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642-813 Q&A – Prepare infrastructure to support advanced services (76-80)

Section4 – Prepare infrastructure to support advanced services

Which three statements are true about the voice VLAN feature on a Catalyst 2950 switch? (Choose three.)
A.The CoS value is trusted for 802.1p or 802.1q tagged traffic.
B.The voice VLAN feature is disabled by default.
C.The IP phone accepts the priority of all tagged and untagged traffic and sets the CoS value to 4.
D.When the voice VLAN feature is enabled, all untagged traffic is sent according to the default CoS priority of the port.
E.PortFast is is automatically disabled when a voice VLAN is configured.
F.The default CoS value for incoming traffic is set to 0.
Answer: B D F

Default Voice VLAN Configuration The voice VLAN feature is disabled by default. When the voice VLAN feature is enabled, all untagged traffic is sent according to the default CoS priority of the port. The default CoS value is 0 for incoming traffic. The CoS value is not trusted for 802.1P or 802.1Q tagged traffic. The IP Phone overrides the priority of all incoming traffic (tagged and untagged) and sets the CoS value to 0.
In software releases earlier than Cisco IOS Release 12.1(13 )EA1 , the CoS value is trusted for all 802.1P or 802.1Q tagged traffic, and the IP Phone does not override the priority of the incoming traffic.

Refer to the exhibit. Which three statements are true about trust boundaries in the campus network? (Choose three.)

A.A device is trusted if it correctly classifies packets.
B.A device is trusted if it correctly declassifies packets.
C.The outermost trusted devices represent the trust boundary.
D.Classification and marking occur using 802.1ab QoS bits before reaching the trust boundary.
E.Network trust boundaries are automatically configured in IOS version 12.3 and later.
F.For scalability, classification should be done as close to the edge as possible.
Answer: A C F
At the trust boundary device, QoS values are trusted if they accurately represent the type of traffic and precedence processing the traffic should receive as it enters the campus network. If untrusted, the traffic is marked with a new QoS value appropriate for the policy in place at the point where the traffic entered the campus network. Ideally, the trust boundary exists at the first switch receiving traffic from a device or IP phone. It is also acceptable to establish the trust boundary where all the traffic from an access switch enters a Building Distribution layer port.

Which three interface commands will configure the switch port to support a connected Cisco phone and to trust the CoS values received on the port if CDP discovers that a Cisco phone is attached? (Select three)
A. switchport voice vlan vlan-id
B. mls qos trust device cisco-phone
C. switchport priority extend cos_value
D. mls qos trust cos
E. mls qos trust override cos
Answer: ABD
1. To configure the IP Phone uplink, just configure the switch port where it connects. The switch instructs the phone to follow the mode that is selected. In addition, the switch port does not need any special trunking configuration commands if a trunk is wanted. If an 802.1Q trunk is needed, a special-case trunk is negotiated by Dynamic Trunking Protocol (DTP) and CDP. Use the following interface configuration command to select the voice VLAN mode that will be used:
Switch( config-if)# switchport voice vlan { vlan-id | dot1p | untagged | none}
2. mls qos trust [ cos ] : Configure the port trust state. By default, the port is not trusted. All traffic is sent through one egress queue. Use the cos keyword to classify ingress packets with the packet CoS values. The egress queue assigned to the packet is based on the packet CoS value
3. mls qos trust device cisco-phone : Configure the Cisco IP Phone as a trusted device on the interface.

Jitter is causing problems with the VOIP application in the Pass4sure network. What causes network jitter?
A. Variable queue delays
B. Packet drops
C. Transmitting too many small packets
D. Compression
Answer: A
Delay variation or jitter is the difference in the delay times of consecutive packets. A jitter buffer is often used to smooth out arrival times, but there are instantaneous and total limits on buffering ability. Any type of buffering used to reduce jitter directly increases total network delay. In general, traffic requiring low latency also requires a minimum variation in latency.
Note: Jitter in Packet Voice Networks:
Jitter is defined as a variation in the delay of received packets. At the sending side, packets are sent in a continuous stream with the packets being spaced evenly apart. Due to network congestion, improper queuing, or configuration errors, this steady stream can become lumpy, or the delay between each packet can vary instead of remaining constant.

The wireless network is using 802.11G access points. Which two statements are true about these access points? (Select two)
A. It supports eight different data rates.
B. It is fully backward compatible with 802.11b
C. It provides the same network throughput with 802.11b clients, 802.11g clients, or a mixed environment where both clients are present.
D. It is only compatibly with the 11 Mbps 802.11b transfer rate.
E. It has three non-overlapping channels in its channel options.
Answer: BE
The 802.11g standard is currently the most popular Wi-Fi standard. 802.11g is the successor to 802.11b, but it is backward-compatible with 802.11b as well . The two standards operate at the same frequency (2.4GHz). With a throughput of about 22Mbps, 802.11g delivers four times the throughput of 802.11b. 802.11g has been the de facto home network standard for a few years and now dominates in retail markets. As it’s been widely adopted, the price of 802.11g products has fallen significantly, making it a cost-effective choice. The only downside to 802.11g is the fact that it uses a crowded slice of the spectrum, with room for only three nonoverlapping channels. This will make 802.11a a better choice for some environments, especially those populated with devices that share the 2.4GHz spectrum, such as cordless phones, baby monitors, microwave ovens, and Bluetooth radios.

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