1.2.a Evaluate proposed changes to a network

1.2.a [vi] Evaluate impact of new traffic on existing QoS design

There can at least be two major scenarios that you need to keep in mind when introducing new traffic to an existing QoS design or implementation.

New traffic could just go to default-class which may be totally undesired

New traffic ends up matching an existing class causing a mix of transport types in a given class. For example, if new traffic is UDP and existing traffic is TCP, it could lead to TCP starvation (also known as UDP dominance).

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1315-1321).  . Kindle Edition.