Cisco CCNP Certification Training For The ONT Exam:

The QoS Models: Differentiated Services (DiffServ)

By Chris Bryant, CCIE #12933

To pass the ONT exam and earn your CCNP certification, you've got to know the ins and outs of the three major QoS models - DiffServ, IntServ, and best-effort. Today's free CCNP training takes a look at Differentiated Services (DiffServ)

Unlike IntServ, DiffServ doesn't use RSVP to reserve bandwidth from the source to the destination. Instead, DiffServ uses Per-Hop Behavior (PHB) to allow each router across the network to examine the packet individually and decide what service level the packet 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". 

There is no advance signaling with DiffServ - no "hey, here comes a really important packet!" advance notice. Each hop along the way from source to destination makes its own decision as to how important a packet is or isn't. This lack of advance signaling is why DiffServ is considered more scalable than IntServ, since no bandwidth is reserved in advance of the actual transmission.

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.

Those of you who have taken my Switching courses know what I'm about to say - when you perform classification with switches, don't perform classification on the core switches! Both Cisco and I recommend that you perform traffic marking and classification as close to the edge of the network as possible, and that means at the Access layer of the Cisco three-layer networking model:

We'll take a look at the IntServ and best-effort models in future CCNP ONT exam tutorials! In the meantime, check these tutorials out, and when you're ready for the ultimate in CCNP certification training for the ONT exam, step up to The Ultimate ONT Study Package!

CCNP Certification Training Tutorial: Priority Queuing Theory

CCNP ONT Exam Training: Configuring PQ

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com