QOS Interview Questions & Answers

Sure! Here are the Cisco QoS interview questions with their answers:

  1. What is QoS and why is it important in networking?
  • QoS stands for Quality of Service. It is a set of techniques used to manage and prioritize network traffic to ensure optimal performance and meet specific requirements. QoS is important in networking because it helps prioritize critical traffic, control congestion, and improve overall network efficiency.
  1. Explain the three main components of QoS.
  • The three main components of QoS are:
    • Classification: Identifying and categorizing network traffic based on specific criteria such as source/destination IP address, port number, protocol, or DSCP/CoS values.
    • Marking: Applying a QoS label (DSCP or CoS value) to classified traffic to indicate its priority or treatment level.
    • Queuing: Managing the order in which packets are transmitted by assigning them to different queues based on their priority or classification.
  1. How does QoS prioritize network traffic?
  • QoS prioritizes network traffic by assigning different levels of priority to different types of traffic. This is done through classification and marking. Once traffic is classified and marked, routers and switches can use queuing mechanisms to give higher priority to critical traffic, ensuring it gets transmitted first.
  1. What are the different queuing mechanisms used in QoS?
  • The different queuing mechanisms used in QoS are:
    • First-In-First-Out (FIFO): Simplest queuing mechanism where packets are transmitted in the order they arrive.
    • Weighted Fair Queuing (WFQ): Provides fair sharing of bandwidth among different flows based on assigned weights.
    • Priority Queuing (PQ): Assigns strict priorities to different queues, transmitting higher priority packets first.
    • Custom Queuing (CQ): Allows administrators to define specific transmission rates for different queues based on priority.
  1. What is the difference between classification and marking in QoS?
  • Classification involves identifying and categorizing network traffic based on specific criteria. Marking, on the other hand, is the process of applying a QoS label (DSCP or CoS value) to classified traffic to indicate its priority or treatment level.
  1. How does congestion avoidance work in QoS?
  • Congestion avoidance in QoS aims to prevent or minimize congestion in the network. This is achieved through mechanisms like Random Early Detection (RED) or Weighted Random Early Detection (WRED). These mechanisms drop or mark packets before congestion occurs, signaling the sending device to slow down transmission.
  1. What is the purpose of policing and shaping in QoS?
  • Policing and shaping are rate-limiting techniques used in QoS to control the flow of traffic. Policing enforces a specific rate limit by dropping or marking excess traffic, while shaping buffers and delays excess traffic to conform to a specific rate.
  1. Explain the concept of bandwidth reservation in QoS.
  • Bandwidth reservation in QoS involves allocating a certain amount of network bandwidth for specific applications or traffic types. This ensures that critical traffic always has the required bandwidth available, even during periods of high network congestion.
  1. How can you verify if QoS is working correctly in a network?
  • You can verify if QoS is working correctly in a network by monitoring network traffic using tools like packet capture or network analyzers. Check if the classified and marked traffic is being prioritized correctly and if the queuing mechanisms are functioning as expected.
  1. What are some common issues that can occur with QoS implementation?
  • Some common issues that can occur with QoS implementation include misconfiguration of classification and marking rules, improper queue sizing or prioritization, inadequate bandwidth allocation, and lack of monitoring or troubleshooting tools for effective QoS management.

Remember to prepare well for your interview and review these answers along with the concepts of QoS thoroughly. Good luck!

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