JNCIA-JUNOS Junos OS Fundamentals — Timed 50 Questions

Question 1

A network engineer is configuring a new Juniper device and needs to ensure that the configuration is saved persistently across reboots. Which command should be used?

Accepted Answer

commit. The `commit` command activates the candidate configuration and saves it to the active configuration database, ensuring it persists across reboots. Without a commit, any changes made in candidate mode are lost when the device restarts.

Answer

save

Answer

request system reboot

Answer

show configuration

Question 2

An administrator needs to quickly revert all uncommitted configuration changes and return the device to the last committed configuration. Which command accomplishes this?

Accepted Answer

rollback 0. The command 'rollback 0' reverts all uncommitted configuration changes and returns the device to the last committed configuration. In Junos, the rollback command uses a numeric argument to specify which previous configuration to load, with 0 always referring to the most recently committed configuration. This effectively discards any pending changes in the candidate configuration without requiring a commit.

Answer

delete configuration

Answer

load override

Answer

rollback 1

Question 3

A Juniper device is experiencing high CPU utilization due to a routing protocol process. The engineer suspects a specific BGP peer is causing the issue. Which operational command can be used to collect diagnostic information about the routing protocol processes?

Accepted Answer

show system processes extensive. Option A is correct because 'show system processes extensive' displays detailed CPU and memory usage for each individual process, including routing protocol daemons like bgpd. This allows the engineer to identify which specific BGP peer or process is consuming excessive CPU resources, rather than just seeing aggregate routing protocol statistics.

Answer

request support information

Answer

show bgp summary

Answer

monitor traffic interface

Question 4

A junior engineer is troubleshooting connectivity issues and wants to trace the path packets take to a remote destination. Which Junos command should be used?

Accepted Answer

traceroute. Option C is correct because the 'traceroute' command in Junos is specifically designed to trace the path packets take to a remote destination by sending UDP probes with increasing TTL values and analyzing ICMP Time Exceeded messages from intermediate routers. This directly addresses the junior engineer's need to map the Layer 3 path and identify where connectivity failures occur.

Answer

monitor traffic

Answer

show route

Answer

ping

Question 5

An engineer needs to view the current active configuration on a Juniper device. Which command will display the configuration that is currently running?

Accepted Answer

show configuration. The 'show configuration' command displays the current active configuration that is committed and running on a Juniper device. Unlike Cisco's 'show running-config', Junos uses a commit model where the candidate configuration is activated only after a 'commit' operation, and 'show configuration' shows that committed, active configuration.

Answer

show chassis hardware

Answer

show interfaces terse

Answer

show system commit

Question 6

A network administrator is configuring a new interface and wants to ensure that the interface is enabled and can pass traffic. Which configuration element is required?

Accepted Answer

set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24. Option B is correct because in Junos, an interface is administratively enabled by default (no explicit 'enable' command is needed), but to pass traffic it requires a logical unit with a configured protocol family and an IP address. The command 'set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24' creates unit 0, assigns the IPv4 address, and implicitly enables the interface for traffic forwarding. Without an address under a family, the interface cannot pass IP traffic even if it is administratively up.

Answer

set interfaces ge-0/0/0 enable

Answer

set interfaces ge-0/0/0 unit 0 family inet

Answer

set interfaces ge-0/0/0 disable

Question 7

During a maintenance window, an engineer needs to apply a series of configuration changes that are stored in a text file. Which command sequence should be used to load and apply the changes from the file?

Accepted Answer

configure; load merge /var/tmp/changes.txt; commit. Option D is correct because the engineer needs to enter configuration mode (configure) and then use the 'load merge' command to merge the contents of the text file into the candidate configuration without replacing the entire configuration. The 'commit' command then activates the changes. This is the standard Junos workflow for applying incremental changes from a file.

Answer

request system configuration rescue save

Answer

load override /var/tmp/changes.txt; commit

Answer

configure; load patch /var/tmp/changes.txt; commit

Question 8

An engineer notices that a Juniper device is not saving configuration changes across reboots. What is the most likely cause?

Accepted Answer

The candidate configuration was not committed.. In Junos OS, configuration changes are stored in a candidate configuration and only become active and persistent across reboots after a 'commit' operation. Without a commit, the changes remain in the candidate buffer and are discarded upon reboot, causing the device to revert to the last committed configuration.

Answer

The rescue configuration is not set.

Answer

The device is booting from factory-default configuration.

Answer

The command 'request system reboot' was used instead of 'commit'.

Question 9

Which TWO statements are true about the Junos OS candidate configuration model? (Choose two.)

Accepted Answer

The candidate configuration can be modified without affecting the active configuration.. Option D is correct because the Junos OS candidate configuration model allows modifications to be made to a separate candidate configuration file without affecting the active configuration currently running on the device. This ensures that changes can be staged, reviewed, and validated before being committed, providing a safe and non-disruptive way to manage configuration changes.

Answer

Each change is applied incrementally to the active configuration.

Answer

The candidate configuration is stored in a separate file from the active configuration.

Answer

After a commit, the candidate configuration is automatically discarded.

Question 10

Which THREE commands are valid Junos operational mode commands for monitoring network diagnostics? (Choose three.)

Accepted Answer

traceroute. Option A is correct because the 'traceroute' command in Junos operational mode sends a series of UDP packets with increasing TTL values to discover the path a packet takes to a destination, which is a standard network diagnostic tool. It is a valid operational mode command used for monitoring network connectivity and path analysis.

Answer

show configuration

Answer

request system reboot

Question 11

Which TWO statements correctly describe the function of the 'commit' command in Junos OS? (Choose two.)

Accepted Answer

It validates the candidate configuration for syntax errors.. Option A is correct because the 'commit' command in Junos OS first validates the candidate configuration for syntax errors before applying it. If syntax errors are found, the commit fails and the candidate configuration is not applied, ensuring the device does not enter an inconsistent state. This validation step is a core part of Junos's transactional configuration model, where changes are staged in a candidate configuration and then committed atomically.

Answer

It schedules the configuration to be applied at a later time.

Answer

It discards the candidate configuration after saving.

Answer

It automatically reboots the device to apply the changes.

Question 12

Based on the exhibit, what is the most likely reason for the ping failure?

Accepted Answer

The neighbor device is not configured or is unreachable.. The ping failure is most likely due to the neighbor device not being configured or unreachable because the output shows the interface is up (Physical link is Up) and the unit is enabled (Unit 0 is enabled), but there is no neighbor discovery or ARP entry. Without a valid next-hop or neighbor reachability, ICMP echo requests cannot be forwarded, resulting in ping failure.

Answer

A firewall filter is blocking ICMP traffic.

Answer

The interface is disabled at the unit level.

Answer

The interface is administratively down.

Question 13

Based on the exhibit, what is the most likely impact on the router?

Accepted Answer

Routing protocol adjacencies will be lost, causing routing instability.. The exhibit shows kernel messages indicating a hardware or software fault (e.g., a FPC crash or PIC restart). In Junos, such critical events cause the Packet Forwarding Engine (PFE) to reset, which tears down all routing protocol adjacencies (OSPF, BGP, IS-IS) because the control plane loses communication with the forwarding plane. This leads to route withdrawal and routing instability until the adjacencies are re-established.

Answer

The CPU will be overloaded due to the kernel messages.

Answer

The router will stop forwarding packets immediately.

Answer

The router will reboot automatically.

Question 14

You are a network engineer at a service provider. You have a Juniper MX router that serves as a BGP route reflector for a large number of customer routes. Recently, the router has been experiencing high CPU utilization, and the BGP process (rpd) is consuming excessive memory. You suspect that the router is receiving too many routes from a specific BGP peer, causing the routing table to grow beyond the available memory. You need to diagnose the issue without affecting production traffic. Which course of action should you take?

Accepted Answer

Use the 'show route summary' command to check the routing table size and memory usage.. Option C is correct because the 'show route summary' command provides a high-level overview of the routing table size, including the total number of routes and the memory utilization of the routing table. This allows the engineer to quickly assess whether the routing table has grown beyond available memory without disrupting production traffic, which is the primary goal of the diagnosis.

Answer

Immediately shut down all BGP peers to stop the influx of routes.

Answer

Configure a prefix limit on all BGP peers.

Answer

Use the 'show bgp summary' command to check the number of prefixes per peer.

Question 15

You are a network administrator for a large enterprise. You have a Juniper SRX firewall that is used as the primary internet gateway. Users are reporting that they are unable to access certain external websites. You check the device and find that the security policies appear correct, and the routing is in place. You suspect that the issue might be related to the DNS resolution. You want to verify that the device can resolve DNS names and that the DNS server configuration is correct. Which command should you use to test DNS resolution from the Juniper device?

Accepted Answer

ping <external website FQDN>. Option B is correct because the `ping` command with a fully qualified domain name (FQDN) forces the Juniper device to perform a DNS resolution before sending ICMP echo requests. If the device cannot resolve the FQDN, the ping will fail with a 'hostname not found' error, directly indicating a DNS configuration or reachability issue. This tests both the DNS server configuration and the device's ability to resolve names, which is essential for verifying DNS functionality.

Answer

show system connections

Answer

traceroute <external website FQDN>

Answer

show dns name-server

Question 16

A network engineer notices that a Junos device is not forwarding traffic for a specific subnet. The routing table shows the route for that subnet, but traffic is still not being forwarded. Which command should the engineer use to verify whether the firewall filter is dropping the traffic?

Accepted Answer

show firewall filter log. Option D is correct because the 'show firewall filter log' command displays the log entries generated by firewall filter terms that have a 'log' action configured. When traffic is not being forwarded despite a valid route, a firewall filter may be silently dropping packets; this command reveals which packets matched the filter and whether they were logged, helping to identify drops.

Answer

show interfaces terse

Answer

show firewall filter counter

Answer

show firewall filter

Question 17

Which THREE of the following statements are true regarding Junos OS commit operations?

Accepted Answer

The 'commit check' command validates the candidate configuration without activating it.. Option A is correct because the 'commit check' command validates the candidate configuration for syntax and semantic errors without activating it. This allows an administrator to verify changes before making them active, preventing potential misconfigurations from disrupting network operations.

Answer

You can run multiple commit scripts during a single commit operation.

Answer

The rollback command reverts to the current active configuration.

Question 18

Refer to the exhibit. An engineer wants to verify that static routes for 10.0.1.0/24 and 10.0.2.0/24 are present in the routing table. Based on the output, which statement is true?

Accepted Answer

Both routes are active and have the same preference value.. The output shows both 10.0.1.0/24 and 10.0.2.0/24 as static routes with a preference of 5. Since the preference value is the same and both are listed as active (the 'A' flag is present in the routing table), both routes are installed and active in the forwarding table. In Junos, a static route's default preference is 5, and any route with a lower preference is preferred; here both have equal preference, so they coexist as active routes.

Answer

Both routes have a preference of 0.

Answer

Only the 10.0.1.0/24 route is active.

Answer

Both routes use the same next-hop IP address.

Question 19

You are administering a Juniper MX240 router that provides connectivity to multiple customer sites. The router uses BGP to exchange routes with two upstream ISPs. Recently, you applied a new firewall filter to the loopback interface to restrict management access. After committing the configuration, you can no longer establish SSH sessions to the router from the management network. You are currently connected via console. The loopback filter is still applied. You suspect the filter is blocking SSH traffic from the management network. What should you do to restore SSH access without losing the other filter rules?

Accepted Answer

Add a new term at the beginning of the filter that accepts SSH traffic from the management network, then reorder the terms so that this term is evaluated first.. Option C is correct because firewall filters in Junos are evaluated in order, and adding a term at the beginning that explicitly accepts SSH traffic from the management network ensures that the SSH packets are permitted before any subsequent deny terms are evaluated. This preserves all existing filter rules while restoring SSH access. The 'insert' command or reordering terms is necessary to place the new term first, as the default behavior appends new terms to the end of the filter.

Answer

Roll back to the previous configuration using 'rollback 0' and commit.

Answer

Add a new term at the end of the filter that accepts SSH traffic from any source.

Answer

Delete the firewall filter from the loopback interface and commit.

Question 20

A network engineer needs to check the operational status of all interfaces on a Juniper device. Which command provides a summary of interface status including link state, protocol state, and error counters?

Accepted Answer

show interfaces terse. Option C is correct because the 'show interfaces terse' command provides a concise summary of all interfaces, displaying the interface name, administrative status (Admin), link state (Link), protocol state (Proto), and a brief description. This command is specifically designed for a quick operational overview, including error counters in the full output when combined with other flags, but the terse output itself focuses on status and protocol state, which directly meets the engineer's requirement.

Answer

show interfaces

Answer

show configuration interfaces

Answer

show interface descriptions