Explanation/Reference:
Section: Considerations for Expanding an Existing Network Explanation
Explanation:
You should implement Quality of Service (QoS) in the core, distribution, and access layers. A network can become congested due to the aggregation of multiple links or a drop in bandwidth from one link to another.
When many packets are sent on a congested network, a delay in transmission time can occur. Lack of bandwidth, end-to-end delay, jitter, and packet loss can be mitigated by implementing QoS. QoS facilitates the optimization of network bandwidth by prioritizing network traffic based on its type. Prioritizing packets enables time-sensitive traffic, such as voice traffic, to be sent before other packets. Packets are queued based on traffic type, and packets with a higher priority are sent before packets with a lower priority.
Because the access layer provides direct connectivity to network endpoints, QoS classification and marking are typically performed in the access layer. Cisco recommends classifying and marking packets as close to the source of traffic as possible and using hardware-based QoS functions whenever possible.
Although classification and marking are typically performed in the access layer, QoS mechanisms must be implemented in each of the higher layers for QoS to be effective.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 3, Campus LAN QoS Considerations, pp. 111-112 Cisco: Campus Network for High Availability Design Guide: General Design Considerations