A corporation has a wireless network where all access points are configured in FlexConnect. The WLC has a Data WLAN and a VoWiFi WLAN implemented where centrally-switched SSID is configured for the APs. Which QoS configuration must be implemented for the wireless packets to maintain the marking across the wired and wireless network?

To maintain Quality of Service (QoS) markings across both the wired and wireless networks in a centrally-switched architecture, the Wireless LAN Controller (WLC) must be configured to trust the Differentiated Services Code Point (DSCP) values from the wired network. When "Trust DSCP" is enabled, the WLC honors the DSCP markings on downstream packets arriving from the wired network. It then maps these values to the appropriate Wi-Fi Multimedia (WMM) User Priority (UP) values for transmission over the air. This ensures that the QoS policy established on the wired network is preserved and extended to the wireless clients, creating a consistent end-to-end QoS model.

A. Set QoS to Platinum: This assigns a specific high-priority QoS profile (typically for voice) to the WLAN but does not, by itself, enable the trust mechanism for end-to-end marking preservation.
B. Enable CAC: Call Admission Control (CAC) is a mechanism to manage and limit the number of active voice/video calls to prevent oversubscription, not to maintain QoS markings.
C. Allow WMM: Allowing WMM is a prerequisite for enabling QoS on the wireless segment, but it does not address how markings from the wired network are handled by the WLC.

1. Cisco Wireless LAN Controller Configuration Guide, Release 8.5, "Configuring QoS" Chapter, "Information About QoS for Downstream Traffic" section: This guide explains that for downstream traffic, the WLC uses the DSCP value from the incoming packet's IP header to determine the QoS treatment and map it to the wireless frame. Enabling trust for this value is the mechanism to preserve the wired marking.

2. Cisco Enterprise Mobility 8.5 Design Guide, "Chapter 5: Wireless QoS Design," section "End-to-End QoS": This document details the end-to-end QoS architecture, stating, "The WLC and APs must be configured to trust the DSCP in the IP packets from the wired network... The WLC uses the DSCP value from the downstream packet to select the WMM user priority (UP) that is used to transmit the packet to the wireless client." (This principle is detailed in various QoS design sections).

3. Cisco Catalyst 9800 Series Wireless Controller Configuration Guide, IOS XE Amsterdam 17.3.x, "QoS for Wireless" Chapter, "QoS Upstream and Downstream" section: While for a different controller, the principle is identical and foundational. The documentation states, "For downstream traffic, the WLC/AP maps the DSCP of the incoming IP packet to a User Priority (UP) value." This mapping relies on trusting the incoming DSCP value.