We will discuss two issues in this recipe:
- How to monitor traffic inside a VLAN?
- How to view tagged frames going through a VLAN-tagged port?
In the first case, a simple configuration is required. In the second case, there are some points to take care of.
While capturing on a VLAN, you won't necessarily see the VLAN tags in packets. The question of whether you will see the VLAN tags actually depends on the operating system you are running, and whether your Network Interface Card (NIC) and the NIC driver support this feature.
The question of whether your OS and NIC supports VLAN tagging entirely depends on the OS and the NIC vendor. Go to the vendor's manuals or Google to find out.
In the following diagram, you can see a typical topology with VLANs. The upper switch is connected by two trunks (these are ports that tag the Ethernet frames) to the lower switches. On this network, you have VLANs 10, 20, and 30, while PCs connected to each of the VLANs will not be able to see PCs from other VLANs.
Figure 8.14: VLAN tagging