784 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
The ip precedence command (or the ip qos dscp command at IDS 12.2T) is used to define the precedence value (or DSCP value) of VoIP packets to the specified dial peer. The command is a subcommand of the dial-peer global configuration command.
28What QoS values can a dial peer mark?
At 12.2 mainline IOS, only the IP precedence. As of 12.2T, DSCP can also be marked.
Chapter 4
“Do I Know This Already?” Quiz
Queuing Concepts
1Describe the benefits of having a single FIFO output queue.
The most basic benefit of queuing is to provide a means to hold a packet while the interface is busy. Without at least a single FIFO queue, routers would have to discard packets if the outgoing interface were busy.
2Explain the purpose of a TX Ring and TX Queue in a Cisco router.
By design, routers want to be able to immediately begin sending the next packet when the preceding packet’s last bit is sent. To do this, the interface hardware must have access to a queue structure with the next packet, and not be impeded by waiting on service from other processes. On Cisco routers, the TX Ring and TX Queue provide queue structures that are available to the interface directly, without relying on the main processor.
3Assume a queuing tool has been enabled on interface S0/0. Describe the circumstances under which the queuing tool would actually be used.
Congestion must occur on the interface first, which causes packets to be held in the TX Ring/Queue. When the TX Ring/Queue fills, IOS enables the queuing function on the interface.
4Explain the circumstances under which it would be useful to enable a queuing tool on a subinterface.
Queues only form on subinterfaces when traffic shaping is enabled on the subinterface.
Chapter 4 785
WFQ and IP RTP Priority
5Characterize the effect the WFQ scheduler has on different types of flows.
Lower-volume flows get relatively better service, and higher-volume flows get worse service. Higher-precedence flows get better service than lower-precedence flows. If lower-volume flows are given higher precedence values, the bandwidth, delay, jitter, and loss characteristics improve even more.
6Describe the WFQ scheduler process. Include at least the concept behind any formulas, if not the specific formula.
Each new packet is assigned a sequence number (SN), which is based on the previous packet’s SN, the length of the new packet, and the IP precedence of the packet. The formula is as follows:
(Previous SN + weight) * New packet length
The scheduler just takes the lowest SN packet when it needs to de-queue a packet.
7You previously disabled WFQ on interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.
Use the fair-queue interface subcommand.
8What commands list statistical information about the performance of WFQ?
The show interfaces and the show queueing fair commands list statistics about WFQ.
CBWFQ and LLQ
9Describe the CBWFQ scheduler process, both inside a single queue and among all queues.
The scheduler provides a guaranteed amount of bandwidth to each class. Inside a single queue, processing is FIFO, except for the class-default queue. In class-default, Flow-Based WFQ can be used, or FIFO, inside the queue.
10Describe how LLQ allows for low latency while still giving good service to other queues.
LLQ is actually a variation of CBWFQ, in which a single class is always serviced first—in other words, the low-latency queue is a strict-priority queue. To prevent the low-latency queue from dominating the link, and to continue to guarantee bandwidth amounts to other queues, the LLQ class is policed.
11Compare and contrast IP RTP Priority and LLQ. In particular, mention what other queuing tools can be used concurrently with each, how each classifies packets, and which is recommended by Cisco.
LLQ is actually a feature of CBWFQ and is always used in conjunction with CBWFQ. IP RTP Priority can be used with WFQ or CBWFQ to add a Low Latency Queue option. IP RTP Priority classifies packets based on even-number UDP ports,
Chapter 4 787
Q&A
Conceptual Questions
1Describe the benefits of having a single FIFO output queue.
The most basic benefit of queuing is to provide a means to hold a packet while the interface is busy. Without at least a single FIFO queue, routers would have to discard packets if the outgoing interface were busy.
2Explain the effects of changing a single FIFO queue’s length to twice its original value. Include comments about how the change affects bandwidth, delay, jitter, and loss.
With a longer queue, more packets can be enqueued before the queue fills. Therefore, the tail-drop process drops packets less often. However, with more packets in the queue, the average delay increases, which also can increase jitter. There is no impact on bandwidth.
3Explain the purpose of a TX Ring and TX Queue in a Cisco router.
By design, routers want to be able to begin immediately sending the next packet when the preceding packet’s last bit is sent. To do this, the interface hardware must have access to a queue structure with the next packet, and not be impeded by waiting on service from other processes. On Cisco routers, the TX Ring and TX Queue provide queue structures that are available to the interface directly, without relying on the main processor.
4Explain how a long TX Ring might affect the behavior of a queuing tool.
Output queuing does not occur until the TX Ring is full. If the TX Ring is long, the queuing tool may not be enabled. Because the TX Ring always uses FIFO logic, packets will not be reordered. With a short TX Ring, output queuing may be queuing the packets, and have an opportunity to reorder the packet exit sequence based on the queuing scheduling algorithm.
5Describe the command output that identifies the length of the TX Ring or TX Queue, and whether the length was automatically lowered by IOS.
The show controllers command lists output that includes the output line that reads something like “tx_limited=0(16).” The first number is 0 or 1, with 0 meaning that the statically-configured value is being used, and the number in parenthesis representing the length of the TX Ring/TX Queue. If the first number is 1, the TX Ring/ TX Queue has been automatically shortened by the IOS as a result of having a queuing tool enabled on the interface.