Page 452 of 7.6 study guide, specifically states " Although you can use the sniffer command to capture traffic on switch ports, the types of packets capture by the sniffer are very limited.
The use of the sniffer command on FortiSwitch CLI can be unreliable on port 23 for specific reasons related to the nature of traffic on the port:
D).The switch port might be used as a trunk member.When a switch port is configured as a trunk, it can carry traffic for multiple VLANs. If the sniffer is set up without specifying VLAN tags or a range of VLANs to capture, it may not accurately capture or display all the VLAN traffic due to the volume and variety of VLAN- tagged packets passing through the trunk port. This limitation makes using the sniffer on a trunk port unreliable for capturing specific VLAN traffic unless properly configured to handle tagged traffic.
References:
For guidelines on how to properly use sniffer commands on trunk ports and configure VLAN filtering, consult the FortiSwitch CLI reference available through Fortinet support channels, including theFortinet Knowledge Base.