Ethernet Provides Quality of Service
It's perhaps not so well known that Ethernet can provide more than just a best-effort broadcast LAN service. Instead, individual Ethernet frames, the term used to describe Ethernet protocol data unitsin other words, the messages exchanged by Ethernet entitiescan be marked to receive a specific service level from the network. The IEEE 802.1p standard provides the mechanism in the form of three priority-related bits in the Ethernet MAC header. This setup is illustrated in Figure 2, where two Ethernet frames arrive at the service provider network edge (for example, PE1 in Figure 1). Because each frame has been marked to receive specific handling inside the network, they're forwarded in the reverse order of arrival: The VoIP frame is forwarded first, ahead of the non-time-critical data frame, in accordance with the required priority for these frames.
Figure 2 Priority-based frame handling.
Many other elements associated with QoS, such as policing, queue handling, egress scheduling, and so on can be provided either by (layer 3) DiffServ or other proprietary technology. In this context, the three priority bits give us a broad-brush layer 2 QoS mechanism. The layer 3 technologies can be used to improve the QoS experience. Let's take a look at the important area of layer 2 (Ethernet) VPNs.