300-365 · Question #173
You have Microsoft Lync, which uses an 802.1p table instead of an 802.11e table for tagging the VoWiFi packets. Which configuration must be used to utilize the priority queue and preserve the…
The correct answer is D. Run the mls qos command and trust DSCP only. When Lync marks VoWiFi packets using 802.1p CoS tags, the wireless infrastructure maps those to DSCP EF, so the Catalyst switch must have MLS QoS enabled and trust DSCP to preserve the EF marking without remapping it.
Question
You have Microsoft Lync, which uses an 802.1p table instead of an 802.11e table for tagging the VoWiFi packets. Which configuration must be used to utilize the priority queue and preserve the correct voice packet marking as EF on a Cisco Catalyst switch?
Options
- ARun the mls qos command and trust either CoS or DSCP
- BRun the no mls qos command and trust nothing on switchport
- CRun the mls qos command and trust CoS
- DRun the mls qos command and trust DSCP only
How the community answered
(50 responses)- A14% (7)
- B6% (3)
- C20% (10)
- D60% (30)
Why each option
When Lync marks VoWiFi packets using 802.1p CoS tags, the wireless infrastructure maps those to DSCP EF, so the Catalyst switch must have MLS QoS enabled and trust DSCP to preserve the EF marking without remapping it.
Trusting either CoS or DSCP is ambiguous and incorrect here; trusting CoS would trigger a CoS-to-DSCP remap on the switch that would overwrite the EF DSCP value rather than preserve it.
Running 'no mls qos' disables the QoS engine entirely, meaning voice packets receive no priority queuing and the EF marking is ignored by the switch.
Trusting CoS causes the switch to derive the internal DSCP from the CoS-to-DSCP mapping table, which overwrites the existing EF marking set by the wireless infrastructure instead of preserving it.
Enabling 'mls qos' activates the QoS engine on the switch and 'trust dscp' instructs it to honor the existing DSCP EF (DSCP 46) value already set by the wireless system, placing packets in the priority queue without alteration. Trusting DSCP is the only trust mode that preserves the end-to-end EF marking that the wireless infrastructure derived from Lync's 802.1p tags.
Concept tested: Cisco Catalyst MLS QoS DSCP trust for VoIP preservation
Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/qos/213373-configure-qos-on-catalyst-switches.html
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