JN0-106 Junos OS Fundamentals Practice Question
During a network traffic storm, a Juniper EX switch's CPU utilization spikes to 100%. Which command would best help identify the cause?
⚠ Common exam trap
Many candidates confuse 'show interfaces extensive' (which shows error counters like CRC errors or giants) with the ability to see live traffic, but it only provides historical statistics, not the packet-level detail needed to pinpoint the storm's source.
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
✓
monitor traffic interface ge-0/0/0
The 'monitor traffic interface' command captures live packet headers on the specified interface, allowing you to see the type and source of traffic causing the CPU spike. During a traffic storm (e.g., broadcast storm), this command reveals excessive broadcast, multicast, or unknown unicast frames, which are typically the root cause of high CPU utilization on Juniper EX switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
monitor traffic interface ge-0/0/0
Why this is correct
`monitor traffic interface ge-0/0/0` runs a real-time packet capture on the specified interface, displaying packet headers so you can see broadcast, multicast, or unknown-unicast traffic flooding the switch. This is the key diagnostic during a storm because it lets you identify the source MAC/IP and the packet type that is overwhelming the CPU. It captures traffic as it arrives, giving immediate evidence for where the storm originates.
- ✗
show spanning-tree bridge
Why it's wrong here
`show spanning-tree bridge` only reports the Spanning Tree Protocol state for the switch, such as the bridge ID, root ID, and per-port STP roles. It does not inspect the data-plane traffic that is causing the CPU spike. Even if a layer-2 loop is the root cause, this command alone cannot show you the actual storm packets or which interface is receiving them; you would need commands that expose traffic behavior.
- ✗
show ethernet-switching table
Why it's wrong here
`show ethernet-switching table` lists the MAC addresses learned on each VLAN and the associated interfaces, which is useful for checking forwarding decisions or MAC flapping. During a traffic storm, this table may be large or unstable, but it does not reveal the rate, type, or source of the packets hitting the CPU. MAC learning happens in the data plane; the CPU spike is driven by traffic that is punting, and this table gives no visibility into packet contents or streams.
- ✗
show interfaces extensive ge-0/0/0
Why it's wrong here
`show interfaces extensive ge-0/0/0` displays cumulative interface counters, error counts, and queue statistics, so you can see high input rates or CRC errors. However, it is a historical counter snapshot, not a live packet analysis; it cannot tell you whether the storm is broadcast, multicast, or a specific source host. To find the actual culprit, you need to see the packets themselves, not just aggregate statistics.
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