The IEEE 802.1p standard defines a class of service (CoS) value (ranging from 0-7) that is included in the Ethernet frame. The Internet Protocol defines the layer-3 QoS values that are carried in the IP packet (Differentiated Services, IP Precedence). The FortiSwitch unit provides configurable mappings from CoS or IP-DSCP values to egress queue values.
Fortinet recommends that you do not enable trust for both Dot1p and DSCP at the same time on the same interface. If you do want to trust both Dot1p and IP-DSCP, the switch uses the latter value (DSCP) to determine the queue. The switch will use the Dot1p value and mapping only if the packet contains no DSCP value. For details, refer to Determining the egress queue .