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Cisco® 642-887 Exam Practice Questions (P. 2)

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Question #6
When configuring LLQ (strict priority queue) on a traffic class using the Cisco IOS XR priority command on a Cisco ASR9K router, which additional QoS command is required for this traffic class?
  1. A
    shape
  2. B
    police
  3. C
    random-detect
  4. D
    bandwidth

Correct Answer:
B
The Low Latency Queueing feature brings strict priority queueing to Class-Based Weighted Fair Queueing (CBWFQ).

Question #7
On the Cisco ASR9K router, when using the bandwidth command to specify the minimum guaranteed bandwidth to be allocated for a specific class of traffic, what will be used as the queuing algorithm?
  1. A
    custom queuing
  2. B
    CBWFQ
  3. C
    WFQ
  4. D
    FIFO
  5. E
    priority queuing

Correct Answer:
B
Class based weighted fair queuing (CB-WFQ) was initially released without the support of a priority queuing system, thus it could not guarantee the delay and jitter
(delay variation) requirements of real-time, interactive voice and video conversations. Since for CBWFQ, the weight for a packet belonging to a specific class is derived from the bandwidth assigned to the class, which in turn determines the order in which packets are sent.
All packets are serviced fairly based on weight and no class of packets may be granted strict priority. This scheme poses problems for voice traffic that is largely intolerant of delay, especially variation in delay

Question #8
When implementing MPLS DS-TE on Cisco IOS XR routers, all aggregate Cisco MPLS TE traffic is mapped to which class type by default?
  1. A
    class-type 0 (bandwidth global pool)
  2. B
    class-type 1 (bandwidth subpool)
  3. C
    class-type 2 (bandwidth priority)
  4. D
    class type class-default (bandwidth best-effort)

Correct Answer:
A
Differentiated Services Traffic Engineering
MPLS Differentiated Services (Diff-Serv) Aware Traffic Engineering (DS-TE) is an extension of the regular MPLS-TE feature. Regular traffic engineering does not provide bandwidth guarantees to different traffic classes. A single bandwidth constraint is used in regular TE that is shared by all traffic. To support various classes of service (CoS), users can configure multiple bandwidth constraints. These bandwidth constraints can be treated differently based on the requirement for the traffic class using that constraint.
MPLS diff-serv traffic engineering provides the ability to configure multiple bandwidth constraints on an MPLSenabled interface. Available bandwidths from all configured bandwidth constraints are advertised using IGP.
TE tunnel is configured with bandwidth value and class-type requirements. Path calculation and admission control take the bandwidth and class-type into consideration. RSVP is used to signal the TE tunnel with bandwidth and class-type requirements.
Diff-Serv TE can be deployed with either Russian Doll Model (RDM) or Maximum Allocation Model (MAM) for bandwidth calculations.

TE Class Mapping -
Each of the eight available bandwidth values advertised in the IGP corresponds to a TE Class. Because the IGP advertises only eight bandwidth values, there can be a maximum of only eight TE classes supported in an IETF DS-TE network.
TE class mapping must be exactly the same on all routers in a DS-TE domain. It is the responsibility of the operator configure these settings properly as there is no way to automatically check or enforce consistency.
The operator must configure TE tunnel class types and priority levels to form a valid TE class. When the TE class map configuration is changed, tunnels already up are brought down. Tunnels in the down state, can be set up if a valid TE class map is found.

Table 4 TE Classes and Priority -

The default mapping includes four classes types.

Question #9
On the Cisco IOS XR, which MQC configuration is different than on the Cisco IOS and IOS XE?
  1. A
    On the Cisco IOS XR, WRED can only be applied in the output direction.
  2. B
    On the Cisco IOS XR, marking can only be applied in the input direction.
  3. C
    On the Cisco IOS XR, LLQ can be applied in the input or output direction.
  4. D
    On the Cisco IOS XR, LLQ can use up to four priority queues: level 1, level 2, level 3, and level 4.

Correct Answer:
C

Question #10
On Cisco routers, how is hierarchical QoS implemented?
  1. A
    Within the parent policy, reference another child policy using the policy-map command.
  2. B
    Within the child policy, reference another parent policy using the policy-map command.
  3. C
    Use the policy-map command within a service-policy to implement nested policy-maps.
  4. D
    Within the parent policy-map, reference another child policy-map using the service-policy command.

Correct Answer:
D

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