CCNA 4 Chapter 6 v6 Exam Answers

What are two approaches to prevent packet loss due to congestion on an interface? (Choose two.)

There are three approaches to prevent sensitive traffic from being dropped:
Increase link capacity to ease or prevent congestion.
Guarantee enough bandwidth and increase buffer space to accommodate bursts of traffic from fragile flows.
Prevent congestion by dropping lower-priority packets before congestion occurs.

Which queuing algorithm has only a single queue and treats all packets equally?

FIFO queuing sends packets out an interface in the order that they had arrived and does not make a decision about packet priority. All packets are treated equally.

Which QoS technique retains excess packets in a separate queue for later transmission?

As network traffic exits an interface it is queued and then shaped to smooth out the packet output rate. Classification and marking should occur early on to identify traffic and classify how the traffic should be treated.

Which QoS technology provides congestion avoidance by allowing TCP traffic to be throttled before buffers become full and tail drops occur?

The weighted random early detection (WRED) algorithm provides congestion avoidance on network interfaces by allowing TCP traffic to be throttled before buffers are exhausted. This maximizes network use and TCP-based application performance while minimizing tail drop.

How does a Cisco router using tail drop handle congestion when a traffic queue becomes full?

Tail drop is also known as packet drop and is used by a router to discard any packet that arrives at the tail end of a queue. Tail drop treats all traffic equally and does not differentiate between delay-sensitive data and other traffic.

When QoS is implemented in a converged network, which two factors can be controlled to improve network performance for real-time traffic? (Choose two.)

Delay is the latency between a sending and receiving device. Jitter is the variation in the delay of the received packets. Both delay and jitter need to be controlled in order to support real-time voice and video traffic.

What are two characteristics of voice traffic? (Choose two.)

Voice traffic does not consume a lot of network resources, such as bandwidth. However, it is very sensitive to delay and dropped packets cannot be retransmitted. For good voice quality, the amount of latency should always be less than 150 milliseconds.

Which QoS model uses the DSCP bits to mark packets and provides 64 possible classes of service?

The DiffServ model uses 6-bits known as the DiffServ Code Point (DSCP) bits to mark traffic and offers a maximum of 64 possible classes of service. Diffserv-aware routers can then implement per-hop behaviors (PHBs) that can control packet forwarding based on the specified class of service.

A network engineer is selecting a QoS method to control congestion on a VPN tunnel link between the headquarters site and a branch office. Which queuing method cannot be used to classify and control VPN traffic?

Weighted fair queuing (WFQ) does not support tunneling and encryption because these features modify the packet content information required by WFQ for classification.

A network engineer performs a ping test and receives a value that shows the time it takes for a packet to travel from a source to a destination device and return. Which term describes the value?

Latency refers to the time it takes for a packet to travel from a source device to a destination device. Jitter is the variation in the latency values of received packets. Bandwidth is the measurement of how much traffic is transmitted in a single second. Priority is the preferential treatment of specific classes of traffic.


What two fields are available in IPv4 and IPv6 headers to mark packets for QoS? (Choose two.)

IPv4 uses an 8-bit Type of Service field to mark packets at Layer 3 and IPv6 uses an 8-bit Traffic Class field to mark packets at Layer 3. The fields are used by receiving devices to forward the packets based on the appropriate assigned QoS policy.

For classifying packets into classes with CBWFQ, what is the purpose of configuring a maximum packet limit for a class?

The maximum packet limit characteristic is the maximum number of packets allowed to accumulate in the queue for a specific class that is defined using CBWFQ.

A network administrator is deploying QoS with the ability to provide a special queue for voice traffic so that voice traffic is forwarded before network traffic in other queues. Which queuing method would be the best choice?

Low latency queuing (LLQ) allows delay-sensitive data, such as voice traffic, to be defined in a strict priority queue (PQ) and to always be sent first before any packets in any other queue are forwarded.

What are two characteristics of the best-effort QoS model? (Choose two.)

The best-effort QoS model provides no guarantees and it is commonly used on the Internet. The best-effort QoS model treats all network packets in the same way.

What is the benefit of deploying Layer 3 QoS marking across an enterprise network?

Marking traffic at Layer 2 or Layer 3 is very important and will affect how traffic is treated in a network using QoS.
Layer 2 marking of frames can be performed for non-IP traffic.
Layer 2 marking of frames is the only QoS option available for switches that are not "IP aware."
Layer 3 marking will carry the QoS information end-to-end.

What are two characteristics of DiffServ QoS model? (Choose two.)

The DiffServ QoS model is commonly used in modern networks and is the easiest QoS model to deploy. TCP flows are grouped into a single class. Network devices are setup to service multiple classes of traffic where each class of traffic has different QoS requirements.

Refer to the exhibit. As traffic is forwarded out an egress interface with QoS treatment, which congestion avoidance technique is used?

Traffic shaping buffers excess packets in a queue and then forwards the traffic over increments of time, which creates a smoothed packet output rate. Traffic policing drops traffic when the amount of traffic reaches a configured maximum rate, which creates an output rate that appears as a saw-tooth with crests and troughs.

What role do network devices play in the IntServ QoS model?

The IntServ QoS model uses resource reservation to guarantee bandwidth and packet-loss rates from end to end. IntServ uses a connection-oriented approach to ensure that available resources are sufficient in the network for the traffic to have a specific level of QoS.

In QoS models, which type of traffic is commonly provided the most preferential treatment over all other application traffic?

Voice traffic from IP phones is commonly provided preferential treatment over all other application traffic such as email, web traffic, and file transfers.

An administrator has mastered the use of access control lists (ACLs) and wants to deploy QoS by defining different traffic classes through the use of ACLs. Which queuing method provides this functionality?

Class-based weighted fair queuing (CBWFQ) provides support for user-defined traffic classes. CBWFQ allows an administrator to define traffic classes based on match criteria such as protocols, access control lists (ACLs), and input interfaces.


Which QoS model is very resource intensive and provides the highest guarantee of QoS?

The IntServ QoS model uses resource reservation and admission control mechanisms to schedule network resources.