IEEE 802.1p is a standard that provides traffic class expediting and dynamic multicast filtering. Essentially, it provides a mechanism for implementing Quality of Service (QoS) at the MAC (Media Access Control) level.
Eight different classes of service are available, expressed through the 3-bit user_priority field in an IEEE 802.1Q header added to the frame. The way traffic is treated when assigned to any particular class is undefined and left to the implementation. The IEEE however has made some broad recommendations.
802.1p is used within the IEEE 802.1D and IEEE 802.1Q standards.
Reference:
http://en.wikipedia.org/wiki/IEEE_802.1p
