Hello Gustavo
First of all, keep in mind that IP precedence and DSCP values are those found within the appropriate field in the IP header. The 802.1Q priority field is part of the VLAN tag that is found in the Layer 2 Ethernet header. These tags are only present for frames traversing a trunk link. Each of these operates at a different layer of the OSI model.
Now both of these features can be used in QoS mechanisms. For example, the Class of Service or CoS can be derived from the 802.1Q priority field. The CoS can then be used to classify frames traversing a trunk. The frames arriving at a trunk port can then be queued and prioritized accordingly.
For the DSCP and IP precedence values, these can be used to classify and mark IP packets that can then subsequently be used by QoS mechanisms to ensure the correct prioritization.
Because DSCP/IP precedence and 802.1Q priorities are constructs that belong to different layers, and are applied and used separately, there is no direct relationship between them. However, it is possible to create a “cos-to-dscp” or a “dscp-to-cos” mapping that will allow an 802.1Q priority field to adopt the priorities indicated in the DSCP of the IP header or visa versa. You can either use a default mapping or create your own. More about this can be found in the following lesson:
I hope this has been helpful!
Laz