Chapter 10 Install and System Administration for FortiOS 5.0 : VLANs : Troubleshooting VLAN issues
  
Troubleshooting VLAN issues
Several problems can occur with your VLANs. Since VLANs are interfaces with IP addresses, they behave as interfaces and can have similar problems that you can diagnose with tools such as ping, traceroute, packet sniffing, and diag debug.
See Also
Asymmetric routing
Layer-2 and Arp traffic
Forward-domain solution
NetBIOS
STP forwarding
Too many VLAN interfaces