Port Mirroring

What is Port Mirroring?

Port mirroring is a network observation method aiming to monitor network traffic. When configuring a switch, reserve a designated port and configure the switch to mirror all traffic passing through the selected port. All packets received and sent on the specified port will be copied and sent to the mirror destination port. The destination port is connected to a data monitoring device, which can diagnose and analyze the mirrored data and troubleshoot problems. The switch can mirror single or multiple source ports to a destination port; that is, the packets from one or more mirrored source ports are copied to a single destination port and then forwarded to the monitoring device. This way, the traffic of one or a group of ports can be observed using only one switch port.

Port mirroring means that the source of mirroring is the port, and other mirroring types include flow mirroring, VLAN mirroring, MAC mirroring, etc. In short, their differences are:

Port mirroring copies all packets in and out of the source port and forwards them to the destination port for observation. Flow mirroring filters the packets in and out of the source port and copies and forwards the matched specific packet stream to the destination port for observation. VLAN mirroring is to copy all packets in and out of the specified VLAN and forward them to the destination port for observation. MAC mirroring is to copy the message designated as the specified MAC address (source MAC address or destination MAC address) in the specified VLAN and forward it to the destination port for observation.

What is the Source Port and Destination Port?

Source port: This port is designated as the monitored message. All traffic passing through it will be copied to the destination port.

Destination port: The destination port is also called the observation port. After receiving the copy of the source message, the observation port sends it to the monitoring device to monitor the data.

How Does Port Mirroring Work?

When the destination port and the monitoring device are directly connected, it is called local port mirroring. The working principle of local port mirroring is shown in the figure below: When the source port of switch S7300-48TE4X2Q receives or sends a message, it creates a copy and sends it to the destination port. The destination port forwards the copied original message to the monitoring device and performs traffic monitoring on the monitoring device.

When the destination port and the monitoring device are not directly connected but forwarded between the destination port and the monitoring device via a Layer 2 or Layer 3 network, this type of port mirroring is remote port mirroring. As shown in the figure, after receiving the copy of the source message sent by the source port, the destination port encapsulates the copy to enable it to pass through the Layer 2 or Layer 3 network smoothly and reach the monitoring end. When the destination port is connected to the monitoring device via the Layer 2 network, it is a Layer 2 remote port mirroring; when the destination port is connected to the monitoring device via the Layer 3 network, it is a Layer 3 remote port mirroring.

What are the Functions of Port Mirroring?

Network troubleshooting

Engineers can quickly locate the cause of network failures, such as packet loss, congestion, and configuration errors.

Traffic analysis and optimization

Real-time traffic monitoring enables network administrators to learn traffic patterns and application behaviors, thus optimizing network performance.

Security monitoring and threat detection

In real time, network administrators can detect abnormal behaviors and potential threats, such as intrusions or malware propagation.

Network planning and capacity assessment

In network expansion, the traffic data collected by port mirroring can help rationally plan resource allocation.

Application performance monitoring

Port mirroring can also monitor application traffic and performance to promptly detect and resolve performance bottlenecks.