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CCNA Junos Monitoring Questions

23 questions · Junos Monitoring topic · All types, answers revealed

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1
MCQeasy

Refer to the exhibit. Which interface has a physical layer connectivity problem?

A.ge-0/0/2
B.ge-0/0/0
C.ge-0/0/3
D.ge-0/0/1
AnswerD

ge-0/0/1 is administratively up (configuration enables the interface) but the link is down, meaning the physical layer is not establishing connectivity. This mismatch—admin up, link down—points to a Layer 1 problem such as a disconnected cable, faulty optics, or an unpowered peer. Unlike an administratively down interface, this is not expected; further troubleshooting like checking the SFP and cable is needed.

Why this answer

Interface ge-0/0/1 shows 'Physical link is Down' in the output, which directly indicates a physical layer connectivity problem. The 'Device present' field is also 'No', confirming that no transceiver or cable is detected, which is a Layer 1 issue.

Exam trap

The trap here is that candidates may focus on the 'Admin' status (up/down) rather than the 'Physical link' status, mistakenly assuming an administratively down interface is a physical layer problem, when in fact the physical layer issue is indicated by the link state being down regardless of administrative configuration.

How to eliminate wrong answers

Option A is wrong because ge-0/0/2 shows 'Physical link is Up' and 'Device present' is 'Yes', indicating no physical layer problem. Option B is wrong because ge-0/0/0 shows 'Physical link is Up' and 'Device present' is 'Yes', so the physical layer is functioning correctly. Option C is wrong because ge-0/0/3 shows 'Physical link is Up' and 'Device present' is 'Yes', meaning there is no physical layer connectivity issue.

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2
MCQhard

A network operator needs to upgrade the Junos OS on the backup RE (re1) of a dual-RE system. Which procedure ensures that only the backup RE is upgraded and rebooted without affecting the forwarding plane?

A.Use 'request system software add <image> re1' and then 'request system reboot re1'
B.Use 'request system software add <image> both' and then 'request system reboot both'
C.Use 'request system software add <image> re0' and then 'request system reboot re0'
D.Copy the image to both REs and reboot the backup RE with 'request system reboot slice 1'
AnswerA

Targeting re1 with the 're1' keyword ensures the image is installed only on the backup Routing Engine, leaving the active primary RE untouched. Rebooting re1 restarts only that RE; the primary continues handling forwarding and control traffic, so there is no service interruption. After re1 returns online as standby, it is ready to assume primary role in a later switchover, which is the correct maintenance procedure.

Why this answer

The 'request system software add <image> re1' command installs the Junos OS image only on the backup Routing Engine (RE1), and the subsequent 'request system reboot re1' reboots only that RE. In a dual-RE system, this procedure isolates the upgrade to the backup RE, ensuring the forwarding plane remains active on the primary RE (RE0) and no traffic disruption occurs.

Exam trap

The trap here is that candidates may confuse the 'both' option with a safe upgrade method, not realizing it reboots both REs simultaneously and disrupts traffic, or they may incorrectly assume that copying the image manually and using a non-existent 'slice' command is a valid procedure.

How to eliminate wrong answers

Option B is wrong because 'request system software add <image> both' and 'request system reboot both' upgrade and reboot both REs simultaneously, which would disrupt the forwarding plane by causing a full system outage. Option C is wrong because 'request system software add <image> re0' and 'request system reboot re0' target the primary RE, which would interrupt the forwarding plane and potentially cause a traffic loss. Option D is wrong because 'request system reboot slice 1' is not a valid Junos command for rebooting a specific RE; the correct syntax uses 're0' or 're1' to specify the Routing Engine, and copying the image to both REs without using the proper 'request system software add' command does not ensure a controlled upgrade.

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3
MCQeasy

A network administrator wants to see the current CPU load average over the last 1, 5, and 15 minutes on a Juniper device. Which command displays this information?

A.show chassis routing-engine
B.show system uptime
C.show system processes
D.show system statistics
AnswerB

The `show system uptime` command is the correct way to view CPU load averages on Junos because its output begins with a `Load averages:` line that shows the 1-, 5-, and 15-minute averages. These three numbers indicate how many threads are runnable or waiting for I/O, providing a historical trend that a simple percentage cannot capture. The command also shows system uptime, time, and active users, but the load average is the key field for this question.

Why this answer

The 'show system uptime' command displays the system's uptime along with the load averages for the last 1, 5, and 15 minutes. This is the standard Junos command for viewing CPU load averages, which are calculated based on the number of processes in the run queue over those time intervals.

Exam trap

The trap here is that candidates often confuse 'show chassis routing-engine' (which shows current CPU utilization) with the load average command, not realizing that load averages are a separate time-weighted metric displayed by 'show system uptime'.

How to eliminate wrong answers

Option A is wrong because 'show chassis routing-engine' displays the status and resource utilization of the Routing Engine (including CPU and memory), but it does not show the load average over 1, 5, and 15 minutes; it shows current CPU utilization percentage. Option C is wrong because 'show system processes' lists all running processes with their PID, memory, and CPU usage, but it does not aggregate the load average over time intervals. Option D is wrong because 'show system statistics' displays various system-wide counters and statistics (e.g., packet counts, memory usage), but it does not include CPU load averages.

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4
MCQeasy

You are managing a Juniper MX240 router at a small ISP. The router has been operating normally for months. This morning, a customer reports intermittent packet loss on their connection, which is served by interface ge-0/0/2. You SSH into the router and run 'show interfaces ge-0/0/2 extensive'. The output shows input errors increasing rapidly, including CRC errors and frame errors. The interface is up/up. You also notice that the interface statistics show a high number of carrier transitions. The cable connecting the router to the customer's CPE was recently replaced by the customer's technician. What is the most likely cause of the errors and the correct action to resolve the issue?

A.Clear the MAC address table on the router to fix possible MAC flapping issues.
B.Administer a 'shutdown' and 'no shutdown' on the interface to reset the line protocol.
C.Replace the cable with a known good one because CRC and frame errors indicate a physical layer problem such as a damaged cable or loose connector.
D.Change the interface speed and duplex settings to auto-negotiate because the errors are due to a mismatch between the router and CPE.
AnswerC

CRC and frame errors indicate that frames are receiving an invalid FCS, which is a classic symptom of a physical-layer issue such as a damaged cable, loose connector, or excessive signal attenuation. The problem appeared after a recent cable change, which strongly implicates the new cable or its seating. Replacing that cable with a known-good one is the correct first step because it directly tests the most likely failure point, and a clean interface afterward confirms the diagnosis.

Why this answer

CRC and frame errors increasing rapidly, combined with a high number of carrier transitions, are classic symptoms of a physical-layer issue. Since the cable was recently replaced by the customer's technician, the most likely cause is a faulty cable or a loose connector. Replacing the cable with a known good one directly addresses the physical layer problem, which is the root cause of these errors.

Exam trap

The trap here is that candidates may confuse CRC/frame errors with a speed/duplex mismatch (Option D) or think a simple interface reset (Option B) will fix the problem, when the rapid increase in carrier transitions and the recent cable replacement clearly indicate a physical cabling fault.

How to eliminate wrong answers

Option A is wrong because MAC address table clearing addresses MAC flapping, which is a Layer 2 issue unrelated to CRC/frame errors or carrier transitions. Option B is wrong because administratively resetting the interface (shutdown/no shutdown) would only temporarily clear counters and restart the line protocol, but it would not fix the underlying physical-layer problem causing the errors. Option D is wrong because while speed/duplex mismatch can cause errors, the presence of carrier transitions and the recent cable replacement strongly point to a physical cabling issue, not a negotiation mismatch; auto-negotiation is already the default on modern Juniper interfaces and would not resolve a faulty cable.

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5
Multi-Selectmedium

Which TWO commands can be used to monitor system log messages in real-time?

Select 1 answer
A.show log messages | last 100
B.show system log messages
C.show configuration system syslog
D.monitor start interactive-commands
E.monitor start messages
AnswersE

The 'monitor start messages' command displays new log messages in real-time as they are written to the /var/log/messages file.

Why this answer

The 'monitor start messages' command streams the contents of the /var/log/messages file in real-time, allowing you to monitor system log messages as they are generated. 'monitor start interactive-commands' (Option D) monitors logs specific to interactive CLI commands, not general system log messages, so it does not satisfy the stem. The other options ('show log messages | last 100', 'show system log messages', 'show configuration system syslog') either display historical data or configuration, not real-time system log monitoring.

Exam trap

The trap here is that candidates confuse static 'show log' commands (which display historical log data) with dynamic 'monitor start' commands (which provide real-time streaming), and may also mistakenly think 'show system log messages' is a valid command when it is not.

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6
MCQeasy

An administrator is asked to check the system log files for any error messages related to interface failures. Which command would be most efficient?

A.monitor start messages | match "error"
B.show log file messages | match "error"
C.show log messages
D.show system syslog messages
AnswerC

show log messages — This is the correct operational command to display the contents of the default system log file /var/log/messages. It shows all messages stored in that file, providing the administrator with the historical log data needed. While it outputs everything without a match filter, it is the only valid command among the options that actually works for viewing existing log entries. Additional filtering could be applied with a pipe, e.g., 'show log messages | match error', but the base command is sufficient.

Why this answer

'show log messages' is the standard JUNOS command to display the contents of the /var/log/messages file. While it does not include a filter, it is the only valid command among the options that allows the administrator to view existing system log messages. The other options are either invalid commands (B and D) or used for real-time monitoring (A), which is not efficient for checking historical log files.

Exam trap

The trap here is that candidates may confuse 'monitor start messages' with 'show log messages', thinking the real-time monitor is more efficient for checking logs, but the question asks for checking existing log files, not monitoring live events.

How to eliminate wrong answers

Option A is wrong because 'monitor start messages' is a real-time monitoring command that continuously streams new log entries to the terminal, not a static search of existing log files; it would require the administrator to wait for errors to occur and does not efficiently check historical logs. Option C is wrong because 'show log messages' displays the entire contents of the messages log file without any filtering, which is inefficient for finding specific error messages related to interface failures, especially on a busy system with many log entries. Option D is wrong because 'show system syslog messages' is not a valid JUNOS command; the correct command to view syslog configuration is 'show configuration system syslog', but this does not display log file contents or error messages.

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7
MCQhard

Your company runs a Juniper MX480 router as the core gateway. The router has been experiencing intermittent connectivity issues with a remote site over a point-to-point OC3 link. Users report that the link goes down for a few seconds several times a day. You suspect that the issue might be related to physical layer errors or framing. You need to determine the cause and collect relevant data to present to the service provider. Which course of action should you take first?

A.Run 'monitor interface so-0/0/0' and observe for any CRC errors or alarms over a period of time.
B.Perform a loopback test on the interface using 'test interface so-0/0/0'.
C.Review the system syslog file using 'show log messages' and search for interface down messages.
D.Immediately replace the serial interface card to rule out hardware failure.
AnswerA

This command provides a continuous real-time view of interface statistics and alarms, allowing you to observe transient physical-layer errors such as CRC errors, framing errors, or loss of signal that might occur intermittently. By watching the output over time, you can correlate error bursts with specific conditions (e.g., nearby equipment, cable movement) without disrupting traffic. It directly confirms the health of the physical layer before moving to more invasive testing or hardware replacement.

Why this answer

'monitor interface so-0/0/0' provides real-time, continuous display of interface counters and alarms, allowing you to observe CRC errors, framing errors, or alarms (e.g., LOS, LOF) as they occur. This is the most direct way to capture transient physical-layer issues on a SONET/SDH OC3 link without disrupting service, making it the appropriate first step before escalating to the service provider.

Exam trap

The trap here is that candidates may choose the loopback test (Option B) because it is a classic troubleshooting tool, but they overlook that it is an intrusive, out-of-service test that should follow passive monitoring to avoid unnecessary downtime.

How to eliminate wrong answers

Option B is wrong because performing a loopback test ('test interface so-0/0/0') would disrupt live traffic and is typically used for out-of-service troubleshooting after initial data collection, not as a first step. Option C is wrong because reviewing the system syslog ('show log messages') may show interface down messages but lacks the granularity to capture brief, intermittent physical-layer errors like CRC or framing alarms that occur over seconds. Option D is wrong because immediately replacing the serial interface card is a premature and invasive action that should only be taken after diagnostic data (e.g., from monitoring or loopback tests) confirms a hardware fault.

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8
Multi-Selectmedium

Which two commands can be used to monitor real-time interface statistics and errors on a Junos device?

Select 2 answers
A.show interfaces terse
B.show interfaces ge-0/0/0
C.show interfaces extensive ge-0/0/0 | refresh 1
D.monitor interface ge-0/0/0
E.monitor security log
AnswersC, D

With the refresh pipe option, it provides continuous updates of extensive interface statistics.

Why this answer

The 'show interfaces extensive ge-0/0/0 | refresh 1' command displays detailed interface statistics and errors, and the 'refresh 1' pipe modifier automatically updates the output every second, enabling real-time monitoring. Option D is correct because the 'monitor interface ge-0/0/0' command is specifically designed for real-time monitoring of interface counters and errors, refreshing the display at a default interval of 1 second. Both commands provide live views of interface statistics such as input/output packets, errors, drops, and CRC errors.

Exam trap

The trap here is that candidates often confuse static 'show interfaces' commands (which provide a point-in-time snapshot) with real-time monitoring commands, leading them to select option B instead of the correct real-time options C and D.

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9
MCQhard

An engineer needs to check if the device has any pending changes that have not been committed. Which operational command should they run?

A.show system configuration
B.show system rollback
C.show configuration | compare
D.show system commit
AnswerC

Shows uncommitted configuration differences.

Why this answer

The 'show configuration | compare' command displays the differences between the candidate configuration (changes made but not yet committed) and the active configuration. This allows the engineer to see any pending changes that have not been committed, which is exactly what the question asks.

Exam trap

The trap here is that candidates often confuse 'show system commit' (which shows commit history) with showing pending changes, or they think 'show configuration' alone shows uncommitted changes, but without the '| compare' pipe it only shows the active configuration.

How to eliminate wrong answers

Option A is wrong because 'show system configuration' is not a valid Junos operational command; the correct command to view the active configuration is 'show configuration'. Option B is wrong because 'show system rollback' displays a list of previous committed configurations (rollback snapshots), not pending uncommitted changes. Option D is wrong because 'show system commit' shows the commit history (log of past commits), not any uncommitted changes in the candidate configuration.

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10
MCQmedium

Which command is used to verify that the current active configuration is the same as the candidate configuration?

A.commit check
B.show configuration
C.show system configuration
D.show | compare
AnswerD

The 'show | compare' command displays the differences between the candidate configuration and the active configuration. It outputs only the lines that would change, in a unified diff format, and if there are no differences, it returns no output. This makes it the ideal command to verify that the candidate exactly matches the active configuration.

Why this answer

The command `show | compare` displays the differences between the candidate configuration and the active (committed) configuration. If there are no differences, the candidate configuration is identical to the active configuration. This is the correct way to verify whether the candidate configuration matches the active configuration in Junos.

Exam trap

The trap here is that candidates often confuse `commit check` (which only validates syntax) with a comparison command, leading them to select Option A, but `commit check` does not compare the candidate to the active configuration.

How to eliminate wrong answers

Option A is wrong because `commit check` validates the syntax and semantics of the candidate configuration but does not compare it to the active configuration; it only ensures the candidate can be committed without errors. Option B is wrong because `show configuration` displays the candidate configuration (or the active if no candidate changes exist), but it does not perform a comparison with the active configuration. Option C is wrong because `show system configuration` is not a valid Junos command; the correct command to view system configuration is `show configuration` or `show system` with specific filters.

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11
MCQhard

You are managing a Juniper MX router that serves as a BGP route reflector for multiple customer VPNs. The router has two routing engines (RE0 and RE1) in a graceful switchover (GRES) configuration. During a routine maintenance window, you need to upgrade the Junos OS from version 18.1R1 to 20.2R2. The upgrade must minimize traffic disruption. You have already staged the new image on both REs. Which sequence of commands ensures minimal impact?

A.On RE0: request system software add jinstall-20.2R2.tgz reboot; then on RE1 same command
B.On RE0: request system reboot (without adding software)
C.On RE1: request system software add jinstall-20.2R2.tgz reboot; after reboot, on RE0: request chassis routing-engine master switch; then request system software add jinstall-20.2R2.tgz reboot
D.On both REs: request system software add jinstall-20.2R2.tgz reboot
AnswerC

Minimizes downtime by upgrading backup first, then switching.

Why this answer

It performs a non-disruptive upgrade by first upgrading the backup RE (RE1), then switching mastership to the upgraded RE, and finally upgrading the original master RE. This sequence leverages Graceful Routing Engine Switchover (GRES) to maintain BGP sessions and VPN forwarding during the upgrade, minimizing traffic disruption.

Exam trap

The trap here is that candidates assume rebooting both REs sequentially with the same command is sufficient, but they overlook the need to switch mastership to the upgraded backup RE before upgrading the original master to avoid a double-reboot scenario that disrupts traffic.

How to eliminate wrong answers

Option A is wrong because rebooting both REs sequentially without first switching mastership causes a full outage when the master RE reboots, as the backup RE is not yet upgraded and may not take over cleanly. Option B is wrong because rebooting without adding the new software does not perform an upgrade, leaving the router on the old version. Option D is wrong because adding the software and rebooting both REs simultaneously or without proper mastership control can cause a complete loss of routing and forwarding, as both REs may reboot at the same time or the backup RE cannot take over gracefully.

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12
MCQeasy

Refer to the exhibit. An administrator notices the /var partition is nearly full. Which command can be used to safely reclaim disk space?

A.request system reboot
B.request system storage cleanup
C.delete /var/log/messages
D.request system software delete
AnswerB

The 'request system storage cleanup' command is the correct procedure because it safely identifies and removes files that are no longer needed, including old log backups, temporary files, and core dumps that have exceeded retention thresholds. It is designed to avoid deleting active or critical files, and it releases space on /var and other partitions in a controlled manner, making it the recommended first step when disk space is low.

Why this answer

The 'request system storage cleanup' command safely removes old log files, core dumps, and other temporary files that are no longer needed, freeing up space on the /var partition without risking system stability. This is the recommended Junos method for reclaiming disk space because it targets only files that can be safely deleted.

Exam trap

The trap here is that candidates may think manually deleting log files (option C) is safe and effective, but Junos relies on its own cleanup mechanisms to avoid breaking log rotation or losing important diagnostic data.

How to eliminate wrong answers

Option A is wrong because 'request system reboot' only restarts the system and does not delete any files, so it will not reclaim disk space. Option C is wrong because manually deleting '/var/log/messages' can cause loss of critical logging data and may not be safe if the file is still being written to; Junos manages log rotation automatically, and direct deletion can disrupt that. Option D is wrong because 'request system software delete' is used to remove installed software packages, not to clean up temporary or log files, and it could inadvertently remove needed software.

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13
Multi-Selecthard

Which TWO commands can be used to view the current routing table entries for IPv4 unicast routes?

Select 2 answers
A.show route protocol static
B.show route table inet.0
C.show forwarding-table
D.show route
E.show route table inet6.0
AnswersB, D

The command 'show route table inet.0' explicitly selects the IPv4 unicast routing table (inet.0) for display. In Junos, inet.0 is the default routing table for IPv4 unicast routes, holding both active and inactive routes from all protocols. This command is functionally equivalent to a plain 'show route' but makes the table selection explicit, ensuring the administrator is viewing the complete IPv4 unicast routing table rather than another table or protocol-specific subset.

Why this answer

`show route table inet.0` explicitly displays the IPv4 unicast routing table (inet.0) in Junos. Option D is correct because `show route` without any filter defaults to showing all routes in the inet.0 table, which is the primary IPv4 unicast routing table.

Exam trap

The trap here is that candidates confuse the routing table (`show route`) with the forwarding table (`show forwarding-table`), or assume a specific protocol filter like `protocol static` shows all routes, when in fact it only shows routes learned via that protocol.

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14
MCQeasy

A technician needs to view the last 50 log messages from the system log file. Which command accomplishes this?

A.show system log messages
B.monitor start messages
C.show system syslog
D.show log messages | last 50
AnswerD

show log messages | last 50 is the correct operational command to view the most recent 50 entries from the default /var/log/messages file. The pipe to 'last' is a Junos CLI filter that displays only the final 50 lines of the output, similar to 'tail -50' in a Unix shell. This is both efficient and reliable because it avoids printing the entire log and directly shows the newest events.

Why this answer

The 'show log messages | last 50' command displays the last 50 lines from the /var/log/messages file on a Junos device. The pipe to 'last' is a Junos CLI filter that outputs only the final N lines of the command output, which is exactly what is needed to view the most recent log entries.

Exam trap

The trap here is that candidates confuse the 'show log' command with 'show system syslog' or 'show system log messages', misremembering the exact syntax for viewing log file contents versus configuration.

How to eliminate wrong answers

Option A is wrong because 'show system log messages' is not a valid Junos command; the correct command to view the system log file is 'show log messages'. Option B is wrong because 'monitor start messages' is used to tail the log file in real time, not to view a static set of the last 50 messages. Option C is wrong because 'show system syslog' displays the syslog configuration, not the actual log file contents.

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15
MCQmedium

A technician notices that the /var partition is filling up on a Juniper device. Which action would be most appropriate to free up space while preserving critical logs?

A.Delete core files manually
B.Run 'request system storage cleanup'
C.Reboot the device
D.Delete all files in /var/log
AnswerB

The 'request system storage cleanup' command is the correct method because it systematically scans well-known system directories and removes only files that are safe to delete, such as stale core files, old log files, and unused software images, while preserving the current system's operational logs and active configuration. It reports how much space was reclaimed and is designed by Juniper to be the standard, nondestructive way to free the /var partition. This avoids the pitfalls of ad-hoc deletion and ensures essential diagnostic data remains intact.

Why this answer

The 'request system storage cleanup' command is the correct action because it safely removes non-essential files such as old log files, core dumps, and temporary files that are no longer needed, while preserving critical logs and configuration files. This command performs a controlled cleanup without risking the deletion of important operational data, making it the most appropriate method for freeing up space on the /var partition.

Exam trap

The trap here is that candidates often assume manual deletion (Option A) or a reboot (Option C) are quick fixes, but they overlook the Junos-specific safe cleanup command that automates the process while preserving essential data.

How to eliminate wrong answers

Option A is wrong because manually deleting core files is risky and inefficient; core files may be needed for debugging, and manual deletion could accidentally remove files that are still in use or miss other space-consuming temporary files. Option C is wrong because rebooting the device does not free up disk space; it only clears temporary memory and may cause unnecessary downtime without addressing the underlying storage issue. Option D is wrong because deleting all files in /var/log would remove critical logs needed for troubleshooting and compliance, and it could also delete active log files that are still being written to, potentially causing system instability or loss of forensic data.

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16
MCQeasy

Which command displays the status of all configured interfaces, including administrative and operational status?

A.show interfaces terse
B.show configuration interfaces
C.show chassis hardware
D.show interface statistics
AnswerA

The 'show interfaces terse' command renders each interface on the device—including all configured interfaces—as a single line in a table with columns for Interface, Admin, Link, Proto, and Local address. Admin reflects the administrative state (up when not administratively disabled, regardless of the physical condition), Link reflects the actual operational state (up only when a valid carrier or signal is detected), and Proto indicates whether a protocol such as inet/IPv4 is active on the interface. Because it combines both the configured administrative state and the real-time operational state in one compact, easy-to-scan output, it is the standard quick check for interface health.

Why this answer

The 'show interfaces terse' command displays a concise summary of all interfaces, including their administrative status (up or down) and operational status (up or down), along with protocol states. This makes it the correct choice for quickly viewing the status of all configured interfaces in a single output.

Exam trap

The trap here is that candidates often confuse 'show interfaces terse' with 'show interface statistics' or 'show configuration interfaces', mistakenly thinking statistics or configuration output will show operational status, but only 'show interfaces terse' provides the concise admin and link status in a single view.

How to eliminate wrong answers

Option B is wrong because 'show configuration interfaces' displays the current configuration of interfaces, not their real-time operational or administrative status. Option C is wrong because 'show chassis hardware' shows physical hardware components (like FPCs, PICs, and power supplies), not interface status. Option D is wrong because 'show interface statistics' shows traffic counters and error statistics for interfaces, but does not explicitly display administrative or operational status in a summary format.

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17
MCQmedium

A network engineer suspects a hardware issue on an interface. Which command provides the most detailed hardware error counters, including CRC errors, framing errors, and runts?

A.show interfaces diagnostics optics
B.show interfaces detail
C.show interfaces extensive
D.show log messages
AnswerC

'show interfaces extensive' is the definitive operational command for hardware error analysis on Junos. It outputs each interface's input breakdown, including CRC errors, frame errors, runts, giants, and resource exhaustion, plus output error subcategories like carrier transitions and errors. It also reveals physical-layer state and multiple logical unit statistics, making it possible to correlate errors with transmit/receive activity. When you suspect a hardware fault, this command gives the most complete and structured view of interface health.

Why this answer

The 'show interfaces extensive' command provides the most detailed hardware error counters for a Junos interface, including CRC errors, framing errors, and runts. This command displays per-interface statistics at the physical layer, such as input errors, output errors, and specific error types like frame, runts, giants, and CRC errors, which are essential for diagnosing hardware issues.

Exam trap

The trap here is that candidates often confuse 'show interfaces detail' with 'show interfaces extensive', assuming 'detail' provides the most granular error counters, but only 'extensive' includes the full hardware error breakdown required for hardware fault diagnosis.

How to eliminate wrong answers

Option A is wrong because 'show interfaces diagnostics optics' is used to display optical transceiver diagnostics (e.g., temperature, voltage, laser bias) and does not show interface error counters like CRC or framing errors. Option B is wrong because 'show interfaces detail' provides more information than the basic output but does not include the extensive hardware error counters; it omits the per-error-type breakdown found in the extensive output. Option D is wrong because 'show log messages' displays system log messages, not real-time interface hardware error counters; it is used for troubleshooting events and errors logged by the system, not for granular interface statistics.

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18
Multi-Selecteasy

Which TWO commands can be used to view the system log messages on a Juniper device? (Select two.)

Select 2 answers
A.show log dcd
B.show log messages
C.monitor start messages
D.show log file
E.show system messages
AnswersB, C

Displays and tails the current log messages.

Why this answer

'show log messages' displays the contents of the default system log file, /var/log/messages, which contains kernel, daemon, and system-level messages. Option C is correct because 'monitor start messages' provides a real-time tail of the same /var/log/messages file, allowing live monitoring of system log entries as they are written.

Exam trap

The trap here is that candidates confuse the generic 'show log' syntax with a nonexistent 'show log file' command, or they mistakenly think 'show system messages' is valid because of similarity to other vendors' commands like 'show logging' on Cisco IOS.

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19
MCQhard

An operator wants to monitor the temperature and voltage sensors on a Juniper chassis. Which command displays current sensor readings?

A.show chassis environment
B.show chassis hardware
C.show system health
D.show log messages | match temperature
AnswerA

The `show chassis environment` operational command in Junos is the standard tool for environmental monitoring: it reads live temperature sensors, voltage rails, and fan speeds across the Routing Engine, line cards, and power supplies. Without any filters it returns current values for all monitored components, and with `detail` it also shows acceptable operating ranges and alarm thresholds. This is the only option that delivers a structured, real-time view of the exact sensors the operator asked about.

Why this answer

The 'show chassis environment' command displays real-time sensor readings for temperature, voltage, and fan status on Juniper devices. It is the standard operational command for monitoring chassis environmental health, directly matching the operator's requirement to check temperature and voltage sensors.

Exam trap

The trap here is that candidates may confuse 'show chassis hardware' (static inventory) with 'show chassis environment' (dynamic sensor data), or assume 'show system health' is a valid Junos command when it is not.

How to eliminate wrong answers

Option B is wrong because 'show chassis hardware' lists installed components (like FRUs, serial numbers, and part numbers) but does not display dynamic sensor readings such as temperature or voltage. Option C is wrong because 'show system health' is not a valid Junos command; the correct command for system health checks is 'request system health' or 'show chassis environment'. Option D is wrong because 'show log messages | match temperature' filters system log messages for temperature-related entries, but it does not show current sensor readings—it only displays historical log events that may be outdated or incomplete.

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20
MCQeasy

A network administrator notices that the CPU utilization on an MX240 router has been consistently above 90% for the past hour. Users are reporting intermittent connectivity issues. The administrator logs in and runs 'show system processes extensive' which shows the Routing Protocol Daemon (rpd) consuming 70% of the CPU. Further investigation reveals that the router has over 5000 BGP prefixes from multiple peers. The administrator suspects that the high CPU is due to continuous route processing. What should the administrator do to immediately reduce CPU load while maintaining network stability?

A.Perform a 'show log messages' to check for errors before taking action
B.Apply a prefix-limit on each BGP session to restrict the number of prefixes received
C.Bounce all BGP sessions by clearing them with 'clear bgp neighbor *'
D.Disable BGP multipath to reduce route processing
AnswerB

Applying a prefix-limit on each BGP session directly caps the number of routes that rpd must read, parse, and install in the RIB and BGP table. By rejecting excess updates (or tearing down the session when a threshold is exceeded), you stop route churn and table bloat at the source, which is the most effective way to lower CPU utilization. It also protects the router from a peer that advertises an abnormally large table due to misconfiguration or a route leak.

Why this answer

Applying a prefix-limit on each BGP session immediately restricts the number of prefixes the router will accept from peers. This prevents the Routing Protocol Daemon (rpd) from processing excessive BGP updates, reducing CPU load while maintaining network stability by keeping existing sessions and routes intact. The prefix-limit action (e.g., with a teardown threshold) ensures the router does not accept more prefixes than it can handle, directly addressing the root cause of high CPU usage.

Exam trap

The trap here is that candidates may confuse diagnostic actions (like checking logs) or disruptive resets (like clearing all BGP sessions) with immediate corrective measures, overlooking the targeted, non-disruptive solution of applying prefix limits to control route processing load.

How to eliminate wrong answers

Option A is wrong because checking logs ('show log messages') is a diagnostic step that does not reduce CPU load; it only gathers information, which is not an immediate action to alleviate the high CPU caused by continuous route processing. Option C is wrong because bouncing all BGP sessions with 'clear bgp neighbor *' would temporarily drop all BGP peers, causing a complete loss of routing information and network instability, and the CPU spike would likely recur when sessions re-establish and process the same 5000+ prefixes again. Option D is wrong because disabling BGP multipath does not reduce the number of prefixes being processed; it only affects how multiple equal-cost paths are used for forwarding, not the volume of route processing by rpd.

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21
MCQhard

Refer to the exhibit. Based on the log messages, what is the most likely cause of the interface flapping?

A.Faulty SFP module
B.MTU mismatch
C.Duplicate IP address
D.Spanning tree topology change
AnswerA

Rapid link up/down events (flapping) are often caused by physical layer issues such as a faulty SFP, loose cable, or bad fiber. This pattern is characteristic of hardware failure.

Why this answer

The log messages show repeated link up/down events (interface flapping). In Junos, interface flapping is most commonly caused by a faulty SFP module, as physical layer issues like degraded optics or loose connections trigger continuous link transitions. The logs typically show 'link up' followed by 'link down' without any protocol-level errors, pointing to a hardware fault rather than configuration mismatches.

Exam trap

The trap here is that candidates often attribute interface flapping to higher-layer issues like MTU or IP conflicts, but Junos logs physical link transitions (up/down) are almost always due to Layer 1 problems such as faulty optics, cables, or hardware, not configuration mismatches.

How to eliminate wrong answers

Option B is wrong because an MTU mismatch causes packet fragmentation or drops but does not cause the physical link state to toggle; it would generate error counters or ICMP messages, not interface up/down events. Option C is wrong because a duplicate IP address results in address conflict logs and connectivity issues, not physical interface flapping; Junos would log 'Duplicate IP address detected' or ARP-related errors. Option D is wrong because spanning tree topology changes affect forwarding state (blocking/forwarding) but do not cause the interface itself to go up/down; STP events would show 'STP topology change' in logs, not link state transitions.

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22
Multi-Selectmedium

Which two events typically trigger a change to the routing table? (Choose two.)

Select 2 answers
A.Interface state change
B.System uptime
C.Log file rotation
D.BGP neighbor reset
E.Configuration commit
AnswersA, D

An interface state change (up or down) is a primary trigger for routing table updates. When an interface transitions, the kernel notifies rpd, which immediately adds or removes the directly connected route for that subnet; this change in turn causes the routing table to recalculate and, if necessary, sends updated route information to dynamic routing protocols like OSPF or IS-IS.

Why this answer

An interface state change (up/down) directly affects the reachability of directly connected networks. When an interface goes up, Junos adds the corresponding directly connected route to the routing table; when it goes down, the route is removed. Similarly, a BGP neighbor reset causes the BGP session to be torn down and re-established, which triggers the withdrawal and re-advertisement of all BGP routes, thereby modifying the routing table.

Exam trap

The trap here is that candidates often confuse a configuration commit (option E) with an automatic routing table change. A commit only activates the configuration; it does not directly modify the routing table unless the configuration itself triggers a protocol event or interface change.

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23
MCQeasy

You are a junior network engineer tasked with upgrading the Junos OS on a pair of EX3400 switches that are in a Virtual Chassis. You have downloaded the new software image to the primary switch. Before performing the upgrade, you must verify the image's integrity and check the current version. Which sequence of commands should you use?

A.Run 'show system uptime' and then 'request system software add /var/tmp/junos-new.tgz'.
B.Run 'show system software' and then 'request system software add validate'.
C.Run 'show version' and then 'request system software validate /var/tmp/junos-new.tgz'.
D.Run 'show configuration system software' and then 'file checksum sha256 /var/tmp/junos-new.tgz'.
AnswerC

This is the correct approach because 'show version' first confirms the currently running Junos image and software version, which is essential for comparing against the new package and verifying upgrade path compatibility. Then 'request system software validate /var/tmp/junos-new.tgz' performs a dry-run validation of the image file, checking its integrity, checksum, and compatibility with the current system without actually installing it. Together, these two commands give the engineer a clear picture of the current state and a verified upgrade package before installation.

Why this answer

'show version' displays the current Junos OS version running on the switch, which is necessary to confirm the starting point before an upgrade. Then 'request system software validate /var/tmp/junos-new.tgz' performs a checksum verification and package integrity check on the image file without installing it, ensuring the image is not corrupted. This sequence directly addresses the two tasks: verifying the image's integrity and checking the current version.

Exam trap

The trap here is that candidates confuse 'show system software' (a non-existent command) with 'show version', or think that 'file checksum sha256' alone is sufficient for integrity verification, missing the fact that Junos requires the full 'request system software validate' command to perform a comprehensive package check before installation.

How to eliminate wrong answers

Option A is wrong because 'show system uptime' shows how long the switch has been running, not the current software version, and 'request system software add /var/tmp/junos-new.tgz' immediately installs the image without any integrity validation. Option B is wrong because 'show system software' is not a valid Junos command (the correct command is 'show version' or 'show system software information'), and 'request system software add validate' is syntactically incorrect—the correct syntax is 'request system software validate <filename>'. Option D is wrong because 'show configuration system software' displays the software-related configuration (e.g., licenses), not the current running version, and while 'file checksum sha256' does verify integrity, it only checks the hash and does not perform the full package validation that 'request system software validate' does, nor does it check the current version.

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