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Cisco Certified Network Associate (CCNA) · Cisco
Select one answer.
Written by other learners. Comments can disagree with the marked answer — check the official documentation when in doubt.
This configuration uses an access port, which is a common and valid setup for connecting a Cisco IP phone with a PC behind it: Untagged frames (e.g., from the PC) are placed in VLAN 20 (access VLAN). Tagged frames from the IP phone, detected via CDP, are placed in VLAN 30 (voice VLAN). Cisco switches support this dual-VLAN capability on access ports when a voice VLAN is configured, making it the simplest and most reliable solution for the requirements.
Per Cisco documentation under the subtopic line Voice VLAN Configuration Guidelines, "Voice VLAN is only supported on access ports and not on trunk ports, even though the configuration is allowed. Quoted directly from the following source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_44_se/configuration/guide/scg/swvoip.html#wp1033848
Only trunk ports support tagged and untagged frames simultaneously. Therefore, option C is the only correct answer.