300-430 Question #249: Real Exam Question with Answer & Explanation

The correct answer is A: DSCP. Cisco Jabber is a software application running on a PC endpoint, which marks traffic at Layer 3 using DSCP in the IP header, so the switch port must be configured to trust DSCP to preserve those markings.

Question

Refer to the exhibit. An engineer must preserve a QoS marking sent by the Cisco Jabber software running on PC1. Which marking value must be trusted on port Fa 0/6?

Exhibit

Options

ADSCP
B802.1p
CCoS
DIP precedence

Explanation

Cisco Jabber is a software application running on a PC endpoint, which marks traffic at Layer 3 using DSCP in the IP header, so the switch port must be configured to trust DSCP to preserve those markings.

Common mistakes.

B. 802.1p markings are embedded in the 802.1Q VLAN tag and are a Layer 2 construct applied by network devices or phones with hardware tagging capability, not by a software application running on a PC.
C. CoS is the common name for the 802.1p priority bits in the 802.1Q tag and is a Layer 2 marking not generated by PC-based software applications like Cisco Jabber.
D. IP Precedence uses only the top 3 bits of the IP ToS byte and is superseded by DSCP; Jabber uses the full 6-bit DSCP field, making IP Precedence trust insufficient to accurately preserve the application's markings.

Concept tested. Trusting DSCP QoS markings from software-based endpoints

Reference. https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/jabber/Windows/12_9/cjab_b_planning-guide-jabber-windows-129/cjab_b_planning-guide-jabber-windows-129_chapter_01110.html

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