Question 1,613 of 2,015

Enterprise Network Design →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the standby switch is processing HSRP hellos for all VLANs, causing CPU spikes. In an HSRP setup, both the active and standby routers must receive and process periodic Hello messages—defaulting to every three seconds—for every VLAN on which HSRP is configured, even though the standby does not forward inter-VLAN traffic. This cumulative CPU overhead from handling hellos across a large number of VLANs can drive high utilization on the standby switch, a common pitfall in designs using SVIs for inter-VLAN routing. On the ENCOR 350-401 exam, this scenario tests your understanding that HSRP control-plane processing is not limited to the active device; a frequent trap is assuming the standby is idle. Remember the memory tip: “Standby still says hello”—the standby switch must process hellos to maintain its role and detect active failures, so high CPU on both switches points to hello overload, not data-plane issues.

350-401 Enterprise Network Design Practice Question

This 350-401 practice question tests your understanding of enterprise network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

An enterprise network is experiencing high CPU utilization on the distribution layer switches. The design uses VLANs with SVIs for inter-VLAN routing, and HSRP for first-hop redundancy. The engineer notices that the standby switch is also experiencing high CPU. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Open the full VLAN trunking answer →

A
The standby switch is processing HSRP hellos for all VLANs, causing CPU spikes.
Correct because HSRP hellos are sent every 3 seconds per group; with many VLANs (e.g., 500), the CPU must process all hellos, leading to high utilization.
B
The standby switch is forwarding all broadcast traffic due to a misconfigured STP root.
Why wrong: Incorrect because STP root placement affects forwarding, but broadcast traffic is not typically the primary cause of high CPU on standby; HSRP hellos are more likely.
C
The standby switch is performing routing for all VLANs because the active switch failed.
Why wrong: Incorrect because if the active failed, the standby would become active and handle routing, which would increase CPU, but the scenario says standby is also experiencing high CPU, not that it became active.
D
The standby switch is processing VTP updates from the distribution layer.
Why wrong: Incorrect because VTP is not commonly used in modern designs and would not cause sustained high CPU on standby unless there are frequent changes.

Full breakdown with real-world context →

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

✓

The standby switch is processing HSRP hellos for all VLANs, causing CPU spikes.

In an HSRP setup, both the active and standby routers process incoming Hello messages for every VLAN on which HSRP is configured. Even though the standby switch does not forward inter-VLAN traffic, it must still receive and process periodic HSRP hellos (default every 3 seconds) to maintain its role and detect active failures. With a large number of VLANs, the cumulative CPU overhead from processing these hellos can cause high utilization on both switches.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
The standby switch is processing HSRP hellos for all VLANs, causing CPU spikes.
Why this is correct
Correct because HSRP hellos are sent every 3 seconds per group; with many VLANs (e.g., 500), the CPU must process all hellos, leading to high utilization.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The standby switch is forwarding all broadcast traffic due to a misconfigured STP root.
Why it's wrong here
Incorrect because STP root placement affects forwarding, but broadcast traffic is not typically the primary cause of high CPU on standby; HSRP hellos are more likely.
✗
The standby switch is performing routing for all VLANs because the active switch failed.
Why it's wrong here
Incorrect because if the active failed, the standby would become active and handle routing, which would increase CPU, but the scenario says standby is also experiencing high CPU, not that it became active.
✗
The standby switch is processing VTP updates from the distribution layer.
Why it's wrong here
Incorrect because VTP is not commonly used in modern designs and would not cause sustained high CPU on standby unless there are frequent changes.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that the standby switch is idle or only processes traffic during failover, when in reality it must continuously process HSRP hellos for every configured group, which can become a significant CPU burden in large VLAN deployments.

Trap categories for this question

Scenario analysis trap
Incorrect because if the active failed, the standby would become active and handle routing, which would increase CPU, but the scenario says standby is also experiencing high CPU, not that it became active.

Detailed technical explanation

How to think about this question

HSRP uses multicast address 224.0.0.2 with UDP port 1985. Each HSRP group sends three Hello packets per second by default (hold time 10 seconds). In a design with 100 VLANs, each switch must process 300 HSRP packets per second just for hellos. This CPU load is often underestimated in large Layer 3 designs, and can be mitigated by increasing the Hello interval or using HSRP version 2 with millisecond timers only when necessary.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 350-401 question test?

Enterprise Network Design — This question tests Enterprise Network Design — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The standby switch is processing HSRP hellos for all VLANs, causing CPU spikes. — In an HSRP setup, both the active and standby routers process incoming Hello messages for every VLAN on which HSRP is configured. Even though the standby switch does not forward inter-VLAN traffic, it must still receive and process periodic HSRP hellos (default every 3 seconds) to maintain its role and detect active failures. With a large number of VLANs, the cumulative CPU overhead from processing these hellos can cause high utilization on both switches.

What should I do if I get this 350-401 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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