642-813 Q&A – Prepare infrastructure to support advanced services (71-75)

Section4 – Prepare infrastructure to support advanced services

QUESTION NO: 71
A Cisco IP phone connects to a host PC and a switch as shown below:

Study the exhibit carefully. Which statement is true about a voice VLAN in this segment?
A. Physically the voice network and the data network are separate.
B. The voice traffic will normally be on a different IP subnet than will the data traffic.
C. End user intervention is necessary to place the phone into the proper VLAN.
D. The same security policy should be implemented for both voice and data traffic.
E. The data VLAN must be configured as the native VLAN.
Answer: B
Explanation:
The new voice VLAN is called an auxiliary VLAN in the Catalyst software command-line interface (CLI). In the traditional switched world, data devices reside in a data VLAN. The new auxiliary VLAN is used to represent other types of devices collectively. Today those devices are IP phones (hence the notion of a voice VLAN), but, in the future, other types of non-data devices will also be part of the auxiliary VLAN. Just as data devices come up and reside in the native VLAN (default VLAN), IP phones come up and reside in the auxiliary VLAN, if one has been configured on the switch.
When the IP phone powers up, it communicates with the switch using CDP. The switch then provides the phone with its configured VLAN ID (voice subnet), also known as the voice VLAN ID or VVID. Meanwhile, data devices continue to reside in the native VLAN (or default VLAN) of the switch. A data device VLAN (data subnet) is referred to as a port VLAN ID or PVID.

QUESTION NO: 72
What does the auto qos voip cisco-phone command do?
A. If a Cisco IP phone is attached, the switch trusts the CoS.
B. The switch assigns a CoS value of 5 to incoming packets.
C. It turns on STP to see if a Cisco IP phone is attached.
D. If a Cisco IP phone is attached and removed, the switch continues to trust the CoS values as long as the switch is not rebooted.
E. It disables the trust boundary feature because the switch knows a Cisco IP phone is attached.
Answer: A
Explanation:
When the “auto qos voip cisco-phone” interface configuration command is entered, the trusted boundary feature is enabled. The trusted boundary feature uses CDP to detect the presence or absence of a Cisco IP Phone. When a Cisco IP Phone is detected, the ingress classification on the interface is set to trust the QoS label received in the packet. When a Cisco IP Phone is absent,
the ingress classification is set to not trust the QoS label in the packet. The egress queues on the interface are also reconfigured. This command extends the trust boundary if an IP Phone is detected.

QUESTION NO: 73
Refer to the exhibit. What is the effect when the switchport priority extend cos 3 command is configured on the switch port interface connected to the IP phone?

A. Effectively, the trust boundary has been moved to the PC attached to the IP phone.
B. The computer is now establishing the CoS value and has effectively become the trust boundary.
C. The IP phone is enabled to override with a CoS value of 3 the existing CoS marking of the PC attached to the IP phone.
D. The switch will no longer tag incoming voice packets and will extend the trust boundary to the distribution layer switch.
E. RTP will be used to negotiate a CoS value based upon bandwidth utilization on the link.
Answer: C
Explanation:
The “switchport priority extend cos <priority>” is used to set the IP phone access port to override the priority received from the PC or the attached device. The CoS value is a number from 0 to 7.
Seven is the highest priority. The default is 0. In this case, it has been set to mark all traffic with a class of service value of 3.

QUESTION NO: 74
Refer to the exhibit. The signal transmitted from the AP is reflected off a wall resulting in multipath interference at the client end. Which statement is true?

A. If signal 1 is in phase with signal 2, the result is essentially zero signal or a dead spot in the WLAN.
B. If signal 2 is close to 360 degrees out of phase with signal 1, the result is essentially zero signal or a dead spot in the WLAN.
C. Multipath interference is solved by using dual antennas.
D. Multipath interference is less of an issue when using a DSSS technology because multipath is frequency selective.
E. The transmitted signal from the AP arrives at the client at slightly different times resulting in phase shifting.
Answer: E
Explanation:
In order to understand diversity using dual antenna’s , you must understand multipath distortion.
When a radio frequency (RF) signal is transmitted towards the receiver, the general behavior of the RF signal is to grow wider as it is transmitted further. On its way, the RF signal encounters objects that reflect, refract, diffract or interfere with the signal. When an RF signal is reflected off an object, multiple wavefronts are created. As a result of these new duplicate wavefronts, there
are multiple wavefronts that reach the receiver. Diversity is the use of two antennas for each radio, to increase the odds that you receive a better signal on either of the antennas. The antennas used to provide a diversity solution can be in the same physical housing or must be two separate but equal antennas in the same location. Diversity provides relief to a wireless network in a multipath scenario. Diversity antennas are physically separated from the radio and each other, to ensure that one encounters less multipath propagation effects than the other. Dual antennas typically ensure that if one antenna is in an RF null then the other is not, which provides better performance in multipath environments. You can move the antenna to get it out of the null point and provide a way to receive the signal correctly.

QUESTION NO: 75
Which statement about the Lightweight Access Point Protocol (LWAPP) is true?
A. LWAPP encrypts control traffic between the AP and the controller.
B. LWAPP encrypts user traffic with a x.509 certificate using AES-CCMP.
C. LWAPP encrypts both control traffic and user data.
D. When set to Layer 3, LWAPP uses a proprietary protocol to communicate with the Cisco Aironet APs.
Answer: A
Explanation:
The LAP and WLC pair use the lightweight access point protocol (LWAPP) as the tunneling mechanism.
LWAPP control messages-Exchanges that are used to configure the LAP and manage its operation. The control messages are authenticated and encrypted so that the LAP is securely controlled by only the WLC.
LWAPP data-Packets to and from wireless clients associated with the LAP. The data is encapsulated within LWAPP, but is not encrypted or otherwise secured between the LAP and WLC.