8021p

IEEE 802.1p is an amendment within the IEEE 802.1 family that defines a basic mechanism for class-of-service at the Ethernet frame level. It provides eight traffic priority levels for frames carried over VLAN-tagged Ethernet, enabling prioritized handling in switches and bridges.

The standard specifies the 3-bit priority code point (PCP) field in the 16-bit Tag Control Information (TCI) portion of the 802.1Q VLAN tag. The TCI also includes a Drop Eligible Indicator (DEI) bit and the VLAN ID. PCP values range from 0 to 7, corresponding to different traffic classes, while DEI marks frames that may be dropped under congestion.

Network devices implement queuing and scheduling that map PCP values to internal queues. Higher PCP values typically receive more favorable treatment during congestion, while the DEI bit indicates frames eligible for dropping if resources are scarce. The standard itself does not prescribe exact queueing algorithms; the implementation defines how PCP-based prioritization is realized.

802.1p provides a data-link layer QoS mechanism that is often used in combination with IP QoS mechanisms such as Differentiated Services (DSCP). Operators may map IP DSCP values to 802.1p PCP levels to achieve end-to-end QoS across heterogeneous networks, aligning Layer 2 and Layer 3 prioritization.

History and scope: 802.1p is an amendment to the 802.1Q framework introduced to support QoS on LANs. It is widely supported in enterprise and data-center equipment and remains a foundational method for VLAN-based QoS, though it does not guarantee specific latency or jitter and depends on path-wide support.