For Network Engineers, the ability to mirror switch traffic and send it to a sniffer for analysis is an essential troubleshooting technique. All Cisco Catalyst switches support the Switched Port Analyzer (SPAN) feature which copies traffic from specified switch source ports or VLANs and mirrors this traffic to a specified destination switch port (SPAN port). Then, you can connect your PC having a sniffer tool (like WireShark) on the destination SPAN port to capture all mirrored traffic. The diagram below shows this:

To utilize the switch SPAN mirroring feature configure the following on the catalyst switch:

Configuration Example – Monitoring an entire VLAN traffic
c3750(config)#monitor session 1 source vlan 5
c3750(config)#monitor session 1 destination interface fastethernet 0/3

The configuration above will capture all traffic of VLAN 5 and send it to SPAN port fastethernet 0/3.

Configuration Example – Monitoring traffic from a specific interface
c3750(config)#monitor session 1 source interface fastethernet 0/1
c3750(config)#monitor session 1 destination interface fastethernet 0/2

The configuration above will capture all traffic from interface 0/1 and send it to SPAN port fastethernet 0/2.

Use the command show monitor session 1 to verify your configuration.

Related posts:

1. Cisco Router-on-a-stick with Switch
2. How to Configure VLANs on a Cisco Switch
3. How to configure a Cisco Layer 3 switch-InterVLAN Routing
4. IOS Packet Capture and Auto Upgrade
5. Cisco Switch Network Design
6. Deleting the VLAN Database from a Cisco Switch
7. Basic Cisco Switch Configuration