Refer to the exhibit. A workstation PC is connected to the Cisco IP phone access port.
Based on the configuration in the exhibit, how will the traffic be managed?
A.
The IP phone access port will override the priority of the frames received from the PC.
B.
The switch port Fa0/4 will trust the priority for the frames received from the PC.
C.
The IP phone access port will trust the priority of the frames received from the PC.
D.
The switch port Fa0/4 will override the priority of the frames received from the PC.
Explanation:
The PC connected to the phone, however, should normally be untrusted and have all
inbound CoS
values set to 0. This is mentioned here to show how trust boundaries also exist at any
connected IP
Phones.
Example:
interface fastethernet 0/1
switchport voice vlan 200
switchport priority extend cos 0
A switch instructs an attached IP Phone through CDP messages as to how it should extend
QoS trust
to its own user data switch port. To configure the trust extension, use the following interface
configuration command:
Switch(config-if)# switchport priority extend {cos value | trust}
Normally, the QoS information from a PC connected to an IP Phone should not be trusted.
This is
because the PC’s applications might try to spoof CoS or Differentiated Services Code Point
(DSCP)
settings to gain premium network service. In this case, use the cos keyword so that the CoS
bits are
overwritten to value by the IP Phone as packets are forwarded to the switch. If CoS values
from the
PC cannot be trusted, they should be overwritten to a value of 0.