Section 5 – Congestion Management Methods
QUESTION 1
Based on the following 2950 switch configurations, which statement is correct?
no wrr-queue cos-map
wrr-queue bandwidth 20 10 70 1
wrr-queue cos-map 4 5
wrr-queue cos-map 1 0 1 2 3
wrr-queue cos-map 3 6 7
A. Queue 1 is setup as the expedite queue.
B. Queue 2 is setup as the expedite queue.
C. Queue 3 is setup as the expedite queue.
D. Queue 4 is setup as the expedite queue.
E. No queue is setup as the expedite queue.
Answer: E
Explanation:
To allocate bandwidth between standard transmit queue 1 (low priority) and standard transmit queue 2 (high priority), use the wrr-queue bandwidth command. Use the no form of this command to return to the default settings.
wrr-queue bandwidth weight-1 weight-2 [ weight-3 ]
no wrr-queue bandwidth
QUESTION 2
Which two commands are typically applied to the voice traffic class within a policy-map? (Choose two.)
A. shape peak {bps}
B. priority {kbps}
C. bandwidth {kbps}
D. compress header ip rtp
E. random-detect ecn
F. random-detect dscp-based
Answer: BD
Explanation:
bandwidth – Allows for the configuration of CBWFQ. The specifics of CBWFQ operation are beyond the scope of this explanation, but this command provides a minimum bandwidth guarantee to this class of traffic. priority – Designates that this class is a Low Latency Queuing (LLQ) class, which should receive strict scheduling priority to minimize delay, jitter and packet loss. Also specifies the amount of bandwidth for this class.
QUESTION 3
What are two benefits of WFQ? (Choose two.)
A. WFQ is very easy to configure, and no manual traffic classification is necessary.
B. WFQ can provide fixed-bandwidth and fixed-delay guarantees.
C. WFQ can provide fixed-bandwidth guarantees.
D. WFQ can provide fixed-delay guarantees.
E. WFQ prevents the large-volume flows with large packet size from starving out the low-volume flows with small packet size.
F. Based on DSCP, WFQ allows weighted, random dropping of packets when the WFQ system is full.
Answer: AE
Explanation:
The Question is WFQ and not CBWFQ, so the Answer should be: A, E
QUESTION 4
Which of the following configurations requires the use of hierarchical policy maps?
A. the use of class-based WRED within a CBWFQ class queue
B. the use of a strict priority-class queue within CBWFQ
C. the use of CBWFQ inside class-based shaping
D. the use of nested class-maps with class-based marking
E. the use of both the bandwidth and shape statements within a CBWFQ class queue
Answer: C
Explanation:
Class-based weighted fair queuing (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes. By using CBWFQ, network managers can define traffic classes based on several match criteria, including protocols, access control lists (ACLs), and input interfaces. A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class. More than one IP flow, or "conversation", can belong to a class. Once a class has been defined according to its match criteria, the characteristics can be assigned to the class. To characterize a class, assign the bandwidth and maximum packet limit. The bandwidth assigned to a class is the guaranteed bandwidth given to the class during congestion. CBWFQ assigns a weight to each configured class instead of each flow. This weight is proportional to the bandwidth configured for each class. Weight is equal to the interface bandwidth divided by the class bandwidth. Therefore, a class with a higher bandwidth value will have a lower weight.
By default, the total amount of bandwidth allocated for all classes must not exceed 75 percent of the available bandwidth on the interface. The other 25 percent is used for control and routing traffic.
The queue limit must also be specified for the class. The specification is the maximum number of packets allowed to accumulate in the queue for the class. Packets belonging to a class are subject to the bandwidth and queue limits that are configured for the class.
QUESTION 5
Which two of the following statements about CBWFQ are correct? (Choose two.)
A. The CBWFQ scheduler provides a guaranteed amount of minimum bandwidth to each class.
B. CBWFQ services each class queue using a strict priority scheduler.
C. The class-default queue only supports WFQ.
D. Each CBWFQ traffic class is policed using a congestion aware policer.
E. Inside a class queue, processing is always FIFO, except for the class-default queue.
Answer: AE
Explanation:
A protocol-dependent switching process handles traffic arriving at a router interface. The switching process includes delivery of traffic to an outgoing interface buffer. First-in, first-out (FIFO) queuing is the classic algorithm for packet transmission. With FIFO, transmission occurs in the same order as messages are received. Until recently, FIFO queuing was the default for all router interfaces. If users require traffic to be reordered, the department or company must establish a queuing policy other than FIFO queuing.
Cisco IOS software offers three alternative queuing options: Weighted fair queuing (WFQ) prioritizes interactive traffic over file transfers in order to ensure satisfactory response time for common user applications. Class-based weighted fair queuing (CBWFQ) in IOS 12.2 prioritizes traffic based on user-defined classes. Low latency queuing (LLQ) (IOS 12.2) brings strict priority queueing to Class-Based Weighted Fair Queuing (CBWFQ).
Class-based weighted fair queuing (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes. By using CBWFQ, network managers can define traffic classes based on several match criteria, including protocols, access control lists (ACLs), and input interfaces. A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class. More than one IP flow, or "conversation", can belong to a class. Once a class has been defined according to its match criteria, the characteristics can be assigned to the class. To characterize a class, assign the bandwidth and maximum packet limit. The bandwidth assigned to a class is the guaranteed bandwidth given to the class during congestion. CBWFQ assigns a weight to each configured class instead of each flow. This weight is proportional to the bandwidth configured for each class. Weight is equal to the interface bandwidth divided by the class bandwidth. Therefore, a class with a higher bandwidth value will have a lower weight.
By default, the total amount of bandwidth allocated for all classes must not exceed 75 percent of the available bandwidth on the interface. The other 25 percent is used for control and routing traffic.
The queue limit must also be specified for the class. The specification is the maximum number of packets allowed to accumulate in the queue for the class. Packets belonging to a class are subject to the bandwidth and queue limits that are configured for the class.