Guide to Configuring Port Mirroring on Cisco Switches

Port mirroring, also known as SPAN (Switched Port Analyzer) on a Cisco switch, allows network administrators to monitor traffic by copying packets from one or more source ports to a destination port. This feature is essential for network analysis, troubleshooting, and security monitoring. Follow this comprehensive guide to configure port mirroring on a Cisco switch.

Steps to Configure SPAN on a Cisco Switch

Access the Switch CLI

To begin, you need to access the Cisco switch's Command Line Interface (CLI).

1. Connect to your Cisco switch using a terminal emulator like PuTTY via an SSH console cable or Telnet.

enable

configure terminal

Define the SPAN Session

Specify a SPAN session. You can create multiple sessions, but for basic setups, using session 1 is common.

monitor session 1 source interface source_interface

Replace source_interface with the port you want to monitor, such as GigabitEthernet0/1 (or another appropriate interface).

If you want to monitor multiple interfaces, use the following command:

monitor session 1 source interface source_interface_1

monitor session 1 source interface source_interface_2

Set the Destination Interface

Specify the destination interface where the mirrored traffic will be sent.

monitor session 1 destination interface destination_interface

Replace destination_interface with the port where your monitoring device, like a packet sniffer, is connected.

Verify the Configuration

To confirm the settings, you can use the following command:

show monitor session 1

Exit Configuration Mode

Once you have verified the settings, you can exit configuration mode:

end

Save the Configuration

To ensure the configuration persists after a reboot, save it:

write memory

Example Configuration

This example demonstrates mirroring traffic from GigabitEthernet0/1 to GigabitEthernet0/2.

enableconfigure terminalmonitor session 1 source interface GigabitEthernet0/1monitor session 1 destination interface GigabitEthernet0/2endwrite memory

Important Considerations

Traffic Type

SPAN can mirror traffic from ingress (incoming) or egress (outgoing) traffic. By default, it mirrors both.

Performance Impact

Be cautious with the number of source interfaces and the amount of traffic being mirrored, as it can impact switch performance.

Destination Port

The destination port should not be part of the source interfaces being monitored. This is to avoid loops and ensure accurate monitoring.

This configuration allows you to effectively monitor traffic on your network for analysis and troubleshooting.