802.1p
802.1p is a specification for giving Layer 2 switches the ability to prioritize traffic (and perform dynamic multicast filtering).
The prioritization specification works at the media access control (MAC) framing layer of the OSI model. To be compliant with 802.1p, Layer 2 switches must be capable of grouping incoming LAN packets into separate traffic classes.
Eight classes are defined by 802.1p. Although network managers must determine actual mappings, IEEE has made broad recommendations. The highest priority is seven, which might go to network-critical traffic such as Routing Information Protocol and Open Shortest Path First table updates.
Values five and six might be for delay-sensitive applications such as interactive video and voice. Data classes four through one range from controlled-load applications such as streaming multimedia and business-critical traffic - carrying SAP data, for instance - down to "loss eligible" traffic. The zero value is used as a best-effort default, invoked automatically when no other value has been set.
In operation, 802.1p calls for the use of priority fields within the packet to signal the switch of the priority-handling requirements.
Additional resources
Connectionless QoS
Discusses the use of 802.1p. Network World on Convergence, 11/19/01.
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