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FAQ: Class of Service (CoS) for Juniper Routing Devices

By Erdem posted 12-28-2015 12:59


When a network experiences congestion and delay, some packets must be dropped. The Juniper Networks Junos operating system (Junos OS) class of service (CoS) enables you to divide traffic into classes and offer various levels of throughput and packet loss when congestion occurs.


Class of service (CoS) is the assignment of traffic flows to different service levels. Service providers can use router-based CoS features to define service levels that provide different delay, jitter (delay variation), and packet loss characteristics to particular applications served by specific traffic flows.


This article provides answers to the most common questions about Junos CoS for routing devices.


  1. What is the difference between CoS and QoS?

  2. How do I get started with CoS or QoS?

  3. What Junos CoS components are used to classify a packet and what order do they operate?

  4. How many forwarding classes does Junos CoS support?

  5. How do schedulers provide different treatment to the different forwarding classes?

  6. How does Juniper use shapers to help manage congestion?

  7. How does Juniper use Packet Loss Priority and RED Drop profiles to manage congestion?

  8. Where can I learn about hierarchical class of service?

  9. What are the default Juniper CoS forwarding classes?

  10. What is the default mapping of code points to Junos CoS forwarding classes?

  11. What is the default scheduler policy for Junos CoS on routing devices?

  12. How do I apply CoS to my existing network?




  • Josef Jahde, Information Dev. Eng. Staff, Juniper Networks, Inc.
  • Walter Goralski, Information Dev. Eng. Sr. Staff, Juniper Networks, Inc.
  • Ulf Bremer, Solution Consultant Staff, Juniper Networks, Inc.
  • Colleen Feerick, Information Dev. Eng. Staff, Juniper Networks, Inc.
  • Kieran Milne, Information Dev. Eng. Sr. Staff, Juniper Networks, Inc.