Which QoS Value Must Be Trusted on WLC Trunk Ports?
Refer to the exhibit. An engineer must maintain consistent parity between incoming and outgoing voice traffic for the trunk switch ports on a wireless controller. Which QoS value must be trusted? - 
Community Votes
67% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question tests which QoS marking is trusted on a Layer 2 trunk interface (CoS inside the 802.1Q tag), not which one is trusted on a routed port (DSCP); the trap is picking the Layer 3 DSCP value that dominates most modern QoS designs.
On a wireless controller's trunk switch ports, voice traffic must keep identical priority in both directions, which requires trusting the Layer 2 CoS value carried in the 802.1Q tag. This page confirms why CoS—not DSCP, IP precedence, or the 802.1p field name—is the QoS value the WLC trunk relies on for wired-to-wireless parity.
Choosing DSCP (A) because it is the default trust value on Catalyst routed ports and on the WLC's own QoS profiles; on a 802.1Q trunk carrying voice VLANs the value actually preserved end-to-end is CoS, so DSCP trust leaves the L2 priority unmatched.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
The controller's switch ports in question are trunks, so every voice frame on them carries an 802.1Q tag with a 3-bit priority field. The value stored in that field is CoS, and trusting it (for example withmls qos trust cos) is what keeps incoming and outgoing voice traffic at the same priority level. Because the wireless side of the controller maps that Layer 2 priority into the voice/platinum QoS profile (UP 6, DSCP EF), trusting CoS gives the wired-to-wireless consistency the engineer is asked to maintain. That is exactly the requirement stated in the stem—parity between ingress and egress voice traffic on the trunk—so CoS is the value that must be trusted. Note that CoS and the 802.1p priority field are the same bits; the exam asks for the name of the trust value, and Cisco QoS doctrine calls it CoS.Why the Other Options Are Wrong
DSCP (A) is a Layer 3 marking in the IP header; trusting DSCP on a trunk port classifies based on the packet's DSCP and does not honour the CoS bits already present in the 802.1Q tag, so the L2 parity across the trunk is lost. IP precedence (C) is the legacy 3-bit TOS field that DSCP replaced, and it is likewise a Layer 3 value that a trunk port does not read for L2 prioritisation. 802.1p (D) is the standard/field that carries the priority bits—the mechanism, not the value a switch trusts—so it is the classic distractor paired against CoS in this question family. Exam keys for 300-430 consistently expect CoS when the interface is a trunk and the requirement is consistent Layer 2 treatment of voice.Community Comment Notes
rrahim gives the accepted reasoning, noting that CoS "is a Layer 2 QoS marking found in the 802.1Q VLAN tag" and is used to prioritise traffic on trunk ports that carry multiple VLANs. 74ca941 summarises the same conclusion bluntly as "CoS Layer 2," while R3DAlert dissents with DSCP and links a Cisco community discussion about when to use CoS instead of DSCP—that thread actually supports CoS for tagged Layer 2 links like this trunk. The vote split (roughly two-to-one for CoS) shows this is a genuine trap, but the interface type named in the stem settles it in favour of CoS.Official Reference
Exam Strategy
First identify the interface type in the exhibit: routed ports and server-facing access ports use DSCP trust, while 802.1Q trunk links between switches and the controller use CoS trust. Then eliminate the two Layer 3 answers (DSCP, IP precedence) immediately, and finally decide between CoS and 802.1p by remembering that 802.1p is the field and CoS is the value trusted.
Frequently Asked Questions
Why is DSCP the wrong choice for a WLC trunk port carrying voice?
Trunk ports are Layer 2 links carrying 802.1Q tags; trusting DSCP ignores the CoS priority bits in those tags and breaks the Layer 2 parity the question requires.
How do CoS and 802.1p differ on an 802.1Q trunk?
802.1p names the 3-bit priority field inside the 802.1Q tag, while the value carried in that field is CoS—which is what a switch or controller actually trusts.