Question 117 of 239 from exam 200-301: Implementing and Administering Cisco Solutions

B

Untagged traffic from the device attached to the Cisco IP Phone passes through the phone unchanged, regardless of the trust state of the access port on the phone.

When an IP phone is connected to a switch, the attached PC can send untagged data traffic to the switch. The behavior of the IP phone in this scenario depends on its configuration.

Option A: It drops the traffic. If the IP phone is configured with a security feature called "port security", it will drop any traffic that is received on the PC port, which is untagged. The reason is that port security is configured to only allow traffic from the phone port, which is already tagged with the voice VLAN. Therefore, the IP phone will not allow untagged traffic from the PC port to pass through.

Option B: It allows the traffic to pass through unchanged. If the IP phone is not configured with port security, it will allow the untagged traffic from the PC port to pass through unchanged. This is because the IP phone does not inspect the traffic on the PC port, and it assumes that any traffic that is received on the PC port is intended for the attached PC.

Option C: It tags the traffic with the native VLAN. If the IP phone is configured with a feature called "voice VLAN", it will tag any untagged traffic that is received on the PC port with the native VLAN. The native VLAN is the VLAN that is configured on the switch port, and it is used for untagged traffic. By tagging the untagged traffic with the native VLAN, the switch can forward the traffic to the correct VLAN.

Option D: It tags the traffic with the default VLAN. If the IP phone is not configured with a voice VLAN, it will tag any untagged traffic that is received on the PC port with the default VLAN. The default VLAN is usually VLAN 1, and it is used for untagged traffic. By tagging the untagged traffic with the default VLAN, the switch can forward the traffic to the correct VLAN.

In summary, the behavior of the IP phone when it receives untagged traffic from an attached PC depends on its configuration. If it is configured with port security, it will drop the traffic. If it is not configured with port security, it will either allow the traffic to pass through unchanged or tag it with the native VLAN or the default VLAN, depending on whether it is configured with a voice VLAN.