Cisco QoS FAQ: QoS Tools and Architectures
Q1. List four classification and marking tools, including the full names and popular acronyms.
Answer: Committed access rate (CAR), policy-based routing (PBR), class-based marking (CB marking), dial peers, network-based application recognition (NBAR), QoS policy propagation with BGP (QPPB).
Q2. List four queuing tools, including the full names and popular acronyms.
Answer: Priority Queuing (PQ), Custom Queuing (CQ), Weighted Fair Queuing (WFQ), IP RTP Priority, Class-Based WFQ (CBWFQ), Low Latency Queuing (LLQ), Modified Deficit Round-Robin (MDRR).
Q3. List four policing and shaping tools, including the full names and popular acronyms.
Answer: Committed access rate (CAR), class-based policing (CB policing), Frame Relay traffic shaping (FRTS), generic traffic shaping (GTS), distributed traffic shaping (DTS), class-based shaping (CB shaping).
Q4. List three congestion-avoidance tools, including the full names and popular acronyms.
Answer: Random Early Detection (RED), Weighted RED (WRED), Flow-Based RED (FRED).
Q5. List four link-efficiency tools, including the full names and popular acronyms.
Answer: Payload compression, RTP header compression (cRTP), TCP header compression, Multilink PPP fragmentation and interleaving (MLPPP LFI), Frame Relay fragmentation (FRF), link fragmentation and interleaving for Frame Relay and ATM VCs.
Q6. List seven VoIP CAC tools, including the full names and popular acronyms.
Answer: Physical DS0 limitation, Max-connections, voice-bandwidth for Frame Relay, trunk conditioning, Local Voice Busy-Out (LVBO), Advanced Voice Busy-Out (AVBO), PSTN Fallback, Resource Availability Indicator (RAI), Gatekeeper Zone Bandwidth (GK Zone Bandwidth), Resource Reservation Protocol (RSVP).
Q7. List four QoS management tools, including the full names and popular acronyms.
Answer: QoS Device Manager (QDM), QoS Policy Manager (QPM), Service Assurance Agent (SAA), Internetwork Performance Monitor (IPM), Service Management Solution (SMS).
Q8. List the QoS tools that perform some classification function.
Answer: This is a bit of a trick question. Almost all IOS QoS tools perform classification—for instance, to place two different types of packets into two different queues, the queue tool performs classification.
Q9. Which of the following tools can be used for classification and marking? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: CB marking, NBAR, CAR, and QPPB.
Q10. Which of the following tools can be used for queuing? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: WFQ, LLQ, PQ, CQ.
Q11. Which of the following tools can be used for policing? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: CAR
Q12. Which of the following tools can be used for shaping? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: CB shaping, FRTS, GTS
Q13. Which of the following tools can be used for link efficiency? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: FRF, cRTP, MLPPP LFI
Q14. Which of the following tools can be used for network management? CAR, CB marking, PQ CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
Answer: QPM, QDM, SAA
Q15. Define the DiffServ term “behavior aggregate.”
Answer: According to RFC 2475, a behavior aggregate is “a collection of packets with the same DS code point crossing a link in a particular direction.” The key points are that the DSCP has been set; the packets all move the same direction; and the packets collectively make up a class.
Q16. Define the DiffServ term “DSCP,” including what the acronym stands for.
Answer: According to RFC 2475, DSCP refers to “a specific value of the DSCP portion of the DS field, used to select a PHB.” The acronym stands for differentiated services code point. It is the 6-bit filed in the redefined ToS byte in the IP header used for marking packets for DiffServ.
Q17. Define the DiffServ term “PHB,” including what the acronym stands for.
Answer: According to RFC 2475, PHB refers to “the externally observable forwarding behavior applied at a DS-compliant node to a DS behavior aggregate.” The acronym stands for per-hop behavior. It is the collection of QoS actions that occur at one router (hop) in a network for a particular BA.
Q18. Define the DiffServ term “MF classifier,” including what the acronym stands for.
Answer: According to RFC 2475, an MF classifier is “a multi-field (MF) classifier which selects packets based on the content of some arbitrary number of header fields; typically some combination of source address, destination address, DS field, protocol ID, source port and destination port.” It is the classification function used to classify packets before the DSCP has been set.
Q19. Define the DiffServ term “DS ingress node,” including what the acronym stands for.
Answer: According to RFC 2475, a DS ingress node is “a DS boundary node in its role in handling traffic as it enters a DS domain.” DS stands for differentiated services. The term defines a node at which packets enter the DiffServ domain.
Q20. Compare and contrast the terms “BA classifier” and “MF classifier,” according to DiffServ specifications. Suggest typical points in the network where each is used.
Answer: A classifier is a DiffServ function that classifies or categories packets based on the contents of fields in the packet headers. A BA classifier performs this function only based on the DSCP field. An MF classifier can look at many fields in the packet header. MF classifiers typically classify ingress traffic near the edge of a network, and work with markers to set the DSCP field. BA classifiers are used at points in the network after an MF classifier and marker have set the DSCP field values.
Q21. Compare and contrast the terms “shaper,” “meter,” and “dropper,” according to DiffServ specifications. Suggest typical points in the network where each is used.
Answer: The meter function measures the packet/bit rate of ingress or egress traffic, as well as the sizes of bursts of traffic, with the goal of determining whether the traffic meets the agreed-upon traffic contract. A shaper delays packets on egress so that the bit rate and burst size do not exceed a configured level. Droppers, called policers by Cisco IOS, drop packets that exceed a configured bit rate and burst size. Shapers are typically used on egress from one DS domain; policers are typically used on ingress boundary nodes.
Q22. Compare and contrast the contents of the IP ToS byte before and after the advent of DiffServ.
Answer: Before DiffServ, the ToS byte contained a 3-bit Precedence field, 4 bits in a ToS field, and 1 reserved bit. DiffServ redefined the ToS byte to contain a 6-bit DSCP field, which contains the DSCP values, and 2 reserved bits.
Q23. Describe the QoS behavior at a single DS node when using the AF PHB. Also explain what the acronym “AF PHB” represents and identify the RFC that defines it.
Answer: The assured forwarding per-hop behavior, as defined in RFC 2597, defines a PHB with two components. The first part defines four BAs or classes, each which should be placed in a separate queue and given a configured guaranteed minimum amount of bandwidth. The second component provides three different drop probabilities for a congestion-avoidance tool such as RED.
24 Explain (by comparing and contrasting) whether AF and CS PHB DSCPs conform to the concept that “bigger DSCP values are better than smaller values.”
Answer: CS uses values that have three binary 0s at the end, and the eight IP precedence values for the first three bits. In other words, CS includes the eight binary values for a 6-bit number for which the last three digits are 0s. CS conforms to the idea that a bigger value is better, to be backward compatible with IP precedence. AF uses 12 different values. Of the three AF DSCPs in each class, the highest of the three values actually receives the worst drop preference.
Q25. Describe the QoS behavior at a single DS node when using the EF PHB. Also explain what the acronym “EF PHB” represents and identify the RFC that defines it.
Answer: The expedited forwarding per-hop behavior, as defined in RFC 2598, defines a PHB with two components. The first part defines queuing, with features that reserve bandwidth for a single BA, with the added feature on minimizing latency, delay, and loss. The other action of the PHB provides a policing/dropper function, disallowing traffic beyond a configured maximum bandwidth for the class.
Q26. Describe the process used by RSVP to reserve bandwidth in a network.
Answer: A host signals to the network using an RSVP reservation request using an RSVP path message. The request passes along the route to the destination host; at each intermediate router, if that router can guarantee the right bandwidth, the request is forwarded. When received by the destination host, it replies with an RSVP resv message. The process is reversed, with each router passing the reserve message if it can guarantee the bandwidth in the opposite direction. If the original host receives the reservation message, the bandwidth has been reserved.
Q27. Imagine an enterprise network, connected to an Internet service provider (ISP), that is connected to a second ISP, which is then connected to another enterprise network. The second ISP does not support IntServ directly. Discuss the two options that allow the other three networks to support IntServ for flows that pass through the nonsupporting ISP.
Figure: IntServ Through the Internet, with Partial Support
Answer: Two options exist for support of RSVP and IntServ. Both require ISP2 to pass the RSVP messages through it just like any other packet. Additionally, with one option, no guarantees are made in ISP2; the packets in the reserved flows are treated as besteffort traffic. The other option maps the RSVP flows to a DSCP, so ISP2 can provide DiffServ QoS treatment.
Q28. List and describe the two main features of IntServ.
Answer: IntServ provides resource reservation and admission control. Resource reservation uses the RSVP protocol to reserve bandwidth and set a maximum delay value. To prevent too many reserved flows from occurring, however, routers can reject new reservations, which is called admission control.
Q29 Compare and contrast DiffServ and IntServ in terms of using classes, flows, and scalability.
Answer: IntServ applies to individual flows, whereas DiffServ differentiates traffic into classes. With large networks and the Internet, the number of IntServ-managed flows does not scale, because information retained about each flow, and the RSVP signaling messages for each flow, continues throughout the life of each flow. DiffServ uses classes, and the number of classes does not increase when packet volumes increase, which allows better scalability.
Q30. Describe the two options available to a router to perform IntServ admission control.
Answer: Routers can perform local admission control by just examining the amount of currently reserved bandwidth, versus the maximum allowed, on each of its interfaces. Routers can also use a Common Open Policy Server (COPS) policy decision point (PDP), asking it to decide whether enough bandwidth is available in the network.
Q31. What is the QoS framework, and what does it define?
Answer: The QoS framework is a drawing and description that lists the major components of QoS functions in a network.
Q32. Which five categories of QoS tools are shown in the bottom half of the Cisco QoS framework?
Answer: Classification and marking, congestion management, link efficiency, traffic conditioners, congestion avoidance.
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