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CCNA 200-301
Online Courses
CCNA 200-301
  • Introduction
  • Interfaces And Cables
    • Ethernet
    • UTP Cables
    • Fiber Optic
    • UTP vs Fiber-Optic
    • Quiz
  • OSI Model & TCP/IP Suite
    • Networking Model
      • OSI Model
        • Application
        • Presentation
        • Session
        • Transport
        • Network
        • Data Link
        • Physical
      • PDUs
      • TCP/IP Suite
    • Quiz
  • Intro to the CLI
    • What is a CLI
      • Global Configuration Mode
      • Enable Password
    • Configuration Files
    • Canceling commands
    • Quiz
  • Ethernet LAN Switching
    • Local Area Network (LAN)
    • MAC Address
    • Ethernet Frame
    • Quiz 1
    • ARP
    • Ping
    • Quiz 2
  • IPv4 Addressing
    • Network Layer Review
    • IPv4
    • Quiz
    • Cisco CLI
    • Quiz
  • Switch Interfaces
    • CLI
    • Full/Half Duplex
    • CSMA/CD
    • Speed / Duplex Auto-Negotiation
    • Interface Errors
    • Quiz
  • IPv4 Header
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  • Static Routing
    • Topology
    • CLI
    • Default Route
    • Static Route
    • Most Specific Matching Route
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  • Subnetting
    • Subnetting
    • Quiz
    • Variable-Length Subnet Masks
  • VLANs
    • What is a LAN ?
    • VLAN Configuration
    • Quiz 1
    • Example
    • Trunk Ports
      • Configuration
    • VLAN Ranges
    • Native VLAN
    • Router on a Stick (ROAS)
    • Quiz 2
    • Native VLAN on Router
    • Layer 3 (Multilayer) Switches
    • Quiz 3
  • DTP/VTP
    • DTP
    • VTP
    • Quiz
  • Spanning Tree Protocol
    • Network Redundancy
    • Layer 2 Loops
    • Spanning Tree Protocol
      • Exercise
    • STP Port Role Selection
      • Exercise
    • Blocking Ports
    • Quiz 1
    • States
    • Timers
    • BPDU
    • STP Toolkit
    • Configurations
    • Load-Balancing
      • Quiz
    • Quiz 2
  • Rapid Spanning Tree Protocol
    • STP Version Comparison
    • RSTP Intro
    • Quiz 1
    • BPDU
    • Link Types
    • Quiz 2
  • EtherChannel
    • Why EtherChannel is needed?
    • Load-Balancing
      • Configuration
    • PAgP, LACP, and Static
      • PAgP
      • LACP
      • Static
      • Manually Configure the Negotiation Protocol
    • EtherChannel Requirements
    • EtherChannel Verification
    • Layer 3 EtherChannel
    • Quiz
  • Dynamic Routing
    • Network Topology
    • Dynamic Routing
    • Types
      • Distance Vector Protocols
      • Link State Protocols
    • Metrics
    • Administrative Distance
      • Floating Static Routes
    • Quiz
    • RIP
      • RIPv1 & RIPv2
      • RIP Configuration
    • EIGRP
      • EGRP Configuration
    • Quiz
    • OSPF
      • Areas
      • Configuration
      • Cost
      • Neighbors
      • Loopback Interfaces
      • Network Types
      • Neighbor Requirements
      • LSA Types
    • Configuration
    • Quiz
  • First Hop Redundancy Protocols
    • Introduction
    • HSRP
    • VRRP
    • GLBP
    • Comparing FHRPs
    • Configuring HSRP
    • Quiz
  • TCP & UDP
    • Basic of Layer 4
      • Port Numbers / Session Multiplexing
    • TCP
    • UDP
    • Comparing TCP & UDP
    • Port Numbers
    • Quiz
  • IPv6
    • What about IPv5?
    • Why IPv6?
    • IPv6
    • Identifying the IPv6 Prefix
    • Configuration
    • EUI-64
      • Configuration
    • Why invert the 7th bit?
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    • Unique Local Addresses
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    • Anycast Addresses
    • Other IPv6 Addresses
    • Representation
    • Header
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    • Neighbor Discovery Protocol
      • SLAAC
      • Duplicate Address Detection (DAD)
    • IPv6 Static Routing
    • Quiz
  • Access Control Lists
    • What are ACLs?
    • How ACLs work
    • Implicit Deny
    • ACL Types
    • Standard Numbered ACLs
    • Standard Named ACLs
    • Numbered ACLs With Subcommands
    • Resequencing ACLS
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    • Quiz
  • Layer 2 Discovery Protocols
    • Introduction
    • Cisco Discovery Protocol
    • Link Layer Discovery Protocol
    • Quiz
  • Network Time Protocol
    • The importance of time
    • Manual Time Configuration
    • Network Time Protocol
    • Reference Clocks
    • NTP Hierarchy
    • NTP Configuration
    • NTP Server mode
    • Symmetric active mode
    • NTP Authentication
    • Quiz
  • Domain Name System
    • Purpose of DNS
    • DNS Cache
    • DNS in Cisco IOS
    • Quiz
  • Dynamic Host Configuration Protocol
    • Purpose of DHCP
    • DHCP Messages
    • Relay
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    • Quiz
  • Simple Network Management Protocol
    • SNMP
    • Versions
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    • Quiz
  • Syslog
    • Syslog
    • Message Format
    • Logging Locations
    • Configuration
    • Syslog vs SNMP
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  • Secure Shell
    • Page
    • Console Port Security
    • L2 Switch Management IP
    • Telnet
    • SSH
    • Quiz
  • FTP & TFTP
    • FTP and TFTP
    • Trivial File Transfer Protocol
    • File Transfer Protocol
    • FTP vs TFTP
    • IOS File Systems
    • Upgrading Cisco IOS
    • Quiz
  • Network Address Translation
    • Private IPv4 Addressess
    • Network Address Translation (NAT)
    • Static NAT
    • Configuration
    • Dynamic NAT
    • Configuration
    • PAT (NAT Overload)
    • Quiz
  • Quality of Service
    • IP Phones
    • Power over Ethernet (PoE)
    • Quality of Service (QoS)
    • Queuing
    • Classification
    • IP Precedence and DSCP
    • RFC 4954
    • Trust Boundaries
    • Queuing/Congestion Management
    • Shaping and Policing
    • Classification
    • Quiz
  • Security Fundamentals
    • Why Security?
    • Concepts
    • Denial-of-service Attack
    • Spoofing Attacks
    • Reflection/Amplification Attacks
    • Man-in-the-middle Attack
    • Reconnaissance Attacks
    • Malware
    • Social Engineering Attacks
    • Password-related attacks
    • Multi-factor Authentication
    • Digital certificates
    • Controlling and Monitoring Users with AAA
    • Security Program Elements
    • Quiz
  • Port Security
    • Port Security
    • Why Port Security?
    • Enabling Port Security
    • Violation Modes
    • Secure MAC Address Aging
    • Sticky Secure MAC Addresses
    • Quiz
  • DHCP Snooping
    • DHCP Snooping
    • DHCP Starvation
    • DHCP Poisoning (Man-in-the-Middle)
    • DHCP Messages
    • DHCP Snooping Operations
    • DHCP Snooping Rate-Limiting
    • DHCP Option 82 (Information Option)
    • Quiz
  • Dynamic ARP Inspection
    • Gratuitous ARP
    • DAI
    • ARP Poisoning (Man-in-the-Middle)
    • Operations
    • Configurations
    • Optional Checks
    • ARP ACLs
    • Quiz
  • LAN Architectures
    • Common Terminologies
    • Two-Tier Campus LAN Design
    • Three-Tier Campus LAN Design
    • Spine-Leaf Architecture
    • SOHO Networks
    • Quiz
  • WAN Architectures
    • WAN
    • Leased Lines
    • MPLS
    • Internet Connections
    • Redundant Internet Connections
    • Internet VPNs
      • Site-to-Site VPNs (IPsec)
      • Remote-Access VPNs
    • Quiz
  • Virtualization & Cloud
    • Server Hardware
    • Servers before Virtualization
    • Virtualization
    • Why Virtualization
    • Connecting VMs to the Network
    • Cloud Services
    • Five Essential Characteristics of Cloud
    • Three Service Models of Cloud
    • Four Deployment Models of Cloud
    • Benefits of Cloud Computing
    • Connecting to Cloud Resources
    • Quiz
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  1. Quality of Service

Queuing/Congestion Management

PreviousTrust BoundariesNextShaping and Policing

Last updated 1 year ago

When a network device receives traffic at a faster rate than it can forward the traffic out of the appropriate interface, packets are placed in that interface's queue as they wait to be forwarded.

When the queue becomes full, packets that don't fit in the queue are dropped (tail drop).

RED and WRED drop packets early to avoid tail drop.

An essential part of QoS is the use of multiple queues.

  • This is where classification plays a role. The device can match traffic based on various factors (for example the DSCP marking in the IP header) and then place it in the appropriate queue.

However, the device is only able to forward one frame out of an interface at once, so a scheduler is used to decide which queue traffic is forwarded from next.

  • Prioritization allows the scheduler to give certain queues more priority than others.

A common scheduling method is weighted round-robin.

  • round-robin = packets are taken from each queue in order, cyclically

  • weighted = more data is taken from high priority each time the scheduler reaches that queue.

CBWFQ (Class-Based Weighted Fair Queuing) is a popular method of scheduling, using a weighted round-robin scheduler while guaranteeing each queue a certain percentage of the interface's bandwidth during congestion.

Round-robin scheduling is not ideal for voice/video traffic. Even if the voice/video traffic receives a guaranteed minimum amount of bandwidth, round-robin can add delay and jitter because even the high priority queues have to wait their turn in the scheduler.

LLQ (Low Latency Queuing) designates one (or more) queues as strict priority queues.

  • This means that if there is traffic in the queue, the scheduler will always take the next packet from that queue until it is empty.

This is very effective for reducing the delay and jitter of voice/video traffic.

However, it has the downside of potentially starving other queues if there is always traffic in the designated strict priority queue.

  • Policing (next slide) can control the amount of traffic allowed in the strict priority queue so that it can't take all of the link's bandwidth.