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CCNP TSHOOT Certification Guide: Understanding the Basics of QoS

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Kevin Wallace, the author of CCNP TSHOOT 642-832 Official Certification Guide, discusses basic quality of service (QoS) mechanisms on Cisco routers, which are important to understand when troubleshooting VoIP issues.
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Cisco's Troubleshooting and Maintaining Cisco IP Networks (TSHOOT) exam (642-832), which is one of three components making up the new CCNP certification, addresses troubleshooting best practices for a variety of technologies (for example, routing, switching, wireless, security, and voice). Interestingly, two of the troubleshooting targets presented in the TSHOOT course (and on the TSHOOT exam) are not covered in the other CCNP courses: ROUTE and SWITCH.

Therefore, the successful TSHOOT exam candidate needs a basic understanding of a collection of topics outside the realm of the ROUTE and SWITCH courses. One such target is the troubleshooting of voice quality. This article considers the fundamental concepts surrounding quality of service (QoS) mechanisms available on Cisco routers, which can recognize voice traffic and treat that special traffic in a special way.

What Is QoS?

To begin, let's consider a definition of quality of service. The best definition I've ever heard (from one of Cisco's older QoS courses) is that the essence of QoS is "managed unfairness." In other words, we're being politically incorrect to certain traffic types. We're saying, "Oh, you're that kind of packet. As a result, you're going to be treated this way." Or, "You're this other kind of packet, and as a result, you're going to be treated this other way."

Cisco defines three categories of QoS:

  • Best-effort. The best-effort approach to QoS is really the absence of QoS. In other words, with best-effort, packets are not prioritized; the first packet to come into a router is the first packet to leave that router.
  • Integrated services (IntServ). IntServ allows an application to make bandwidth reservations for the duration of that application. Resource Reservation Protocol (RSVP) is the primary protocol used when working with IntServ. Interestingly, RSVP doesn't share. For example, if an application has reserved 128 kbps of bandwidth, any currently unused bandwidth cannot be shared with another application that's in need of extra bandwidth—even if the reserving application doesn't currently need all 128 kbps.
  • Differentiated services (DiffServ). DiffServ is the primary approach to QoS addressed in the TSHOOT curriculum, and it's the most common type of QoS you'll encounter in the real world. As its name suggests, with DiffServ a router's QoS mechanisms differentiate between different packet types. Specifically, QoS mechanisms can classify packets and then mark those packets. Once a packet is marked, the next router or the next switch along the packet's path can very quickly and efficiently examine that marking and make a decision to forward or drop the packet based on that marking. Unlike with IntServ, DiffServ mechanisms can share bandwidth. For example, imagine we allocate 128 kbps of bandwidth for web traffic and 256 kbps for FTP traffic. If the network's web traffic doesn't need all of its allocated 128 kbps at the moment, and the FTP traffic needs more than its allocated 256 kbps, the FTP traffic can use some of the web traffic's unused bandwidth.
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