Quality of Service


QoS is an umbrella term that includes many tools and techniques to improve the performance of network, it is very complex topic as well.

QoS is intended to prioritize the traffic on the basis of protocols. For example, the VoIP traffic take 64Kb bandwidth for entire session the call is up and if there is quite a much other traffic in your service then it could affect the VoIP call and there could be dozens of glitters in call. The sole purpose of this technology is to prioritize certain protocol over other protocols.

The Cisco network architecture divides into three tiers Core, Distribution, and Access and QoS technology works on access level where your switches are placed. It is really run on switches because switches do not do much than sending traffic based on destination MAC addresses.

The Core layer should be quite fast because it is connected to the WAN and there should be no filtering and intelligence work on this layer. It consists of routers, and firewalls intended to move the traffic as fast as possible.
The Distribution layer works hard on intelligence such as traffic filtering and more and this layer consist routers, layer-3 switches, and firewalls as well.


QoS Components

QoS has three different components that we have to worry about at CCNA level.

Tagging System

Tagging system puts the tags on traffic such as telnet, VoIP, SSH, HTTP, and more. It is also called classification because it classifies the different data and it happens at access layer. DiffServe Code Point or Differentiated Services Code Point (DSCP) is a mechanism of traffic classification it consists of two digit hexadecimal value. The most high prioritize value is EF and there are 4 levels of this value such as, 11, 12, 13, 14.

There are 4 tier and each tier has 4 levels in it and the most top priority level is EF-11 (Tier-1, Level-1).

Trust System

It is trustworthiness of packets prioritization tag, it is to make sure that the packets are classified (prioritized) by intermediated devices (switches, routers) not by client.

Prioritize System

It works at the Core layer which just takes the packet reads it tag and sends it immediately if it is high prioritized.

Normal Traffic Flow in Switches

Normally switches receive the data they store the traffic in buffer and they forward the traffic in First In First Out (FIFO) algorithm. We can describe it in different stages as following:

At very first movement when the switches boot up there is no traffic and memory buffer is completely empty at this stage.


Now over switch starts receiving traffic and some traffic is video and other is web and all are put in the queue to get transmitted and each packet has to wait for a little in order to get its turn.


Now the packet at 1st place of memory gets transmitted and all other packets moving forward to get their turn.


Now everything is working fine and VoIP packet comes into buffer at very last place and it will have to wait for its turn in this normal network flow.


Traffic Flow with Basic QoS

Let’s say we have classified the VoIP traffic and now the switch will not place this traffic at free space but the packet will be placed at very first box and all other packets will be paused for a little while.


Buffer Management

It is the most complex topic that you are taught in CCNP level where you work with the memory buffer management.

Traffic Shaping / Traffic Policing

Traffic policing is the technique to drop the traffic when it exceeds certain threshold. For example you set the threshold for web traffic for 80% of the bandwidth and when traffic exceeds the limit it is discarder. It is used to make sure there is always some space in the buffer for the important traffic. For example if the buffer is full then the prioritize data will never get prioritized because buffer is full and there is no space for any new packet. This technology helps to keep the certain portion of buffer empty so when the high priority traffic gets into the buffer it gets room and it is sent with priority.


Traffic shaping is another hand is also defines the threshold but it does not drops the packets it manages the packets so they can get to their destination.

Traffic Shaping and Classification work together in order to implement the QoS.


Tail Down is the mechanism that switches perform when there is not free memory in buffer and this mechanism simply discards the new packets completely and packets should be regenerated at client-side in order to make communication.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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