Comprehensive Detailed Explanation with all CompTIA Server+ SK0-005 Study Guide References:The most likely cause of a speed mismatch (100Mbps instead of 1Gbps) is a defective or incompatible network cable. Gigabit Ethernet requires at least Cat5e or Cat6 cables; using older cables (e.g., Cat5) or damaged cables can limit speeds to 100Mbps. * Port securityandVLAN misconfigurationwould typically prevent communication entirely, not reduce speed. * Incorrect IP subnetdoes not affect link speed.Checking and replacing the network cable is a logical first troubleshooting step in this scenario.