Chapter 4 789
12Describe the process and end result of the scheduling feature of Priority Queuing.
Always service higher-priority queues first; the result is great service for the High queue, with the potential for 100 percent of link bandwidth. Service degrades quickly for lower-priority queues.
13Describe the process and end result of the scheduling feature of Custom Queuing.
Services packets from a queue until a byte count is reached; round-robins through the queues, servicing the different byte counts for each queue. The effect is to reserve a percentage of link bandwidth for each queue.
14List the maximum number of queues used by Priority Queuing and Custom Queuing.
PQ has 4 queues, and CQ has 16. CQ also has a reserved queue, into which only IOS can schedule packets.
WFQ
15Characterize 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.
16Describe 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, 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.
17You 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.
18What commands list statistical information about the performance of WFQ?
The show interfaces and the show queueing fair commands list statistics about WFQ.
19Define what comprises a flow in relation to WFQ.
A flow consists of all packets with the same source and destination IP address, transport layer protocol, and transport layer source and destination port. Some references also claim that WFQ includes the ToS byte in the definition of a flow.
790 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
20You just bought and installed a new 3600 series router. Before adding any configuration to the router, you go ahead and plug in the new T1 Frame Relay access link to interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.
No commands are required. WFQ is the default on E/1 and slower interfaces in a Cisco router.
CBWFQ, LLQ, IP RTP Priority
21Describe 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.
22Describe how LLQ allows for low latency while still giving good service to other queues.
LLQ is actually a variation of CBWFQ, in which the LLQ classes are always serviced first—in other words, the low-latency queues are a strict-priority queues. To prevent the low-latency queues from dominating the link, and to continue to guarantee bandwidth amounts to other queues, the LLQ classes are policed.
23Compare 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, in a specified range. LLQ can classify on anything that can be matched using MQC classification commands, making it much more flexible. Given a choice, Cisco recommends LLQ.
24Compare and contrast the CBWFQ command that configures the guaranteed bandwidth for a class with the command that enables LLQ for a class.
The bandwidth command enables you to define a specific bandwidth, or a percentage bandwidth. The priority command, which enables LLQ in a class, appears to reserve an amount or percentage of bandwidth as well. However, it actually defines the policing rate, to prevent the LLQ from dominating the link. The priority command enables you to set the policing burst size as well.
