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4.4 QoS Fundamentals

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Cisco Certified Network Associate CCNA

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1 Introduction

About the CCNA 200-301 Exam Domains and Weights Study Strategies and Tips

2 Chapter 1: Network Fundamentals (20%)

1.1 Network Components 1.2 Network Topology Architectures 1.3 Physical Interfaces and Cabling 1.4 TCP vs. UDP 1.5 IPv4 Addressing and Subnetting 1.6 IPv6 Addressing and Prefix 1.7 Wireless Principles 1.8 Virtualization Fundamentals 1.9 Switching Concepts 1.10 OSI and TCP/IP Reference Models 1.11 Verifying IP Parameters on Client Operating Systems

3 Chapter 2: Network Access (20%)

2.1 VLANs — Configuration and Verification 2.2 Trunking and 802.1Q 2.3 Layer 2 Discovery Protocols (CDP and LLDP) 2.4 EtherChannel (Link Aggregation) 2.5 Spanning Tree Protocol (STP) and RSTP 2.6 Wireless Architectures and AP Modes 2.7 Network Access Troubleshooting 2.8 Inter-VLAN Routing — Deep Dive

4 Chapter 3: IP Connectivity (25%)

3.1 Routing Table Components 3.2 Router Forwarding Decisions 3.3 IPv4 and IPv6 Static Routing 3.4 Single-Area OSPFv2 — Concepts 3.5 Single-Area OSPFv2 — Configuration 3.6 First Hop Redundancy Protocols (FHRP) 3.7 Troubleshooting IP Connectivity 3.8 OSPF Troubleshooting Scenarios

5 Chapter 4: IP Services (10%)

4.1 NAT (Network Address Translation) 4.2 DHCP and DNS 4.3 NTP, SNMP, and Syslog 4.4 QoS Fundamentals 4.5 TFTP/FTP and Remote Access 4.6 IP Services — Comprehensive Review

6 Chapter 5: Security Fundamentals (15%)

5.1 Key Security Concepts 5.2 Device Access Control and Password Policies 5.3 VPN Fundamentals 5.4 Access Control Lists (ACLs) 5.5 Layer 2 Security Features 5.6 Network Security Best Practices and Hardening 5.7 Threat Mitigation and Incident Response

7 Chapter 6: Automation and Programmability (10%)

6.1 Impact of Automation on Network Management 6.2 Controller-Based and Software-Defined Architectures 6.3 AI and Machine Learning in Network Operations 6.4 REST-Based APIs 6.5 Configuration Management Tools 6.6 JSON-Encoded Data 6.7 Automation and Programmability — Comprehensive Review

4.4 QoS Fundamentals

Key Takeaways Explain This Topic AI

QoS manages bandwidth, delay, jitter, and loss so latency-sensitive traffic is protected during congestion.

Voice needs under 150 ms one-way delay, under 30 ms jitter, and under 1% loss.

DSCP is a 6-bit Layer 3 marking (0-63) carried end-to-end; CoS is a 3-bit Layer 2 marking that exists only on trunks.

Per-Hop Behaviors: EF (DSCP 46) for voice, the AF classes for prioritized data, and BE (DSCP 0) as default.

The trust boundary is where the network begins believing incoming QoS markings — usually the access switch.

Last updated: June 2026

Why QoS Exists

Quality of Service (QoS) is the set of tools that prioritizes traffic when a link is congested. With no QoS, every packet is best-effort and drops are indiscriminate during congestion — fine for a file download that simply retransmits, fatal for a phone call that cannot wait. QoS does not create bandwidth; it decides who suffers first when bandwidth runs out.

The four impairments QoS controls

  • Parameter Definition What it ruins
  • Bandwidth Capacity of the link Total throughput
  • Delay (latency) One-way travel time Conversation feels laggy
  • Jitter Variation in delay between packets Choppy, robotic audio
  • Loss Packets that never arrive Gaps and dropouts
  • Application requirements
  • Traffic Bandwidth One-way delay Jitter Loss
  • Voice (VoIP) 30-128 Kbps/call < 150 ms < 30 ms < 1%
  • Video conferencing 384 Kbps-20 Mbps < 150 ms < 30 ms < 1%
  • Streaming video High < 4-5 s (buffered) tolerant < 2%
  • Web / email / file Variable tolerant n/a 0%

The headline numbers to memorize are the voice trio: 150 ms delay, 30 ms jitter, 1% loss — interactive video uses the same targets because a human is in the conversation.

Classification and Marking

QoS first classifies traffic (identifies what it is) then marks it so every downstream hop can act without re-inspecting deep into the packet.

CoS (Class of Service) — a 3-bit field (values 0-7) inside the 802.1Q VLAN tag. Because it lives in the tag, it survives only on trunk links and is lost the moment a frame leaves a trunk.

DSCP (Differentiated Services Code Point) — a 6-bit field (values 0-63) in the IP header's ToS byte. Being Layer 3, it travels end-to-end across routed networks, which is why CCNA treats DSCP as the preferred marking.

Per-Hop Behaviors (PHB) and DSCP Values

A Per-Hop Behavior is the forwarding treatment a router gives a packet based on its DSCP mark.

  • PHB DSCP bits Decimal Use
  • EF Expedited Forwarding 101110 46 Voice — strict low latency
  • AF41 100010 34 Interactive video
  • AF31 011010 26 Mission-critical data
  • AF21 010010 18 Transactional data
  • AF11 001010 10 Bulk data
  • BE / CS0 Best Effort 000000 0 Default, no priority
  • Reading an AF name

Assured Forwarding values are written AFxy : x = class (1-4, higher is more important) and y = drop precedence (1-3, higher drops first under congestion). So AF43 is high class but most-droppable; AF11 is low class, drop-last within its class.

  • Low drop (1) Med drop (2) High drop (3)
  • Class 4 AF41 (34) AF42 (36) AF43 (38)
  • Class 3 AF31 (26) AF32 (28) AF33 (30)
  • Class 2 AF21 (18) AF22 (20) AF23 (22)
  • Class 1 AF11 (10) AF12 (12) AF13 (14)
  • Queuing and Trust Boundary

After marking, the egress interface sorts traffic into queues:

LLQ (Low Latency Queuing) — a strict priority queue, the proper home for EF voice; it is serviced first but policed so it cannot starve everything else.

CBWFQ (Class-Based Weighted Fair Queuing) — guarantees a minimum bandwidth share per traffic class.

WFQ — proportionally fair sharing for unclassified flows.

The trust boundary is the point where the network starts believing a device's QoS marks. A PC could set DSCP 46 to cheat its way into the voice queue, so PC markings are not trusted; a Cisco IP phone is trusted, and the access switch is the usual boundary — re-marking or trusting at the first managed port.

Congestion Tools: Shaping, Policing, and Drop

QoS has two ways to handle traffic that exceeds a rate. Policing measures the rate and immediately drops (or re-marks down) anything over the limit — it is harsh but adds no delay, suited to enforcing a contract at a network edge. Shaping instead buffers the excess and releases it later at the allowed rate, smoothing bursts at the cost of added delay and the risk of buffering. The exam-level takeaway: policing drops, shaping delays; policing is typically applied inbound and shaping outbound.

When queues fill, the default behavior is tail drop — once the queue is full, every new packet is discarded regardless of importance. Tail drop can cause TCP global synchronization , where many flows back off and ramp up in lockstep, wasting capacity. Weighted Random Early Detection (WRED) counters this by dropping lower-priority (higher AF drop-precedence) packets probabilistically before the queue is completely full, signaling a few TCP senders to slow down early and keeping the link smoothly utilized. This is the practical reason AF drop precedence exists: under WRED, AF13 is discarded before AF11 within the same class.

Putting Voice Through the Network End to End

Trace a VoIP packet to see every concept connect. The IP phone marks its voice RTP stream EF (DSCP 46) and its signaling CS3 , sending them on a dedicated voice VLAN. The access switch, sitting at the trust boundary , trusts the phone's markings but re-marks any traffic from the daisy-chained PC down to best-effort. Each router along the path reads the DSCP value, applies the EF Per-Hop Behavior , and places the packet in its LLQ strict-priority queue so it jumps ahead of bulk data.

Because every hop honors the same Layer 3 DSCP mark, the call meets its under-150 ms / under-30 ms / under-1% budget end to end — something CoS alone could never guarantee, since CoS evaporates the moment the frame leaves a trunk.

On the exam: CCNA tests QoS concepts, not full config. Lock in EF = voice = DSCP 46, BE = DSCP 0, CoS is Layer 2 / trunk-only while DSCP is Layer 3 / end-to-end, the voice 150/30/1 targets, that policing drops while shaping delays, and what a trust boundary is.