Chapter 5

"Do I Know This Already" Quiz

Cisco Router Queuing Concepts Questions

1.
What is the main benefit of the hardware queue on a Cisco router interface?
Prioritizes latency-sensitive packets so that they are always scheduled next
Reserves a minimum amount of bandwidth for particular classes of traffic
Provides a queue so that as soon as the interface is available to send another packet, the packet can be sent, without requiring an interrupt to the router CPU
Allows configuration of a percentage of the remaining link bandwidth, after allocating bandwidth to the LLQ and the class-default queue
Answer:
C

2.
A set of queues associated with a physical interface, for the purpose of prioritizing packets exiting the interface, are called which of the following?
Hardware queues
Software queues
Shaping queues
TX-queues
Answer:
B

3.
Which of the following commands could change the length of a hardware queue?
hardware queue-length 10
tx-queue length 10
hardware 10
tx-ring-limit 10
Answer:
D

Scheduling Concepts: FIFO, PQ, CQ, and MDRR Questions

4.
What is the main benefit of having FIFO queuing enabled on a Cisco router interface?
Prioritizes latency-sensitive packets so that they are always scheduled next
Reserves a minimum amount of bandwidth for particular classes of traffic
Provides a place to hold packets in RAM until space becomes available in the hardware queue for the interface.
Provides a queue so that as soon as the interface is available to send another packet, the packet can be sent, without requiring an interrupt to the router CPU
Allows configuration of a percentage of the remaining link bandwidth, after allocating bandwidth to the LLQ and the class-default queue
Answer:
C

5.
What are the main benefits of CQ being enabled on a Cisco router interface?
Prioritizes latency-sensitive packets so that they are always scheduled next
Reserves a minimum amount of bandwidth for particular classes of traffic
Provides a place to hold packets in RAM until space becomes available in the hardware queue for the interface.
Provides a queue so that as soon as the interface is available to send another packet, the packet can be sent, without requiring an interrupt to the router CPU
Allows configuration of a percentage of the remaining link bandwidth, after allocating bandwidth to the LLQ and the class-default queue
Answer:
B, C

6.
What is the main benefit of enabling PQ on a Cisco router interface?
Prioritizes latency-sensitive packets so that they are always scheduled next
Reserves a minimum amount of bandwidth for particular classes of traffic
Provides a place to hold packets in RAM until the interface becomes available for sending the packet
Provides a queue so that as soon as the interface is available to send another packet, the packet can be sent, without requiring an interrupt to the router CPU
Allows configuration of a percentage of the remaining link bandwidth, after allocating bandwidth to the LLQ and the class-default queue
Answer:
A

Concepts and Configuration: WFQ, CBWFQ, and LLQ Questions

7.
Which of the following are reasons why WFQ might discard a packet instead of putting it into the correct queue?
The hold-queue limit for all combined WFQ queues has been exceeded.
The queue length for the flow has passed the WRED minimum drop threshold.
The WFQ queue length for the queue where the newly-arrived packet should be placed has exceeded the CDT
ECN feedback has been signaled, requesting that the TCP sender slow down
Answer:
A, C

WFQ may discard the newly arriving packet, or it may discard a previously-enqueued packet in another queue, depending on sequence numbers.

8.
Which of the following settings cannot be configured for WFQ on the fair-queue interface subcommand?
CDT
Number of queues
Number of RSVP-reserved queues
Hold Queue limit
WRED thresholds
Answer:
D, E

9.
Examine the following configuration snippet. If a new class, called class3, was added to the policy-map, which of the following commands could be used to reserve 25 kbps of bandwidth for the class?
policy-map fred class class1 priority 20 class class2 bandwidth 30 ! interface serial 0/0 bandwidth 100 service-policy output fred

priority 25
bandwidth 25
bandwidth percent 25
bandwidth remaining-percent 25
Answer:
A, B

Multiple classes can be configured as LLQs with the priority command. Also, only one style of bandwidth command is allowed in a single policy map, making the last two answers incorrect in this case.

10.
Examine the following configuration snippet. How much bandwidth does IOS assign to class2?
policy-map fred class class1 priority percent 20 class class2 bandwidth remaining percent 20 interface serial 0/0 bandwidth 100 service-policy output fred

10 kbps
11 kbps
20 kbps
21 kbps
Not enough information to tell
Answer:
B

To find the answer, take interface bandwidth (100 kbps), subtract 25 percent of the bandwidth for the class-default bandwidth (based on max-reserved-bandwidth of 75 percent). That leaves 75 kbps. Then subtract 20 percent of the interface bandwidth (20 percent of 100 kbps) for the LLQ, leaving 55 kbps. Bandwidth remaining percent allocates percentages of the 55 kbps.

11.
What is the largest number of classes inside a single policy map that can be configured as an LLQ?
1
2
3
more than 3
Answer:
D

A policy map allows 256 classes, with a maximum of 64 classes defined for CBWFQ. Multiple LLQ classes are allowed.

12.
To prevent non-LLQ queues from being starved, LLQ can police the low-latency queue. Looking at the configuration snippet below, what must be changed or added to cause this policy-map to police traffic in class1?
policy-map fred class class1 priority 20 class class2 bandwidth remaining percent 20 interface serial 0/0 bandwidth 100 service-policy output fred

Change the priority 20 command to priority 20 500, setting the policing burst size
Add the police 20000 command under class1
Nothing the priority command implies that policing will also be performed
Add the LLQ-police global configuration command
Answer:
C

Q&A

1.
Describe the benefits of having a single FIFO output queue.
Answer:
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.

2.
Explain 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.
Answer:
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.

3.
Explain the purpose of a TX Ring and TX Queue in a Cisco router.
Answer:
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.

4.
Explain how a long TX Ring might affect the behavior of a queuing tool.
Answer:
Output queuing does not occur until the TX Ring is full. If the TX Ring is long, the Queuing tool might 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.

5.
Describe the command output that identifies the length of the TX Ring or TX Queue, and whether the length was automatically lowered by IOS.
Answer:
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.

6.
Explain under what circumstances the TX Ring, interface output queues, and subinterface output queues both fill and drain, and to where they drain.
Answer:
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 queues begin to fill. The subinterface output queues 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 queues, the interface queues into the TX Ring, and the TX Ring onto the physical interface.

7.
Assume a queuing tool has been enabled on interface S0/0. Describe the circumstances under which the queuing tool would actually be used.
Answer:
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.

8.
Explain the circumstances under which it would be useful to enable a queuing tool on a subinterface.
Answer:
Queues only form on subinterfaces when traffic shaping is enabled on the subinterface.

Scheduling Concepts: FIFO, PQ, CQ, and MDRR

9.
Describe the process and end result of the scheduling feature of Priority Queuing.
Answer:
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, with possile total starvation of the lower queues.

10.
Describe the process and end result of the scheduling feature of Custom Queuing.
Answer:
Scheduler 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.

11.
Describe how the Modified Deficit Round-Robin scheduler works, and specifically why the word "deficit" refers to part of the scheduler logic.
Answer:
DRR schedules some number of bytes per pass through the queues. MDRR takes packets from the queue, which means it may take more than the allotted number of bytes; this excess is called the deficit. The deficit is subtracted from the number of bytes taken from that queue in the next round. As a result, MDRR can accurately predict the percentage bandwidth assigned to a queue.

Concepts and Configuration: WFQ, CBWFQ, and LLQ

12.
WFQ classifies packets based on their flow. Other than a typical flow from an end user device, identify the other two types of flows recognized by WFQ.
Answer:
WFQ reserves 8 flow queues for system overhead traffic. It also adds flows in conjunction with RSVP, helping to reserve bandwidth for those flows.

13.
Characterize the effect the WFQ scheduler has on different types of flows.
Answer:
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.

14.
Describe the WFQ scheduler process. Include at least the concept behind any formulas, if not the specific formula.
Answer:
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.

15.
You previously disabled WFQ on interface S0/0. List the minimum number of commands required to enable WFQ on S0/0.
Answer:
Use the fair-queue interface subcommand.

16.
What commands list statistical information about the performance of WFQ?
Answer:
The show interfaces and the show queueing fair commands list statistics about WFQ.

17.
Define what comprises a flow in relation to WFQ.
Answer:
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.

18.
You 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.
Answer:
No commands are required. WFQ is the default on E/1 and slower interfaces in a Cisco router.

19.
Describe the CBWFQ scheduler process, both inside a single queue and among all queues.
Answer:
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.

20.
Describe how LLQ allows for low latency while still giving good service to other queues.
Answer:
LLQ is actually a variation of CBWFQ, in which the LLQ classes are always serviced firstin 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.

21.
Compare and contrast the CBWFQ command that configures the guaranteed bandwidth for a class with the command that enables LLQ for a class.
Answer:
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.

22.
Describe the CBWFQ classification options. List at least five fields that can be matched without using an ACL.
Answer:
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.

23.
Name 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.
Answer:
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.

24.
Examine the following configuration (Example 5-10). Which of the five policy maps would certainly enable LLQ for voice payload traffic, based only of the information in the configuration?

Example 5-10. 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 class class1 priority 60 policy-map pmap2 class class2 priority 60 policy-map pmap3 class class3 priority 60 policy-map pmap4 class class4 priority 60 policy-map pmap5 class class5 priority 60 ! interface Serial0/0 service-policy output ????? ! access-list 101 permit udp any gt 16383 any gt 16383 access-list 102 permit udp any range 16383 32767 any range 16383 32767 !

Answer:
All the policy maps except pmap4 would perform LLQ on voice payload. In some cases, the policy map would match more than just voice payload. Only pmap1 would match just RTP voice payload traffic.

25.
Using the same exhibit as in the preceding example, describe what must also be true for pmap4 to queue voice payload traffic successfully, and only voice payload traffic, in a low-latency queue.
Answer:
If some other classification and marking tool were configured, and it marked all voice payload traffic as DSCP EF, pmap4 would match all voice packets in the low-latency queue.

26.
Which of the following queuing tools can always service a particular queue first, even when other queues have packets waiting? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ).
Answer:
PQ and LLQ.

27.
Which of the following queuing tools allows for a percentage bandwidth to be assigned to each queue? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ).
Answer:
CBWFQ and LLQ. CQ effectively does this as well, but you cannot specify the exact percentage.

28.
Which queuing tools could be configured to provide the lowest possible latency for voice traffic? Of these, which does Cisco recommend as the best option for voice queuing today?
Answer:
PQ and LLQ. PQ would probably not be a good option in many networks today, but it could provide the lowest possible latency for voice. Cisco currently recommends LLQ.

29.
Which of the following queuing tools can use flow-based classification? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ).
Answer:
WFQ and CBWFQ in the class-default queue.

30.
Which of the following queuing tools uses the Modular QoS CLI? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ).
Answer:
CBWFQ, LLQ.

31.
Which of the following queuing tools allows for a value to be configured, which then results in a specific number of bytes being taken from each queue during a round-robin pass through the queues? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ).
Answer:
CQ.

32.
What model of Cisco router supports WFQ inside CBWFQ classes other than class-default?
Answer:
7500 series routers.

33.
Give an explanation for the following comment: "WFQ can become too fair when it has a large number of active flows"?
Answer:
With many flows, WFQ will give some bandwidth to every flow. In an effort to give each flow some of the link bandwidth, WFQ may actually not give some or most of the flows enough bandwidth for them to survive.

34.
Imagine the following commands in Example 5-11 were typed in configuration mode, in order. Also assume that class maps class1, class2, and class3 have already been correctly defined. How much bandwidth will class class3 be assigned on interface S0/0?

Example 5-11. Exhibit for CBWFQ Configuration Questions

! policy-map pmap1 class class1 priority 60 class class2 bandwidth percent 30 class class3 bandwidth percent 45 ! policy-map pmap2 class class1 priority percent 20 class class2 bandwidth remaining percent 30 class class3 bandwidth remaining percent 70 ! policy-map pmap3 class class1 priority percent 20 class class2 bandwidth 30 class class3 bandwidth percent 30 ! interface Serial0/0 service-policy output pmap1 ! interface Serial0/1 bandwidth 512 service-policy output pmap2 ! interface Serial0/2 bandwidth 256 service-policy output pmap3 !

Answer:
Actually, none. The service-policy command would be rejected, because the 60 kbps in class1, plus the 75 percent total bandwidth in class2 and class3 would exceed the maximum bandwidth allowed in the policy map.

35.
In the same example, what could be done so that the service-policy output pmap1 command would be accepted under interface serial0/0without changing the policy map? Assuming that was done, what actual bandwidth could be assigned to class3?
Answer:
The 60 kbps for class1 is 4 percent of the configured interface bandwidth. So, the max-reserved-bandwidth 79 interface subcommand could be used on interface s0/0, and then the service-policy output pmap1 would be accepted. Then, class3 would get 45 percent of 1544 kbps, or 695 kbps.

36.
In the same example, how much bandwidth would class1 be assigned on interface serial 0/1?
Answer:
class1, the LLQ, would get 20 percent of interface bandwidth, which is defined as 512 kbps. So, class1 would get 102 kbps.

37.
In the same example, how much bandwidth would class2 and class3 be assigned on interface serial 0/1?
Answer:
class2 and class3 use the bandwidth remaining percent command. So, you would subtract 25 percent of the link bandwidth from the configured 512 kbps interface bandwidth, as well as the 102 kbps reserved for class1. That leaves 280 kbps total remaining bandwidth. class2 gets 30 percent of the remainder, or 84 kbps, and class3 gets 70 percent of the remainder, or 196 kbps.

38.
In the same example, how much bandwidth would class2 and class3 be assigned on interface serial 0/2?
Answer:
Actually, class2 would get 30 kbps, and class3 would get none. When policy map pmap3 is configured as shownrecall, the example shows the commands as typed, not the output of a show running-config commandthe bandwidth percent 30 class subcommand would be rejected, because you cannot mix different styles of bandwidth class subcommands inside a single policy map. In effect, you would not have a bandwidth class subcommand configured for class3 at this point.