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There are some tricky scenarios when capturing data that you should be aware of.

Monitoring a VLAN—when monitoring a VLAN, you should be aware of several important issues. The first issue is that even when you monitor a VLAN, the packet must physically be transferred through the switch you are connected to in order to see it. If, for example, you monitor VLAN-10 that is configured across the network, and you are connected to your floor switch, you will not see traffic that goes from other switches to the servers on the central switch. This is because in building networks, the users are usually connected to floor switches, in single or multiple locations in the floor, that are connected to the building central switch (or two redundant switches). For monitoring all traffic on a VLAN, you have to connect to a switch on which all traffic of the VLAN goes through, and this is usually the central switch:

In the preceding diagram, if you connect Wireshark to Switch SW2, and configure a monitor to VLAN30, you will see all packets coming in and out of P2, P4, and P5, inside or outside the switch. You will not see packets transferred between devices on SW3, SW1, or packets between SW1 and SW3.

Another issue when monitoring a VLAN is that you might see duplicate packets. This is because when you monitor a VLAN and packets are going in and out of the VLAN, you will see the same packet when it is coming in and going out of the VLAN.

You can see the reason in the following illustration. When, for example, S4 sends a packet to S2 and you configure the port mirror to VLAN30, you will see the packet once sent from S4 to the switch and entering the VLAN30, and then when leaving VLAN30 to S2: