In the previous chapter, we discussed the QOS toolkit that is available as part of a QOS deployment on a router. We now move on, leaving behind the perspective of an isolated router and considering a network-wide QOS deployment. Such deployments always have peculiarities depending on the business requirements, which make each single one unique. However, the challenges that are likely to be present across most deployments are the subject of this chapter.

Within a QOS network and on each particular router in the network, multiple traffic types compete for the same network resources. The role of QOS is to provide each traffic type with the behavior that fits its needs. So the first challenge that needs to be considered is how providing the required behavior to a particular traffic type will have an impact and will place limits on the behavior that can be offered to the other traffic types. As discussed in Chapter 2, the inherent delay in the queuing and scheduling operation can be minimized for traffic that is placed inside a particular queue. However, that is achieved at the expense of increasing the delay for the traffic present in other queues. The unavoidable fact that something will be penalized is true for any QOS tool that combines a greater number of inputs into a fewer number of outputs.

This description of QOS behavior can also be stated in a much more provocative way: a network in which all traffic is equally “very important and top priority” has no room ...

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