Cisco CCNP Certification Exam Training For The ONT Exam: QoS Models

A large part of your CCNP training for the ONT certification exam should be spent studying the various ways we can implement Quality of Service (QoS) on Cisco routers and switches.  Before you start configuring your network’s devices, though, you’ve got to understand the three QoS models and their impact on your network.  Let's take a look at the pros and cons of each.

If you don't have a QoS model in place, you actually do.  Best-effort QoS is just that - best-effort.  No priority is given to any traffic.  If your network is carrying voice or video traffic, best-effort is definitely not the way to go.

The Integrated Services model, more popularly known as IntServ, uses the Resource Reservation Protocol (RSVP) to reserve network resources in advance of the data actually traveling across the network.  Once the end-to-end bandwidth reservation is in place, the data is transmitted.

That sounds great, but there are some drawbacks.  It's a waste of bandwidth to have the entire end-to-end path reserved in advance.  Additionally, IntServ isn't as scalable a solution as we'd like.  Everything we do on a router or switch has a cost of some kind, and in this case it's RSVP overhead.  One or two paths won't cause much overhead, but as the number of reserved paths increases as a network becomes larger, the RSVP overhead can take its toll on the routers involved.

Differentiated Services (DiffServ) is the latest of the three models, and many would agree that it's also the greatest.  DiffServ doesn't use RSVP, but instead uses Per-Hop Behavior (PHB) to allow each router across the network to examine the packet and decide what service level it should receive.  With DiffServ, one router along the path from source to destination could consider a packet to be of the highest priority, while another router could consider it "just another packet". 

A term you hear often with DiffServ is "marking and classification".  Marking a packet is the process of assigning the packet a value reflecting the level of QoS it should receive, while classification is placing that packet into a queue in accordance with that level of QoS.

When it comes to marking, there are different values we can use to decide what value to mark the frame or packet with.  In my experience, here are the four that are used most often:

• IP Precedence (IP Prec)
• Differentiated Services Code Point (DSCP)
• CoS value
• Interface that received the data (ingress interface)

Which one you choose depends on your particular network's needs, and of course, the OSI layer at which the marking is taking place.  We’ll take a look at each of these methods in future CCNP ONT exam training tutorials!