Cisco QoS FAQ: Congestion Management

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.

Q2. 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.

Q3. 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.

Q4. 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 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.

Q5. 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.

Q6. 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 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

Q7. 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.

Q8. 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.

Q9. Describe the classification feature of Priority Queuing, including the list of items that can be examined for classification decisions.

Answer: 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.

Q10. Describe the classification feature of Custom Queuing, including the list of items that can be examined for classification decisions.

Answer: 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.

Q11. List the classification options available to Custom Queuing that are not also available to Priority Queuing.

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

Q12. 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.

Q13. Describe the process and end result of the scheduling feature of Custom Queuing.

Answer: 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.

Q14. List the maximum number of queues used by Priority Queuing and Custom Queuing.

Answer: PQ has 4 queues, and CQ has 16. CQ also has a reserved queue, into which only IOS can schedule packets. WFQ

Q15 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.

Q16 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.

Q17. 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.

Q18. What commands list statistical information about the performance of WFQ?

Answer: The show interfaces and the show queueing fair commands list statistics about WFQ.

Q19. 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.

Q20. 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. CBWFQ, LLQ, IP RTP Priority

Q21. 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.

Q22. 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 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.

Q23. Compare 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.

Answer: 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.

Q24. 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.

Q25. 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.

Q26. 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.

Q27. List the command used to configure IP RTP Priority on a serial link.

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

Q28. Characterize 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.

Answer: 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.

Q29. Examine 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: Exhibit for CBWFQ Configuration Questions
cisco-qos-faq-congestion-management

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.

Q30. 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 lowlatency 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 lowlatency queue. Comparing Queuing Tool Options

Q31. Which of the following queuing tools allows for WRED inside a single 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); and IP RTP Priority.

Answer: CBWFQ.

Q32. 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); and IP RTP Priority.

Answer: PQ, LLQ, and IP RTP Priority.

Q33. 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); and IP RTP Priority.

Answer: CBWFQ and LLQ. CQ effectively does this as well, but you cannot specify the exact percentage.

Q34. 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, IP RTP Priority, 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.

Q35. Which of the following queuing tools can use flow-based classification? First-In, FirstOut Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.

Answer: WFQ and CBWFQ in the class-default queue.

Q36. 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); and IP RTP Priority.

Answer: CBWFQ, LLQ.

Q37. 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); and IP RTP Priority.

Answer: CQ.

Q38. Which of the following queuing tools allow the largest number of queues for a flow-based tool? For a class-based tool? What are the maximum values? First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.

Answer: WFQ allows 4096 (flow based), and CBWFQ allows 64 queues (class based). DQOS.book Page 793 Wednesday, July 16, 2003 3:54 PM 794 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections

Q39. Which queuing tools could be configured to provide the lowest possible latency for voice signaling traffic?

Answer: PQ and LLQ. PQ would probably not be a good option in many networks today. IP RTP Priority does not match voice signaling traffic.

Cisco QoS FAQ: Congestion Management

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