642-813 Q&A – Prepare infrastructure to support advanced services (51-55)

Section4 – Prepare infrastructure to support advanced services

QUESTION NO: 51
Which two statements are true about trust boundaries? (Choose two.)
A. Setting trust boundaries at the edge of the network allows intermediate hop devices to administer QoS policies without detailed packet identification.
B. Trust boundaries are used to determine which QoS mechanism will be applied to the traffic.
C. At the trust boundaries, the untrusted traffic will be marked with a new QoS value appropriate for the policy in place at the point where the traffic entered the campus network.
D. Trust boundaries are configured in the core of the network to provide the most efficient forwarding based upon QoS markings.
Answer: AC

Explanation:
The packets that enter your network or hardware can be marked into different classes; you can define the trust boundaries in your network. You can define some devices as trusted devices and some as untrusted devices. The packets that come from trusted devices are considered trusted because the trusted devices classify the packets correctly. The packets that come from untrusted devices are considered untrusted because they might not classify the packets correctly. After you have marked the packets and defined the trust boundaries, you can force the scheduling of the packets into different queues. These queues invoke at the time of congestion.
Defining trust boundaries is important in your network. Setting the trust boundary at the IP phone means that you can accept all the IP phone markings into the network without modifications.
You should always try to do classification close to the edge of the network, for scalability.
On an IP phone, the tagged data (802.1Q/p) from the PC or any other device that is attached to the access port of the IP phone can be trusted or untrusted. In trusted mode, the IP phone passes all the data unchanged. In untrusted mode, the IP phone re-marks the Layer 2 CoS value to the new value (if configured on the access layer switch) or changes it to 0, if nothing is configured. The default is untrusted mode, which is the recommend method.

QUESTION NO: 52
Refer to the exhibit. Which statement is true when voice traffic is forwarded on the same VLAN used by the data traffic?:

A. Quality of service cannot be applied for the voice traffic.
B. The voice traffic cannot be forwarded to the distribution layer.
C. Port security cannot be enabled on the switch that is attached to the IP phone.
D. The voice traffic cannot use 802.1p priority tagging.
Answer: D
Explanation:
You can configure a port connected to the Cisco IP Phone to send CDP packets to the phone to configure the way in which the phone sends voice traffic. The phone can carry voice traffic in 802.1Q frames for a specified voice VLAN with a Layer 2 CoS value. It can use 802.1P priority tagging to give voice traffic a higher priority and forward all voice traffic through the native (access) VLAN. The IP phone can also send untagged voice traffic or use its own configuration to send voice traffic in the access VLAN. In all configurations, the voice traffic carries a Layer 3 IP precedence value (the default is 5).
If the Cisco IP Phone and a device attached to the Cisco IP Phone are in the same VLAN, they must be in the same IP subnet. These conditions indicate that they are in the same VLAN:
– They both use 802.1p or untagged frames.
– The Cisco IP Phone uses 802.1p frames and the device uses untagged frames.
– The Cisco IP Phone uses untagged frames and the device uses 802.1p frames.
– The Cisco IP Phone uses 802.1Q frames and the voice VLAN is the same as the access VLAN.

QUESTION NO: 53
Which three QoS mechanisms can be configured to improve VoIP quality on a converged network? Select three.
A. The use of a queuing method that will give VoIP traffic strict priority over other traffic
B. The use of RTP header compression for the VoIP traffic.
C. The proper classification and marking of the traffic as close to the source as possible
D. The use of 802.1QinQ trunking for VoIP traffic
E. The use of WRED for the VoIP traffic
Answer: ACE
Explanation:
In order to optimize the quality of VOIP calls, QoS should be implemented to ensure that VOIP traffic is prioritized over other traffic types.
By providing a strict queue for VOIP traffic, you will ensure that voice calls take precedence over the other traffic types.
In order to properly provide for QoS across the network, the voice traffic should be marked to give priority as close to the source as possible. This will ensure that the traffic is prioritized end to end.
Finally, WRED (Weighted Random Early Detection) could be configured to prevent congestion.
WRED can be used to selectively drop less important traffic types, instead of dropping the voice packets when links become busy.
Incorrect Answers:
B: Compression can be used to lower the bandwidth required to transmit VOIP calls, but it will not help with improving the voice quality. In general, compression of any kind lowers the quality of VOIP.
D: The trunking method used will have no bearing on the VOIP quality.

QUESTION NO: 54
Which two types of activities does the Lightweight Access Point Protocol (LWAPP) define? (Choose two.)
A.access point certification and software control
B.compression and Layer 3 address mapping
C.Layer 3 addressing and distribution
D.packet encapsulation, fragmentation, and formatting
E.SNMP monitoring services
Answer: A D
Explanation:
The LWAPP specification works to address these issues by defining the following types of activities:
Access point device discovery, information exchange, and configuration
Access point certification and software control
Packet encapsulation, fragmentation, and formatting
Communications control and management between access point and wireless system device

QUESTION NO: 55
Refer to the exhibit. Which statement is true about the configuration that is shown?

A.Untagged ingress traffic will be dropped.
B.Untagged ingress traffic will be marked with the default CoS value of the port.
C.Tagged and untagged ingress traffic will be carried on VLAN 1.
D.Ingress traffic from the host will be tagged with the CoS value of 5.
Answer: B
Explanation:
The “Switchport voice vlan vlan-id dot1q” command enables voice VLAN on switch port and associated VLAN.
The “mls qos trust cos” command enables QoS (Quality of Services ) based on COS (Class of service)
The “switchport priority cos <priority>” value sets the priority values to frame coming from PC to switch port. Since this has not been explicitly configured, the default CoS value of the port will be used.