Cisco QOS Basics

In today’s digital landscape, where data is being exchanged at lightning speed, efficient network traffic management is critical. Quality of Service (QoS) ensures that your network can handle different types of data with grace and efficiency. In this blog post, we will explore QoS, its importance and delve into the two key models for implementing QoS: Integrated Services (IntServ) and Differentiated Services (DiffServ).

A Primer on Quality of Service (QoS)

Quality of Service (QoS) is a suite of technologies to manage network traffic. It allocates different priorities to various applications, users, or data flows. QoS is indispensable in dense data traffic environments, such as businesses and services that depend on swift and reliable data transfer.

By employing QoS, networks benefit from:

  • Reduced packet loss
  • Diminished network jitter
  • Enhanced application performance

Companies like Cisco incorporate QoS into their networking equipment, including routers and switches, to ensure that networks operate smoothly and efficiently.

The Nitty-Gritty: Understanding Bandwidth and Delay

Before diving into QoS models, let’s clarify two important networking terms: bandwidth and delay.

  • Bandwidth: Bandwidth refers to the maximum data transfer capacity of a network. A network with high bandwidth can transmit more data in a given period. For instance, a 100 Mbps connection can send 100 megabits of data each second.
  • Delay (Latency): Delay is the time it takes for a data packet to travel from one point to another within a network. High delays can be detrimental for real-time applications like VoIP or video conferencing, causing lags or interruptions.

The Two Pillars: IntServ and DiffServ

When implementing QoS, there are two predominant models – Integrated Services (IntServ) and Differentiated Services (DiffServ).

Integrated Services (IntServ)

IntServ is designed for fine-grained, per-flow QoS, ensuring that each data flow gets its needed resources.

  • Function: IntServ reserves network resources for each traffic flow. This guarantees bandwidth on an end-to-end basis, ensuring critical applications receive the required service.
  • Method: IntServ primarily uses the Resource Reservation Protocol (RSVP) for reserving bandwidth. Devices across the data flow’s path use RSVP to set aside resources to meet the flow’s QoS requirements.
  • Example: Consider a company using video conferencing for critical meetings. By employing IntServ, the company can reserve bandwidth specifically for video traffic, ensuring smooth, uninterrupted conferences.

Differentiated Services (DiffServ)

DiffServ, on the other hand, is designed for scalability by classifying traffic into broader classes.

  • Function: Traffic is grouped into classes based on Differentiated Services Code Point (DSCP) values in the IP header. Each class can then be given a different level of service. This is done without reserving bandwidth for individual flows, making it more scalable than IntServ.
  • Method: DiffServ uses Per-Hop Behaviors (PHBs), which dictate how traffic is treated as it travels through the network.
  • Example: An internet service provider might use DiffServ to prioritize web traffic over email traffic during peak usage, ensuring that web pages load swiftly for all users.

Best Practices for Implementing QoS

  1. Identify the Traffic: Understand the types of traffic flowing through your network. This is fundamental before implementing any QoS policies.
  2. Define QoS Policies: Establish policies that align with your business needs. For instance, a call center might assign the highest priority to VoIP traffic.
  3. Avoid Complexity: While tempting to use all features, a simpler QoS configuration is easier to manage and troubleshoot.
  4. Monitor and Adjust: Networks evolve. Regularly monitoring traffic and adjusting QoS policies ensures your network stays optimized.
  5. Know Your Models: Choose between IntServ and DiffServ based on your network’s scale and requirements. Smaller networks with critical applications might benefit from IntServ, while larger networks might find DiffServ more scalable.
  6. Document and Seek Support: Maintain records of your QoS configurations, and don’t hesitate to consult vendor documentation or support services.

Wrapping Up

Quality of Service is vital for any modern network. By understanding and wisely implementing QoS models like IntServ and DiffServ, you can ensure your network is robust and agile, catering effectively to all types of data traffic. Whether running a small business with critical data needs or a large enterprise with varied traffic, QoS is the unsung hero that keeps the digital gears turning smoothly.


Below are example configurations using Cisco IOS command-line interface (CLI) for both IntServ and DiffServ models:

IntServ Configuration Example (Using RSVP)

IntServ uses RSVP (Resource Reservation Protocol) to reserve bandwidth for specific flows. Here’s an example configuration on a Cisco router for a video conference application:

! Enter global configuration mode
Router# configure terminal

! Enable RSVP on the interfaces through which the traffic will pass
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip rsvp bandwidth 1000
Router(config-if)# exit

Router(config)# interface GigabitEthernet0/1
Router(config-if)# ip rsvp bandwidth 1000
Router(config-if)# exit

! Create an access list to match video conferencing traffic
Router(config)# access-list 101 permit ip any any eq 5000

! Create a policy for RSVP reservation
Router(config)# ip rsvp sender-host udp 5000
Router(config-rsvp-sender)# flow-spec 1000
Router(config-rsvp-sender)# exit

! Apply RSVP policy to an interface
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip rsvp reservation 101 sender udp dport 5000

! Exit configuration mode
Router(config)# end

This example configuration sets up RSVP to reserve 1 Mbps of bandwidth for a video conference application that uses UDP port 5000.

DiffServ Configuration Example

DiffServ classifies traffic into different classes and assigns different treatment to each class. Below is an example configuration for prioritizing VoIP traffic over regular traffic on a Cisco router:

! Enter global configuration mode
Router# configure terminal

! Define a traffic class
Router(config)# class-map match-all VOIP
Router(config-cmap)# match ip dscp ef

! Define a QoS policy
Router(config)# policy-map QOS_POLICY
Router(config-pmap)# class VOIP
Router(config-pmap-c)# priority 1000
Router(config-pmap-c)# exit

Router(config-pmap)# class class-default
Router(config-pmap-c)# fair-queue

! Apply the QoS policy to an interface
Router(config)# interface GigabitEthernet0/0
Router(config-if)# service-policy output QOS_POLICY

! Exit configuration mode
Router(config)# end

This example configuration defines a class for VoIP traffic marked with DSCP value ef (Expedited Forwarding) and assigns 1 Mbps of priority bandwidth for this traffic. Other traffic will use fair queuing.

Please note that these examples are illustrative and are meant to give you an idea of how to configure IntServ and DiffServ. The actual configuration might vary based on the network design, requirements, and the devices used. It’s also essential to thoroughly plan and understand the implications before applying any QoS configurations in a production environment.