An Example of a Class of Service Mapping
Class of Service IP Precedence
Platinum (Typically Voice Traffic) 5
Gold 4
Silver 3
Bronze 2
Iron 1
Best-Effort (default) 0
In this example, we can realize the economy of using CBWFQ within the
DiffServ model. Using packet classification at the edge of the network to mark IP
precedence, we have effectively divided all our applications into five classes of service,
plus a default class. Except for the edge devices, no other classification may
be necessary to place a packet into the proper queue as it traverses the network.
By marking applications at the edge and allowing internal routers to queue
packets according to these classes, we not only assure consistent QoS for that
application across the entire network, but we also reduce the resource load on
both the routers and the network administrator.The routers do not have to process
lengthy ACLs at every hop, and the administrators have to worry about classification
only at the edge of the network. Additionally, it is at these edge devices
that packet rates are the smallest, and processor utilization according to packet
marking is manageable.To classify packets at the hub site where many circuits are
being aggregated might be too much for the router to handle.
NOTE
Remember that QoS is never a substitute for bandwidth. On the other
hand, even a gigabit link can drop packets if the queues fill up.
Congestion management rations the limited bandwidth to the most
important applications, or in the case of CBWFQ, ensures that certain
applications get at least the percentage of total bandwidth allocated.
The important point here is that QoS mechanisms will help prioritize
traffic on a congested link (and drop the least important traffic first and
most often) but, at some point, a link may become so congested that
packet drops reach an unacceptable level. When this point is reached, a
bandwidth upgrade is in order.