CCDA 640-864 Q&As – WAN Technologies and Design (31-35)

March 12, 2012

Section 6 – WAN Technologies and Design

Which queuing mechanism establishes four interface output queues that will be used for traffic scheduling?
A.    Priority queuing (PQ)
B.    First-in, first-out (FIFO)
C.    Weighted fair queuing (WFQ)
D.    Custom queuing (CQ)
Answer: A

Congestion Management
Two types of output queues are available on routers: the hardware queue and the software queue. The hardware queue uses the strategy of first in, first out (FIFO). The software queue schedules packets first and then places them in the hardware queue. Keep in mind that the software queue is used only during periods of congestion. The software queue uses QoS techniques such as priority
queuing, custom queuing, weighted fair queuing, class-based weighted fair queuing, low-latency queuing, and traffic shaping and policing.
Priority Queuing
Priority queuing (PQ) is a queuing method that establishes four interface output queues that serve different priority levels: high, medium, default, and low. Unfortunately, PQ can starve other queues if too much data is in one queue because higher-priority queues must be emptied before lower- priority queues.
Custom Queuing
Custom queuing (CQ) uses up to 16 individual output queues. Byte size limits are assigned to each queue so that when the limit is reached, it proceeds to the next queue. The network operator can customize these byte size limits. CQ is fairer than PQ because it allows some level of service to all traffic. This queuing method is considering legacy due to the improvements in the queuing methods.
Weighted Fair Queuing
Weighted fair queuing (WFQ) ensures that traffic is separated into individual flows or sessions without requiring that you define ACLs. WFQ uses two categories to group sessions: high bandwidth and low bandwidth. Low-bandwidth traffic has priority over high-bandwidth traffic. High- bandwidth traffic shares the service according to assigned weight values. WFQ is the default QoS mechanism on interfaces below 2.0 Mbps.
Cisco Press CCDA 640-864 Official Certification Guide Fourth Edition, Chapter 6

Which option is not valid for using the public Internet as a backup WAN medium?
A.    IP Security (IPSec) tunnels
B.    Shared PVC
C.    IP routing without constraints
D.    Generic Routing Encapsulation (GRE) tunnels
Answer: B

Which one of the following QoS mechanisms is recommended for VoIP networks?
A.    Low-latency queuing (LLQ)
B.    Switched-based queuing
C.    Fast queuing
D.    Custom queuing
Answer: A
Low-Latency Queuing
Low-latency queuing (LLQ) adds a strict priority queue (PQ) to CBWFQ. The strict PQ allows delay sensitive traffic such as voice to be sent first, before other queues are serviced. That gives voice preferential treatment over the other traffic types. Unlike priority queuing, LLQ provides for a maximum threshold on the PQ to prevent lower priority traffic from being starved by the PQ.
Without LLQ, CBWFQ would not have a priority queue for real-time traffic. The additional classification of other traffic classes is done using the same CBWFQ techniques. LLQ is the standard QoS method for many VoIP networks.
Cisco Press CCDA 640-864 Official Certification Guide Fourth Edition, Chapter 6

Which statement is true about WANs?
A.    Switches or concentrators often relay information through the WAN.
B.    WANs typically encompass broad geographic areas.
C.    In general, WAN technologies function at the middle three layers of the Open System Interconnection (OSI) model.
D.    Users of WANs do not typically own all transmission facilities.
Answer: B

Which packet-switching topology approach typically requires the greatest level of expertise to implement?
A.    Hub and spoke
B.    Point-to-point
C.    Star
D.    Partial mesh
Answer: D

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