A server is only able to connect to a gigabit switch at 100Mb.

The first thing that the administrator should check is the network cable. The network cable is a physical medium that connects a server to a switch or other network device. The network cable can affect the speed and quality of the network connection, depending on its type, length, and condition. If the network cable is damaged, faulty, or incompatible, it can cause the server to connect at a lower speed than expected. Therefore, the administrator should check the network cable for any signs of wear, tear, or mismatch, and replace it if necessary.