Quality of service

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

Quality of service is the classification, marking, queuing, and scheduling of network traffic to manage performance characteristics such as delay, jitter, bandwidth, and loss.

Quality of service (QoS) prioritizes important traffic during congestion and de-prioritizes less important traffic. It protects delay-sensitive applications such as voice and video when bandwidth is limited, reducing delay, jitter, and packet loss. section 10.0 section 10.2.2

Use these terms to describe QoS behavior:

• Classify packets by traffic type, such as real-time, business-critical, default, or scavenger traffic. section 10.3.1
• Mark packets so later devices can classify them efficiently from header fields rather than repeating complex inspection. section 10.3.2
• Queue packets when they arrive faster than an interface can transmit them, then schedule their transmission order. section 10.2.3 section 10.4
• Apply scheduling methods such as Priority Queuing (PQ), Class-Based Weighted Fair Queuing (CBWFQ), or Low Latency Queuing (LLQ). section 10.2.3 section 10.4

Recognize the principal marking mechanisms:

• Use DSCP in IPv4 or IPv6 headers; its six bits provide 64 values and carry end to end unless modified. section 10.3.3
• Use PCP, also called CoS, in the three-bit 802.1Q tag; voice commonly uses decimal 5, while best effort uses 0. section 10.3.2
• Recognize standardized DSCP examples: DF is decimal 0 for best effort, and EF is decimal 46 for delay-, jitter-, and loss-sensitive traffic. section 10.3.3

Separating voice and data on an access port

Configure a PC and IP phone to share a switch port while keeping their traffic in separate VLANs: interface range f0/1-3 switchport mode access switchport access vlan 10 switchport voice vlan 20 The data VLAN is untagged, the voice VLAN is tagged, and the port remains an access port. Verify the result with show interfaces f0/1 switchport; the output should show Access Mode VLAN: 10 and Voice VLAN: 20. section 10.1.1

Figure 10.7 The QoS big picture. A router receives packets and makes a forwarding decision for each, classifies the packets based on their type, queues them for transmission, and uses scheduling methods to determine the order of transmission.

Trace the QoS pipeline from forwarding through classification and separate queues to scheduled transmission. section 10.2.3

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes

Explains classification, marking, queuing, and scheduling, but stops short of detailed policy configuration and wireless QoS internals.

Examples

Uses IP telephony, Zoom calls, file downloads, voice VLAN configuration, DSCP and PCP markings, and FIFO queue behavior.

What it assumes

Assumes basic VLANs, access ports, Ethernet framing, IP headers, and congestion while defining QoS terminology explicitly.

What you can do after

Configure and verify a voice VLAN, and evaluate how QoS classifies, marks, queues, and schedules congested traffic.

How it connects

Connects QoS with VoIP, voice VLANs, DSCP, PCP, ACLs, NBAR, congestion, queuing, and scheduling.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 2 Advanced Networking and Security on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  Acing the CCNA Exam: Volume 1 Fundamentals and ProtocolsQuality of Service is the classification and prioritization of network traffic to provide differentiated service levels.

Compare how 2 books explain quality of service →

Related concepts

• markinghas part · Marking is the process of setting packet or traffic header fields to encode classification information for network policy enforcement.
• classifierhas part · A classifier is a predictive computational model that assigns inputs to one or more discrete classes or categories.
• differentiated services code pointuses · The Differentiated Services Code Point is a six-bit field in an IP header that identifies a packet’s desired forwarding treatment.
• meanaddresses · A mean is a measure of central tendency, calculated as the arithmetic average of values or, for a random variable, as the probability-weighted average of its possible values.
• class-based weighted fair queuinguses · Class-Based Weighted Fair Queuing is a packet-scheduling technique that assigns traffic classes to queues and guarantees each queue a configured minimum share of bandwidth during congestion.
• low latency queuinguses · Low Latency Queuing is a QoS scheduling technique that gives selected traffic strict priority treatment while providing bandwidth allocation for other classes.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.