Class-based weighted fair queuing (CBWFQ) builds upon WFQ by adding the concept of traffic
classes. Classes can be defined by a tag within the frame such as type of service (ToS) or differentiated
services code point (DSCP). These tags are added by the end station or the access router
and are used to forward packets through the network core without each router re-examining the
packet to determine that datagram’s priority. We are not defining the methodology used but
simply explaining the fact that you can use this information for CBWFQ.
Common implementations of CBWFQ establish three or four classes of application services,
typically described as gold, silver, bronze, and other. This categorization does not include network
traffic such as routing updates, which should always have priority over user application traffic.
Although some users will take exception to their traffic being described as a low priority, the network
administrator needs to constrain the total number of classes to keep administration manageable
and negate a situation in which bandwidth is being managed to the bit via QoS policy.
One of the strongest benefits to CBWFQ is the ability to define a specific amount of bandwidth
to an application. For example, Financial Information Exchange (FIX) is a common financial systems
protocol that might warrant special attention. Perhaps this application requires a guaranteed
256Kbps to prevent application failure on a T-1 link. CBWFQ can provide this guarantee, and,
perhaps more importantly, will allow the application to use more than the 256Kbps if bandwidth
is available. This is different from CAR, discussed in the next section, which establishes a hard
limit on the bandwidth available to a specific protocol. Please note that by default you cannot allocate
more than 75 percent of the link’s total bandwidth for management by CBWFQ.
With regard to traffic classes, the model is fairly straightforward. When congestion occurs,
the queue will process packets in the gold class before those in the silver class within the constraints
of WFQ. As such, the administrator is defining that the queue should be fair to all applications,
but that gold traffic is the most important. This will lead to the managed unfairness that
is the basis for all QoS policies; under congestion, the network will have to discard something
to stay within the available resources.
ToS and DSCP are not commonly accepted from end nodes because many
applications and some operating systems will automatically tag all packets for
the highest priority. It is recommended that you configure your edge routers to
ignore the end station and tag based on address or port information.