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788 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections

6Explain under what circumstances the TX Ring, interface output queues, and subinterface output queues both fill and drain, and to where they drain.

The TX Ring fills when the packets needing to exit an interface exceed the line (clock) rate of the interface. When the TX Ring fills, the interface output queue(s) begin to fill. The subinterface output queue(s) only fill if traffic shaping is enabled on the subinterfaces or individual VCs, and if the offered traffic on a subinterface or VC exceeds the shaped rate. The VC or subinterface queues drain into the interface queue(s), the interface queue(s) into the TX Ring, and the TX Ring onto the physical interface.

Priority Queuing and Custom Queuing

7Assume 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/TX Queue. When the TX Ring/TX Queue fills, IOS enables the queuing function on the interface.

8Explain 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.

9Describe the classification feature of Priority Queuing, including the list of items that can be examined for classification decisions.

PQ can classify packets using access-control lists (ACLs) for most Layer 3 protocols, matching anything allowed by any of the types of ACLs. PQ can also directly match, without using an ACL, the incoming interface, packet length, and TCP and UDP ports numbers.

10Describe the classification feature of Custom Queuing, including the list of items that can be examined for classification decisions.

CQ performs class-based queuing, in that it classifies packets on a large variety of packet header fields. CQ can classify packets using access-control lists (ACLs) for most Layer 3 protocols, matching anything allowed by any of the types of ACLs. CQ can also directly match, without using an ACL, the incoming interface, packet length, and TCP and UDP ports numbers.

11List the classification options available to Custom Queuing that are not also available to Priority Queuing.

There are none. PQ and CQ classification options are identical.

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.

Chapter 4 791

25Describe the CBWFQ classification options. List at least five fields that can be matched without using an ACL.

CBWFQ uses the Modular QoS CLI, and therefore can match on any fields that can be matched with other MQC tools, like CB marking. Other than referring to an ACL, CBWFQ can classify based on incoming interface, source/destination MAC, IP Precedence, IP DSCP, LAN CoS, QoS group, MPLS Experimental bits, and anything recognizable by NBAR.

26Name the two CBWFQ global configuration commands that define classification options, and then the per-hop behaviors, respectively. Also list the command that enables CBWFQ on an interface.

The class-map command names a class map and places the user into class-map configuration mode. Classification parameters can be entered at that point. The policy-map command names a policy and enables you to refer to class maps and then define actions. The service-policy command enables the policy map for packets either entering or exiting the interface.

27List the command used to configure IP RTP Priority on a serial link.

ip rtp priority starting-rtp-port-number port-number-range bandwidth

28Characterize the type of traffic that can be queued using both IP RTP priority and LLQ. List specific port numbers and IP addresses as applicable, and describe the type of traffic.

Both tools can classify and queue traffic using even-numbered UDP ports from 16384 to 32767. These UDP ports are used to transport RTP traffic, which holds voice payload, but not signaling, traffic.

29Examine the following configuration (Example 4-10). Which of the five policy maps would certainly enable LLQ for voice payload traffic, based only of the information in the configuration?

Example 4-10 Exhibit for CBWFQ Configuration Questions

!

class-map match-all class1 match ip rtp 16384 16383 class-map match-all class2 match access-group 101 class-map match-all class3 match ip rtp 16384 32767 class-map match-all class4

match ip dscp ef class-map match-all class5

match access-group 102

!

policy-map pmap1

continues