CCNP Voice FAQ: Congestion Management and Queuing
Q1. Which of the following is not a common reason for congestion?
A. Speed mismatch
Q2. Which of the following is a congestion management tool?
D. Fast Reroute
Q3. Which of the following is not a function within a queuing system?
A. Creating one or more queues
C. Assigning arriving packets to queues
D. Scheduling departure of packets from queues
Q4. How many queuing subsystems exist in an interface queuing system?
B. Two: a software queue and a hardware queue
C. Three: a software, a transmit, and a hardware queue
D. Four: a software, a hold, a transmit, and a hardware queue
Q5. What is the default queuing discipline on all but slow serial interfaces?
Q6. How many queues does PQ have?
B. Two: High and Low
C. Three: High, Medium, and Low
D. Four: High, Medium, Normal, and Low
Q7. Custom queuing is a modified version of which queuing discipline?
Q8. Which of the following is not a goal or objective of WFQ?
A. Provide high bandwidth to high-volume traffic
B. Divide traffic into flows
C. Provide fair bandwidth allocation to the active flows
D. Provide faster scheduling to low-volume interactive flows
Q9. Which of the following is not used to recognize and differentiate flows in WFQ?
A. Source and destination IP address
B. Packet size
C. Source and destination TCP/UDP port number
D. Protocol number and type of service
Q10. Which of the following is an advantage of WFQ?
A. WFQ does not starve flows and guarantees throughput to all flows.
B. WFQ drops/punishes packets from most aggressive flows first.
C. WFQ is a standard queuing mechanism that is supported on most Cisco platforms.
D. All of the above.
Q11. Which of the following is not a disadvantage of WFQ?
A. WFQ does not offer guarantees such as bandwidth and delay guarantees to traffic flows.
B. FQ classification and scheduling are not configurable and modifiable.
C. You must configure flow-based queues for WFQ, and that is a complex task.
D. Multiple traffic flows may be assigned to the same queue within the WFQ system.
Q12. Which of the following is not true about CBWFQ?
A. CBWFQ allows creation of user-defined classes.
B. CBWFQ allows minimum bandwidth reservation for each queue.
C. CBWFQ addresses all of the shortcomings of WFQ.
D. Each of the queues in CBWFQ is a FIFO queue that tail drops by default.
Q13. Which of the following is not true about LLQ?
A. LLQ includes a strict-priority queue.
B. The LLQ strict priority queue is given priority over other queues.
C. The LLQ strict-priority queue is policed.
D. LLQ treats all traffic classes fairly.
Q14. Why does congestion occur?
Answer: Congestion occurs when the rate of input (incoming traffic switched) to an interface exceeds the rate of output (outgoing traffic) from an interface. Aggregation, speed mismatch, and confluence are three common causes of congestion.
Q15. Define queuing.
Answer: Queuing is a congestion management technique that entails creating a few queues, assigning packets to those queues, and scheduling departure of packets from those queues.
Q16. What are three main tasks that congestion management/queuing mechanisms might perform?
Answer: Congestion management/queuing mechanisms might create queues, assign packets to the queues, and schedule a departure of packets from the queues.
Q17. What is the default queuing algorithm on Cisco router interfaces?
Answer: On fast interfaces (faster than E1 or 2.048 Mbps), the default queuing is FIFO, but on slow interfaces (E1 or less), the default queuing is WFQ.
Q18. In what situation might FIFO be appropriate?
Answer: FIFO might be appropriate on fast interfaces and when congestion does not occur.
Q19. Describe priority queuing.
Answer: PQ has four queues available: high-, medium-, normal-, and low-priority queues. You must assign packets to one of the queues, or the packets will be assigned to the normal queue. Access lists are often used to define which types of packets are assigned to the four queues. As long as the high-priority queue has packets, the PQ scheduler only forwards packets from that queue. If the high-priority queue is empty, one packet from the medium-priority queue is processed. If both the high- and medium-priority queues are empty, one packet from the normal-priority queue is processed, and if high-, medium-, and normal-priority queues are empty, one packet from the low-priority queue is processed.
Q20. Cisco custom queuing is based on which queuing mechanism?
Answer: Cisco custom queuing is based on weighted round-robin (WRR).
Q21. What are the Cisco router queuing components?
Answer: The Cisco router queuing components are software queue and hardware queue (also called transmit queue).
Q22. List the steps that a packet takes when it goes through an interface queuing system.
Answer: The software queuing mechanism usually has several queues.Packets are assigned to one of those queues upon arrival.If the queue is full, the packet is dropped (tail drop).If the packet is not dropped, it joins its assigned queue, which is usually a FIFO queue.The scheduler dequeues and dispatches packets from different queues to the hardware queue based on the particular software queuing discipline that is deployed. After a packet is classified and assigned to one of the software queues, it might be dropped if a technique such as weighted random early detection (WRED) is applied to that queue.
Q23. Describe WRR queuing.
Answer: A modified version of RR called weighted round-robin (WRR) allows you to assign a “weight” to each queue. Based on that weight, each queue effectively receives a portion of the interface bandwidth, not necessarily equal to the others.
Q24. Describe WFQ and its objectives.
Answer: WFQ has these important goals and objectives: divide traffic into flows, provide fair bandwidth allocation to the active flows, provide faster scheduling to low-volume interactive flows, and provide more bandwidth to the higher-priority flows.
Q25. How does WFQ define traffic flows?
Answer: WFQ identifies flows based on the following fields from IP and either TCP or UDP headers: Source IP Address, Destination IP Address, Protocol Number, Type of Service, Source TCP/ UDP Port Number, Destination TCP/UDP Port Number.
Q26. Describe WFQ early dropping and aggressive dropping.
Answer: WFQ has a hold queue for all the packets of all flows (queues within the WFQ system). If a packet arrives while the hold queue is full, it is dropped. This is called WFQ aggressive dropping.
Each flow-based queue within WFQ has a congestive discard threshold (CDT). If a packet arrives and the hold queue is not full but the CDT of that packet flow queue is reached, the packet is dropped. This is called WFQ early dropping.
Q27. What are the benefits and drawbacks of WFQ?
Benefits: Configuring WFQ is simple and requires no explicit classification, WFQ does not starve flows and guarantees throughput to all flows, and WFQ drops packets from most aggressive flows and provides faster service to nonaggressive flows.
Drawbacks: WFQ classification and scheduling are not configurable and modifiable, WFQ does not offer guarantees such as bandwidth and delay guarantees to traffic flows, and multiple traffic flows might be assigned to the same queue within the WFQ system.
Q28. What are the default values for CDT, dynamic queues, and reservable queues?
Answer: The default values for CDT, dynamic queues, and reservable-queues are 64, 256, and 0. The dynamic queue’s default is 256 only if the interface’s bandwidth is more than 512, but is based on the interface bandwidth.
Q29. How do you adjust the hold queue size?
You adjust the hold queue size by entering the following command in interface configuration mode:
Q30. List at least two problems associated with PQ/CQ/WFQ.
Answer: To use PQ and CQ, you must define traffic classes using complex access lists. PQ might impose starvation on packets of lower-priority queues. WFQ does not allow creation of userdefined classes. WFQ and CQ do not address the low delay requirements of real-time applications
Q31. Describe CBWFQ.
Answer: CBWFQ allows the creation of user-defined classes, each of which is assigned to its own queue. Each queue receives a user-defined amount of (minimum) bandwidth guarantee, but it can use more bandwidth if it is available.
Q32. What are the three options for bandwidth reservation within CBWFQ?
Answer: The three options for bandwidth reservation within CBWFQ are bandwidth, bandwidth percent, and bandwidth remaining percent.
Q33. How is available bandwidth calculated?
Answer: Available bandwidth is calculated as follows:
Available bandwidth = (interface bandwidth x maximum reserved bandwidth) × (sum of all existing reservations)
Q34. What are the benefits and drawbacks of CBWFQ?
Answer: CBWFQ has a couple of benefits. First, it allows creation of user-defined traffic classes. You can define these classes conveniently using MQC class maps. Second, it allows allocation/ reservation of bandwidth for each traffic class based on user policies and preferences. The drawback of CBWFQ is that it does not offer a queue that is suitable for real-time applications such as voice or video over IP applications.
Q35. How is CBWFQ configured?
Answer: CBWFQ is configured using Cisco modular QoS command-line interface (MQC) class map, policy map, and service policy
Q36. Describe low-latency queuing.
Answer: Low-latency queuing (LLQ) adds a strict-priority queue to CBWFQ. The LLQ strict-priority queue is given priority over other queues, which makes it ideal for delay- and jitter-sensitive applications. The LLQ strict-priority queue is policed so that other queues do not starve.
Q37. What are the benefits of LLQ?
Answer: Low-latency queuing offers all the benefits of CBWFQ, including the ability of the user to define classes and guarantee each class an appropriate amount of bandwidth and to apply WRED to each of the classes (except to the strict-priority queue) if needed. In both LLQ and CBWFQ, the traffic that is not explicitly classified is considered to belong to the class-default class. You can make the queue that services the class-default class a WFQ instead of FIFO, and if needed, you can apply WRED to it, too. The benefit of LLQ over CBWFQ is the existence of one or more strict-priority queues with bandwidth guarantees for delay- and jitter-sensitive traffic.
Q38. How do you configure LLQ?
Answer: Configuring LLQ is almost identical to configuring CBWFQ, except that for the strict priority
queue(s), instead of using the keyword/command bandwidth, you use the keyword/command priority within the desired class of the policy map.