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Figure 2. Example of a VLAN
Traffic Prioritization and Class of Service (CoS)
Not all traffic is equal. A financial company, for example, might place greater importance on
financial transactions and real-time production data than on other less-critical traffic. Realizing
this concern, the IEEE recently ratified its 802.1p specification that defines the standards for
traffic prioritization.
Traffic prioritization gives preferential treatment to critical data, enabling the network to speed
the delivery of prioritized data to its destination. In fact, the IEEE 802.1p specification defines
eight levels of priority to be defined, according to business needs.
Traffic prioritization is especially useful for:
Converged network applications
organizations that use the same network infrastructure
for voice and data may require a higher priority for video and audio traffic to ensure jitter-
free video and audio, while relegating Web-browsing video to a lower priority.
Data-intensive applications
where users need priority connections to server farms and
other devices to transfer bandwidth-intensive files.
Financial applications
where the accounting staff needs instant access to large files and
spreadsheets at critical times of the month.
While the IEEE specification has been a long-awaited advancement for switch vendors and
users alike, there is a catch: all the applications and devices along a data flow's path must also
support CoS to meet the goal of providing end-to-end CoS. This means that, besides switches,
devices that must work together can include hosts (clients and servers), routers, and firewalls.
Any CoS policy will only be as good as the weakest link in that very lengthy chain. For this
reason, complete end-to-end CoS implementations are difficult to achieve and often come at a
steep price.
