What is preventing the server from seeing all network traffic when hooked up to a mirrored port?

When a network interface card (NIC) is set to promiscuous mode, it allows the NIC to receive all traffic that passes over the network segment, regardless of the intended destination address. This is especially important when monitoring network traffic, such as when using a mirrored port on a switch.

If the NIC is not in promiscuous mode, it will only capture packets that are specifically addressed to its MAC address or broadcast packets. This means that if a server is connected to a mirrored port but the NIC is not configured to operate in promiscuous mode, it will fail to see all of the traffic that is being mirrored. This behavior directly explains why the server is unable to view all network traffic, as it limits the traffic that the NIC can process.

Other configurations, such as NIC filtering or port security, typically restrict traffic in a different manner, potentially blocking specific types of packets or limiting access based on certain criteria, but the fundamental requirement for capturing all mirrored traffic is enabling promiscuous mode on the NIC.