Quality of Service Part – 2

Random Early Detection – RED

Random Early Detection (RED) is the mechanism that detects the memory buffer size before it gets full. It looks for the hosts that are sending more data and it drops the packet of host that is transmitting more data and it lets go the host who is sending low data. For example, Host – A is sending 100Mb traffic and Host – B is sending 100Kb traffic now when RED detects the buffer is getting full it will start dropping the Host – A because it is transmitting more data. RED mechanism defines the equal network resources usage for all users in the network.

Waited Random Early Detection – WRED

Waited Random Early Detection (WRED) is more advanced form of RED that not only looks for the traffic but the protocols behind the traffic as well to drop the packets. For example, Host – A is sending 100Mb traffic and Host – B is sending 100Kb traffic and in Host – A traffic there is VoIP traffic as well and RED would not detect this but WRED will detect this and it will allow VoIP traffic and drop other low prioritized traffic.

QoS Interfaces

The QoS is done at egress (outbound) interface meaning where the traffic will leave exit from network, because it is outbound interface that sends traffic to other network resources not inbound. However, the classification and prioritization of traffic is performed at ingress (inbound) interface.


Nowadays many networks are using MPLS technology that support QoS as well but if you are using tunnel of MPLS then the QoS is really difficult and in this case you can make use of Frame-relay over MPLS to perform QoS.
Forward Early Congestion Notifications (FECNs) and Backward Early Congestion Notifications (BECN s) are the technologies used by Frame-relay to notify users to slow down the traffic rate in order to avoid traffic drop. These technologies work on WAN networks.

Auto QoS

It is set of best practices for QoS from Cisco, we usually implement this technology and then tweak it regarding our own network to get the best performance.

QoS Configuration

Checking Neighbours

At first we have to see the neighbors at our switch to determine whether there is any IP phone or not.


Checking QoS Status

By default the QoS is turned off on most of the Cisco devices but we can always check the status whether it is on or off.


Enable QoS

We need to run only mls qos command in global configuration to enable QoS.


After turning on QoS it does not do much because we have not made any configuration.

Now we just need to move on the interface where we want QoS implementation and we use the auto qos to enable QoS.


Now when we start enabling auto we will see 4 major templates defined by Cisco.


Classify, trust, VoIP, and Video are Cisco best practices template and in the voip configuration we get three sub-categories where trust means the third party VoIP phones.

Now when after applying auto qos we look at interface it has policies and other configuration to set up QoS.


The AutoQoS-Police-CiscoPhone is the policy that have been applied on this interface.
priority-queue out is telling that the qos is on outbound traffic.
mls qos trust device cisco-phone says that this policy trust Cisco phones.

Now when we see running-configuration we can see a huge amount of list and multiple class and policy map that have been automatically created for us by using auto qos technology to apply QoS on VoIP phones.


Look at the above screenshot it is huge configuration for QoS that have been automatically generated for us.


Above is the auto-generated policy map for our QoS that is defining that voip traffic can take upto 320000 bandwidth and more 8000 burst if it exceeds the limit after this the traffic will be policed (dropped)and we discuss the classification level where ef-11 is the highest notice we have attached this level to VoIP policy. The another class is for other type of traffic.

Caution: It is highly recommended that do not use QoS on your production network and the network that you are not completely aware of because it could be a nightmare. Create a lab practice it and then implement it on your production environment.

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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