Chapter 2

"Do I Know This Already" Quiz

QoS Tools Questions

1.	Which of the following are not Queuing tools? LLQ CBPQ CBWFQ CBCQ WRR
Answer:	B
2.	Which of the following tools monitors the rate at which bits are sent out an interface? LLQ CB Shaping WRED CB Policing MLP LFI
Answer:	B, D
3.	Which of the following tools can mark IP packet's DSCP field? CB Marking WRED CB Policing MLP LFI NBAR
Answer:	A NBAR cannot actually mark packets, although it can be used to classify packets in conjunction with CB Marking.
4.	Which of the following tools chooses to discard packets even though the router either has memory to queue the packets, or available physical bandwidth to send the packets? CB Marking WRED CB Policing MLP LFI NBAR ECN
Answer:	B, C WRED discards packets in order to get TCP senders to slow down, and CB Policing discards packets when overall defined bit rates are exceeded. ECN does not actually discard packets, but instead signals the TCP sender to slow down in a different way than just discarding packets.

Classifying Using Flows or Service Classes Questions

5.	Which of the following are not used to identify a flow? ECN bits Source port Layer 4 protocol type Destination IP address TCP acknowledgment number
Answer:	A, E
6.	Which of the following are likely places at which to mark packets in a network using good QoS design practices? In client PCs As close to the client as possible without allowing end users to mark packets As packets enter an SP from a customer network As packets enter the LAN switch to which the destination host is attached
Answer:	B, C

The Differentiated Services QoS Model Questions

7.	What does DSCP stand for? Diffserv Static Code Point Diffserv Standardized Configuration Process Differentiated Services Code Point Differentiated Services Configuration Point
Answer:	C
8.	According to the DiffServ, which PHB defines a set of three DSCPs in each service class, with different drop characteristics for each of the three DSCP values? Expedited Forwarding Class Selector Assured Forwarding Multi-class-multi-drop
Answer:	C
9.	Which of the following is true about the location of DSCP? High order 6 bits of ToS byte Low order 6 bits of ToS byte Middle 6 bits of ToS byte High order 5 bits in ToS byte
Answer:	A
10.	Imagine a packet is marked with DSCP CS3. Later, a QoS tool classifies the packet. Which of the following classification criteria would match the packet, assuming the marking had not been changed from the original CS3 marking? Match of DSCP CS3 Match of precedence 3 Match on DSCP AF32 Match on DSCP AF31 Match on DSCP decimal 24
Answer:	A, B, E
11.	Imagine a packet is marked with AF31. Later, a QoS tool classifies the packet. Which of the following classification criteria would match the packet, assuming the marking had not been changed from the original AF31 marking? Match of DSCP CS3 Match of precedence 3 Match on DSCP AF32 Match on DSCP AF31 Match on DSCP 24
Answer:	B, D

The Integrated Services QoS Model Questions

12.	Which of the following are reasons why IntServ does not scale as well as DiffServ? Keeps flow state on each node Flow-based architecture Requires complex configuration for matching criteria for each possible flow Continuous signaling to maintain QoS reservation
Answer:	A, B, D

Q&A

1.	List the two classification and marking tools mentioned in this chapter, including the full names and popular acronyms.
Answer:	Class-Based Marking (CB Marking), Network-Based Application Recognition (NBAR).
2.	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).
3.	List the two shaping tools mentioned in this chapter, including the full names and popular acronyms.
Answer:	Frame Relay traffic shaping (FRTS) and Class-Based shaping (CB shaping).
4.	List three Congestion Avoidance tools, including the full names and popular acronyms.
Answer:	Random Early Detection (RED), Weighted RED (WRED), Explict Congestion Notification (ECN).
5.	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.
6.	List the QoS tools that perform some classification function.
Answer:	This is a bit of a trick question. Almost all IOS QoS tools perform classificationfor instance, to place two different types of packets into two different queues, the queue tool performs classification.
7.	Which of the following tools can be used for classification and marking? CB marking, PQ, CB shaping, WFQ, WRED, FRTS, LLQ, MLPPP LFI, NBAR, QPM, cRTP
Answer:	CB marking. NBAR can be used for classification in conjunction with CB Marking.
8.	Which of the following tools can be used for queuing? CB marking, PQ, CB shaping, WFQ, WRED, FRTS, LLQ, MLPPP LFI, NBAR, QPM, cRTP
Answer:	WFQ, LLQ, PQ.
9.	Which of the following tools can be used for shaping? CB marking, PQ, CB shaping, WFQ, WRED, FRTS, LLQ, MLPPP LFI, NBAR, QPM, cRTP
Answer:	CB shaping and FRTS
10.	Which of the following tools can be used for link efficiency? CB marking, PQ, CB shaping, WFQ, WRED, FRTS, LLQ, MLPPP LFI, NBAR, QPM, cRTP
Answer:	cRTP, MLPPP LFI
11.	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.
12.	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.
13.	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.
14.	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.
15.	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.
16.	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.
17.	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.
18.	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.
19.	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.
20.	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.
21.	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.
22.	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.
23.	List and describe the two key advantages of the Best Effort model for QoS.
Answer:	Best Effort (BE) scales well, because routers and switches do not have to perform any extra work for each packet or frame. And because Best Effort does no specific PHB, it also requires no specific QoS tools.
24.	List and describe the two key advantages of the DiffServ model for QoS.
Answer:	DiffServ scales well mainly due to its Class-Based operation. Also, DiffServ provides a large number of different classes, ensuring that most networks will have plenty of different classes for their network traffic.
25.	List and describe the two key disadvantages of the DiffServ model for QoS.
Answer:	DiffServ tools can be complicated, which requires more training and higher skill levels. Also, DiffServ does attempt to provide the appropriate bandwidth, delay, jitter, and loss characteristics, but it does not absolutely guarantee those characteristics.
26.	List and describe the two key disadvantages of the IntServ model for QoS.
Answer:	The main problem is poor scalability. IntServ scales poorly because it is flow-based, it signals repetitively for each flow, and the nodes must keep flow state information for each flow.