JN0-106 Practice Questions 1 — 50 Questions

Question 1

A network engineer needs to make a change to a Junos device and ensure the change can be reverted if it causes issues. Which feature should be used?

Accepted Answer

commit confirmed. The `commit confirmed` command allows an engineer to commit a configuration change with a timer (default 10 minutes). If the change causes issues and the engineer does not confirm the commit within the timer, the device automatically reverts to the previous active configuration. This provides a safety net to revert changes without manual intervention.

Answer

configure exclusive

Answer

commit check

Answer

rollback 0

Question 2

Order the steps to configure a user account with a password in Junos.

Accepted Answer

Set the class (set system login user class ), then set the authentication method to plain-text-password (set system login user authentication plain-text-password), then commit the configuration.. The correct order to configure a user account in Junos involves setting both the class and authentication before committing. Options A and B are both valid sequences because the order of setting the class and authentication does not matter; the configuration is applied only upon commit. Option C is wrong because committing before setting the authentication leaves the user without a password. Option D is wrong because committing before setting the class results in an incomplete user configuration with no privilege level.

Answer

Set the class, commit the configuration, then set the authentication method to plain-text-password.

Answer

Set the authentication method to plain-text-password, commit the configuration, then set the class.

Question 3

A junior network administrator is setting up a Juniper MX router for the first time. After powering on the device, the administrator notices that the LED on the front panel blinks amber and the device does not complete the boot process. The console displays messages reporting file system errors. The administrator has no previous configuration changes and the device was shipped with factory defaults. Which action should the administrator take to resolve the boot issue?

Accepted Answer

Boot into single-user mode and run file system checks.. The amber blinking LED and file system errors indicate a corrupted file system, which is common on factory-default devices if the flash memory was not properly initialized. Booting into single-user mode (option A) allows the administrator to run 'fsck' (file system check) to repair the root file system without loading the full Junos OS, which is the standard recovery procedure for such boot failures.

Answer

Perform a password recovery procedure to gain access.

Answer

Use the 'request system software add' command from the boot loader.

Answer

Reinstall the Junos OS using a USB flash drive with the software image.

Question 4

While in configuration mode, an administrator wants to see the difference between the candidate configuration and the active configuration. Which command accomplishes this?

Accepted Answer

show | compare. The 'show | compare' command, when executed in configuration mode, displays the differences between the candidate configuration and the active (committed) configuration. This pipe filter compares the current candidate configuration against the last committed configuration, highlighting additions, deletions, and changes, which is the exact requirement for seeing the difference.

Answer

show configuration

Answer

run show configuration

Answer

show system commit

Question 5

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

Accepted Answer

show interfaces extensive ge-0/0/0 | refresh 1. 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.

Answer

show interfaces terse

Answer

show interfaces ge-0/0/0

Answer

monitor security log

Question 6

Which TWO statements are true about the Junos file system? (Choose two.)

Accepted Answer

The /config directory contains configuration files.. The /config directory in the Junos file system stores the active and backup configuration files, including juniper.conf and juniper.conf.gz. These files are critical for device operation and are loaded during the boot process.

Answer

The /altroot directory is a backup root filesystem.

Answer

The /kernel directory contains the operating system kernel.

Answer

The root directory (/) is a RAM disk.

Question 7

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

Accepted Answer

show log messages. '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.

Answer

show log dcd

Answer

show log file

Answer

show system messages

Question 8

Refer to the exhibit. A frame with destination MAC 00:0c:29:2a:3b:4d arrives on interface ge-0/0/0. What action will the switch take?

Accepted Answer

Forward the frame out of ge-0/0/2 only. The switch learns MAC addresses and their associated VLANs from incoming frames. Since the destination MAC 00:0c:29:2a:3b:4d is already in the MAC address table and mapped to interface ge-0/0/2 within the same VLAN, the switch forwards the frame only out of ge-0/0/2. This is the fundamental behavior of transparent bridging: unicast frames are forwarded only to the port where the destination MAC was last seen.

Answer

Flood the frame to all ports except ge-0/0/0

Answer

Drop the frame because the VLAN does not match

Answer

Forward the frame out of both ge-0/0/1 and ge-0/0/2

Question 9

Which THREE statements are true about commit operations in Junos OS?

Accepted Answer

Multiple users can make configuration changes simultaneously, but only one can commit at a time.. Junos OS allows multiple users to enter configuration mode and make changes simultaneously, but the commit operation is serialized: only one user can commit at a time. This prevents conflicting changes from being applied concurrently, ensuring configuration consistency.

Answer

The 'commit confirmed' command immediately discards changes if not confirmed.

Answer

The 'commit' command always requires 'commit synchronize' when using dual Routing Engines.

Question 10

Which two statements about static routes in Junos OS are correct?

Accepted Answer

Static routes have a default preference of 5.. Static routes have a default preference of 5 in Junos OS. Option C is correct: when a static route uses an interface as the next-hop (e.g., ge-0/0/0), the router uses ARP to resolve the MAC address of the next-hop device. Option D is correct: a static route with a next-hop of 0.0.0.0 is a discard route that silently drops packets. Option A is incorrect: a static route with a next-hop of 127.0.0.1 creates a reject route (sends ICMP unreachable), not an unreachable route. Option E is incorrect: static routes cannot have a preference value of 0 because preference 0 is reserved for directly connected routes.

Answer

A static route with a next-hop of 127.0.0.1 is an unreachable route.

Answer

Static routes can have a preference value of 0.

Question 11

You are troubleshooting an OSPF adjacency issue between two Juniper MX series routers, R1 and R2, both running Junos 18.2. They are directly connected via a Gigabit Ethernet link. R1's interface ge-0/0/0 is configured with IP 192.168.1.1/24, and R2's ge-0/0/0 is configured with IP 192.168.1.2/24. Both interfaces are in OSPF area 0.0.0.0. The physical connectivity is confirmed up, and both interfaces are administratively enabled. You have checked that the hello and dead intervals match (hello 10, dead 40), the area IDs are identical, and the subnet masks are /24. No firewall filters are blocking OSPF. The OSPF configuration on R1 includes 'authentication-type md5' and 'authentication-key juniper123' under the interface. On R2, the configuration also includes 'authentication-type md5' but the key is 'juniper321'. Both routers have loopback addresses as router IDs: 1.1.1.1 for R1 and 2.2.2.2 for R2. No explicit OSPF network type is set, so the default is broadcast. Despite these configurations, the OSPF adjacency state on both routers remains in INIT. What is the most likely cause of this issue?

Accepted Answer

The OSPF authentication keys do not match.. OSPF authentication keys must match exactly for the adjacency to form. R1 uses key 'juniper123' while R2 uses 'juniper321', causing the MD5 digest computed on each Hello packet to differ. Since OSPF authentication is validated per-packet, mismatched keys prevent the routers from moving past the INIT state.

Answer

The interface MTU size is mismatched.

Answer

The OSPF network type is mismatched.

Answer

The router IDs are not properly configured.

Question 12

During a network traffic storm, a Juniper EX switch's CPU utilization spikes to 100%. Which command would best help identify the cause?

Accepted Answer

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

show spanning-tree bridge

Answer

show ethernet-switching table

Answer

show interfaces extensive ge-0/0/0

Question 13

A network engineer is configuring a Juniper device in a production network. They have applied several changes to the candidate configuration and run 'commit check', which passes. After a brief pause, they run 'commit' but receive the error: 'commit failed: configuration database modified by another user. Please reload the configuration and try again.' The engineer is the only person currently logged into the device, but they know that another engineer was working on the same device earlier and may have left a commit pending. What is the most likely reason for the commit failure?

Accepted Answer

The other engineer committed their changes between the commit check and the commit.. The error message 'configuration database modified by another user' indicates that another user (the earlier engineer) had a pending commit that was not yet applied or discarded. When the current engineer ran 'commit check', it validated the candidate configuration against the current active configuration, but between that check and the actual 'commit', the other engineer's pending commit was applied (or their session was closed, causing their changes to be committed automatically if they had left a commit pending). This changed the configuration database, causing the commit to fail due to a version mismatch.

Answer

The commit check did not validate the syntax of the new changes.

Answer

The candidate configuration has timed out and must be re-entered.

Answer

The device has insufficient storage to write the new configuration.

Question 14

You are responsible for a Juniper MX router that connects two customer sites over a Layer 3 VPN. The router is configured with BGP for VPN routes and uses MPLS to forward traffic. Recently, the customer reported that traffic from Site A (10.0.1.0/24) to Site B (10.0.2.0/24) is intermittently failing. You check the routing table on the router and see that both routes are present with valid next-hops. However, when you ping from the router's loopback to the remote site's loopback, the ping succeeds. MPLS labels are being assigned and the LSP is up. You also notice that when the failure occurs, the router's BGP session to the remote PE is still established. The failure seems random and lasts a few seconds before recovering. Which troubleshooting step is most likely to identify the root cause?

Accepted Answer

Review the forwarding table for the affected prefixes and check for any discrepancies.. The issue is intermittent packet loss with valid routes in the routing table but successful pings from the loopback. This points to a forwarding table (FIB) inconsistency, where the control plane (routing table) has the correct next-hop, but the forwarding plane (PFE) may have a stale or incorrect entry for the specific prefixes. Checking the forwarding table with 'show route forwarding-table' will reveal if the next-hop or label information differs from the routing table, which is a classic symptom of a hardware programming issue or a transient PFE problem.

Answer

Check the interface error counters for CRC errors or drops.

Answer

Verify the BGP session state and check for route flapping.

Answer

Check the MPLS label switching table to ensure labels are correctly assigned.

Question 15

A network engineer needs to view the operational status of all interfaces on a Junos device, including those that are disabled. Which command should be used?

Accepted Answer

show interfaces terse. The 'show interfaces' command displays detailed information for all interfaces, including those that are administratively disabled. However, 'show interfaces terse' provides a concise, one-line-per-interface summary that is more efficient for quickly viewing the operational status of all interfaces, including disabled ones. For the JNCIA exam, 'show interfaces terse' is the recommended command for this purpose because it offers a clear and compact overview of interface status across the device.

Answer

show interfaces

Answer

show configuration interfaces

Answer

monitor interface traffic

Question 16

What is the primary function of the fxp0 interface on a Juniper device?

Accepted Answer

Management interface. The fxp0 interface is a dedicated out-of-band management Ethernet port on Juniper devices, used exclusively for management traffic such as SSH, SNMP, and syslog. It is separate from the data plane and control plane forwarding interfaces, ensuring administrative access remains available even if the routing or forwarding planes are disrupted.

Answer

Internal routing

Answer

Loopback testing

Answer

Data plane forwarding

Question 17

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

Accepted Answer

show log messages. '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.

Answer

monitor start messages | match "error"

Answer

show log file messages | match "error"

Answer

show system syslog messages

Question 18

A network administrator accidentally deleted a vital part of the configuration while in configuration mode. They need to revert to the previous configuration without losing recent changes that are correct. Which action should they take?

Accepted Answer

rollback 0. The 'rollback 0' command reverts the candidate configuration to the most recently committed configuration (the current active configuration), discarding any uncommitted changes. This allows the administrator to undo the accidental deletion without affecting any recently committed correct changes, as those are already part of the active configuration. Option B, 'rollback 1', would revert to the previous commit, which could remove recently committed correct changes if they were made in the most recent commit. Therefore, 'rollback 0' is the safer choice when you want to preserve correct changes that have already been committed.

Answer

deactivate

Answer

rollback 1

Answer

delete

Question 19

A host in VLAN 10 can ping a host in VLAN 20 without a default gateway configured. What feature is likely enabled on the router that interconnects the VLANs?

Accepted Answer

Proxy ARP. Proxy ARP allows a router to respond to ARP requests on behalf of hosts in different subnets. When a host in VLAN 10 sends an ARP request for a host in VLAN 20, the router responds with its own MAC address if it has a route to the destination. This enables the host to send traffic to the router, which then forwards it to the destination VLAN, even without a default gateway configured on the host.

Answer

VLAN translation is configured on the trunk

Answer

IP routing is enabled on the router

Answer

Dynamic ARP Inspection

Question 20

You are a network engineer at a company that operates a pair of Juniper SRX firewalls in an active/passive cluster (Chassis Cluster). The cluster has been running Junos 15.1X49-D100 for over a year. Management has mandated an upgrade to a newer version to address security vulnerabilities. You follow the recommended upgrade procedure and successfully upgrade the primary node (node0) first, then failover to make node0 the backup, and upgrade the new primary (node1). After the upgrade, both nodes have the same version and appear to be in the cluster, but you notice that the backup node (node0) is stuck in 'ineligible' state and does not synchronize configuration changes. What is the most likely cause?

Accepted Answer

The cluster control link is down or misconfigured on the backup node. The backup node being stuck in 'ineligible' state after a chassis cluster upgrade indicates that the cluster control link (control port) is not functioning correctly. In a Juniper SRX active/passive cluster, the control link is used for heartbeat and cluster state synchronization; if it is down or misconfigured on the backup node, the node cannot participate in the cluster election process and remains ineligible. This is a common issue after upgrades if the control link interfaces are not properly re-established or if the cable is faulty.

Answer

The backup node has not been configured with 'commit synchronize'

Answer

The 'set chassis cluster reth-count' statement is missing on the backup node

Answer

The 'monitor interface' configuration is causing a mis-match between nodes