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350-401 Practice Question: Examine the following configuration snippet on a…
Examine the following configuration snippet on a Cisco IOS-XE router:
interface GigabitEthernet0/1
service-policy output QOS_POLICY
policy-map QOS_POLICY
class VOICE
priority percent 10
class VIDEO
bandwidth percent 30
class class-default
fair-queue
What is the effect of this configuration?
⚠ Common exam trap
Cisco often tests the misconception that 'priority' and 'bandwidth' cannot coexist in the same policy-map, but they are allowed as long as they are in different classes; the trap is that candidates think the policy-map is invalid, when in fact it is a standard LLQ 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
✓
VOICE traffic is guaranteed 10% of the interface bandwidth with strict priority queuing, VIDEO traffic is guaranteed 30%, and all other traffic shares the remaining bandwidth using fair-queuing.
The 'priority percent 10' command under class VOICE enables strict priority queuing, guaranteeing that VOICE traffic is served first up to 10% of the interface bandwidth. The 'bandwidth percent 30' under class VIDEO provides a minimum bandwidth guarantee of 30% during congestion. The 'fair-queue' under class-default ensures that all other traffic shares the remaining bandwidth fairly using flow-based queuing, which is the default behavior when no explicit bandwidth is configured.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VOICE traffic is guaranteed 10% of the interface bandwidth with strict priority queuing, VIDEO traffic is guaranteed 30%, and all other traffic shares the remaining bandwidth using fair-queuing.
Why this is correct
This is correct. The 'priority' command places VOICE in a strict-priority (LLQ) queue and, during congestion, guarantees it the configured 10% of interface bandwidth so it cannot be starved by other traffic. The 'bandwidth' command reserves 30% of bandwidth for VIDEO, ensuring its minimum service under load. The default class with 'fair-queue' applies flow-based WFQ to all other traffic, which fairly shares the remaining bandwidth without dropping any class outright.
- ✗
VOICE traffic is limited to 10% of bandwidth, VIDEO to 30%, and all other traffic is dropped if the interface is congested.
Why it's wrong here
This is incorrect because 'fair-queue' in the default class does not drop traffic when congested; it applies weighted fair queuing so all unclassified flows can share whatever bandwidth remains after the guaranteed classes. Additionally, 'priority' and 'bandwidth' do not act as hard policers that 'limit' traffic to the configured percentages—they are scheduling guarantees. Under congestion, the default class still gets a share, not zero, so traffic is not dropped merely because the interface is congested.
- ✗
VOICE traffic is given priority over VIDEO, but VIDEO can use up to 30% of bandwidth only if VOICE is not using its allocation.
Why it's wrong here
This is wrong because the VIDEO class's 30% bandwidth guarantee is unconditional; it does not depend on whether VOICE is using its priority allocation. Even if VOICE fully utilizes its 10% strict-priority bandwidth, the scheduler must still reserve 30% for VIDEO from the remaining capacity. What the configuration actually does is give VOICE lower latency and a small guaranteed queue, while VIDEO receives a larger guaranteed minimum share regardless of VOICE traffic.
- ✗
The policy-map is invalid because 'priority' and 'bandwidth' cannot be used together in the same policy-map.
Why it's wrong here
The policy-map is perfectly valid because 'priority' and 'bandwidth' are used in different classes—VOICE has 'priority' and VIDEO has 'bandwidth'. Cisco only prohibits placing both commands in the same class, since that would double-guarantee resources in one queue. In this policy, each class has its own scheduling mechanism, so no conflict arises and the policy-map will be accepted.
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