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WAN Availability and QoS

Chapter Description

This sample chapter from CCNP Enterprise Design ENSLD 300-420 Official Cert Guide, 2nd Edition covers the selection and use of WAN designs in enterprise networks in addition to QoS strategies and designing end-to-end QoS policies. This chapter covers topics from the ENSLD 300-420 exam.

QoS Strategies

Quality of service (QoS) is a fundamental network technology that has been around for over 20 years and is still relevant in today’s networks, even though bandwidth has been increasing rapidly over the years. QoS gives network operators techniques to help manage the contention for network resources and in turn provide better application experiences for end users. To help us with this, Cisco supports three main models for providing QoS service differentiation: best-effort (BE), Differentiated Services (DiffServ), and Integrated Services (IntServ). These three models are different in how they enable applications to be prioritized throughout the network and how they handle the delivery of data packets with a specified level of service.

Best-Effort QoS

The best-effort (BE) QoS model is typically the default QoS model and does not implement any QoS behaviors to prioritize traffic before other QoS traffic classes. This is the easiest of the three models because there is nothing you really need to do for it to work. You would not want to use best-effort QoS for any real-time applications such as voice or video traffic. It is a last-resort QoS model that you use after you have already prioritized all other important traffic classes that are sensitive to delay, jitter, and/or bandwidth within the network.


The DiffServ QoS model separates traffic into multiple classes that can be used to satisfy varying QoS requirements. A packet’s class can be marked directly inside the packet that classifies packets into different treatment categories.


With the DiffServ model, packets are classified and marked to receive a per-hop behavior (PHB) at the edge of the network. Then the rest of the network along the path to the destination uses the DSCP value to provide proper treatment. Each network device then treats the packets according to the defined PHB. The PHB can be specified in different ways, such as by using the 6-bit Differentiated Services Code Point (DSCP) setting in IP packets or by using ACLs with source and destination addresses.

Priorities are marked in each packet using DSCP values to classify the traffic according to the specified QoS policy for the traffic class. Typically, the marking is performed per packet at the QoS domain boundaries within the network. Additional policing and shaping operations can be implemented to enable greater scalability.

Table 9-6 maps applications to DSCP and decimal values.


Table 9-6 DSCP Mapping Table



Decimal Value

Network control



Internetwork control






Broadcast video



Multimedia conferencing



Real-time interaction



Multimedia streaming






Transactional data



Network management



Bulk data










The IntServ QoS model was designed for the needs of real-time applications such as video, multimedia conferencing, and virtual reality. It provides end-to-end QoS treatment that real-time applications require by explicitly reserving network resources and giving QoS treatment to user packet flows. The IntServ model applications ask the network for an explicit resource reservation per flow and use admission control mechanisms as key building blocks to establish end-to-end QoS throughout the network.


IntServ uses Resource Reservation Protocol (RSVP) to explicitly request QoS for the application along the end-to-end path through devices in the network. Before an application begins transmitting, it requests that each network device reserve the necessary bandwidth along the path. The network, in turn, accepts or rejects the reservation per flow based on available network resources.

IntServ requires several functions on each of the routers and switches between the source and destination of the packet flow:

  • Admission control: Determines whether the requested flows can be accepted without impacting existing reservations

  • Classification: Identifies traffic that requires different levels of QoS

  • Policing: Allows or drops packets when traffic does not conform to the QoS policy

  • Queuing and Scheduling: Forwards traffic for permitted QoS reservations

8. Designing End-to-End QoS Policies | Next Section Previous Section

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