration between Core-1 and Core-2. In typical network configurations involving STP and MCLAG, the absence of MCLAG can lead to STP blocking one of the redundant paths to prevent loops, which is a critical function of STP. Port1 on Core-2 being in a discarding state suggests that it has been identified as providing a redundant path that could potentially create a network loop, hence STP has placed this port in a blocking (discarding) state to maintain a loop-free topology.
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For a deeper understanding of STP operations and MCLAG configurations in FortiGate managed environments, consult the Fortinet knowledge base:Fortinet Knowledge Base.