easyMultiple Choice
350-401 Practice Question: Is designing a QoS policy for a Cisco router that…
A network engineer is designing a QoS policy for a Cisco router that connects to an MPLS VPN. The service provider expects all traffic to be marked with IP Precedence values. The engineer wants to ensure that voice traffic (DSCP EF) is mapped to IP Precedence 5. What configuration is required on the router to perform this mapping?
⚠ Common exam trap
Cisco often tests the misconception that DSCP and IP Precedence are automatically synchronized or that a single command like 'set dscp ef' will implicitly set the IP Precedence field, when in fact they are independent markings that require separate configuration.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a policy-map that sets the IP precedence to 5 using 'set ip precedence 5'.
The 'set ip precedence 5' command in a policy-map explicitly marks the IP Precedence field to 5, which corresponds to the same value as DSCP EF (46) in the IP header. This ensures that voice traffic is marked with IP Precedence 5 as required by the service provider, regardless of any existing DSCP markings.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure a policy-map that sets the IP precedence to 5 using 'set ip precedence 5'.
Why this is correct
The MQC policy-map action `set ip precedence 5` explicitly writes the 3-bit IP precedence value in the Type of Service byte. Precedence 5 is the standard value associated with the EF PHB (DSCP 46), so this command satisfies the marking requirement directly. In Cisco IOS/IOS-XE, this is the correct method when the requirement is specifically to mark IP precedence, and attaching the policy map to the interface (or globally) makes the classification and marking take effect.
- ✗
Configure a policy-map that sets the DSCP to EF, and the router will automatically set IP precedence to 5.
Why it's wrong here
This is incorrect because `set dscp ef` and `set ip precedence 5` are distinct policy-map actions; configuring one does not automatically configure the other. Although DSCP EF (46) has binary bits 101110, whose first three bits are 101 (precedence 5), the Cisco QoS engine only updates the ToS byte according to the specific set command used and does not populate the separate precedence marker for later policy match. Therefore, a device matching or trusting IP precedence may not see precedence 5 if only DSCP was set.
- ✗
Use the 'qos map dscp-ip-precedence' command to create a mapping table.
Why it's wrong here
The `qos map dscp-ip-precedence` command is a platform-specific table used to translate existing DSCP values into precedence values for internal QoS or classification purposes; it is not a marking command found in standard IOS policy-map configuration. In MQC, the correct syntax is `set precedence 5` or `set dscp ef`, not a table definition. Even where this command exists, creating a mapping table does not, by itself, cause the router to mark packets; an explicit policy-map action is still required.
- ✗
The router will automatically map DSCP EF to IP precedence 5 without any configuration.
Why it's wrong here
There is no automatic packet-marking behavior that re-writes the IP precedence field when a packet arrives with DSCP EF. While many platforms can use the DSCP value as a classification key, classification does not equal marking, and the outgoing packet's precedence bits remain unchanged unless an explicit `set` action is configured. The engineer must configure a policy-map with `set ip precedence 5` (or `set dscp ef`) to implement the required marking.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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