JN0-105 Junos - Associate (JNCIA-Junos) 2025 Exam Questions and Answers

The Routing Engine (RE) in Juniper Networks devices plays a critical role in the control plane operations. One of its functions includes responding to network utility commands like ping and traceroute, which are essential for diagnosing network connectivity and path issues. Furthermore, the RE is responsible for maintaining the routing tables, which contain information about network paths and destinations. These tables are vital for making forwarding decisions but are distinct from the actual forwarding of packets, which is handled by the Packet Forwarding Engine (PFE).

CSMA/CD is the access method used in classic Ethernet (half-duplex). Its main functions:

Carrier Sense : A device listens to the medium to check if another device is transmitting before sending (avoiding collisions).

Collision Detection : If two devices transmit at the same time, they detect the collision, stop, and wait a random backoff period before retrying.

Devices can either transmit or receive at a given time (half-duplex) , not both simultaneously.

Thus, correct answers:

✔ A – Listen before transmit.

✔ B – Devices can either transmit or receive.

Options C and D are incorrect:

CSMA/CD is a half-duplex protocol , not full-duplex.

Full-duplex eliminates collisions and no longer uses CSMA/CD.

From JNCIA-Junos Study Guide :

“CSMA/CD requires that devices sense the wire before transmitting. Devices can either transmit or receive, but not both at the same time.”

To advertise a default static route into OSPF on router R1, a routing policy should be configured. This policy would typically include a statement to match the default route (0.0.0.0/0) and then apply an action to set the route as an OSPF external type, which would then be redistributed into the OSPF domain. The routing policy is a set of conditions and actions that determine how routes are imported into or exported from the routing table and how routes are shared between routing instances or routing protocols. After defining the policy, it must be applied to OSPF under the export section of the OSPF configuration on R1. This process will allow R1 to announce the default route to other OSPF routers in the network, which then can use it as a gateway of last resort to reach the Internet or other networks not explicitly known to the OSPF domain.

In Junos OS, the request system configuration rescue save command is used to save the current active configuration as a rescue configuration. This feature is particularly useful for preserving a known good configuration state that can be quickly reverted to in case of configuration errors or issues. By saving a rescue configuration, administrators can ensure that they have a reliable fallback option that can be loaded in the future to restore the device ' s operation without having to reconfigure from scratch. This is an essential practice for maintaining network stability and quick recovery.

The error message in the exhibit indicates that the root-authentication statement is missing, which is mandatory for committing the configuration. In Junos OS, it is required to set a password for the root account to commit any configuration changes. This is a security measure to ensure that unauthorized users cannot access the device ' s configuration mode. To solve the error shown in the exhibit, configuring a password for the root account is necessary. This can be done by using the set system root-authentication plain-text-password command, after which the user will be prompted to enter a new password for the root account.

Junos OS can store up to 50 rollback configuration files, making B the correct answer. These rollback files allow administrators to revert to previous configurations, providing a safety net that facilitates recovery from configuration errors or undesired changes

In the Junos OS, multiple software processes run independently in their own protected memory spaces. This design ensures that if one process crashes or encounters an issue, it does not affect the other processes running on the system. This architecture enhances the stability and reliability of the network device, as each process operates independently and securely.

One of the key advantages of Junos OS is its ability to roll back to previous configurations. This feature allows administrators to revert to an earlier configuration state, which is invaluable for quickly recovering from configuration errors or undesired changes. Junos OS maintains an archive of previous configurations, enabling easy rollback to any saved state. Another significant advantage of Junos OS is its modular design. The operating system is structured so that different processes and services run in separate protected memory spaces, enhancing the stability and reliability of the system. If one process fails, it does not affect the others, thereby minimizing the risk of system-wide failures.

In Junos OS, the prompt employee@R1 > indicates the current context of the user interface. The ' R1 ' part of the prompt signifies the hostname of the device, which in this case is ' R1 ' . The absence of a ' # ' symbol at the end of the prompt suggests that the user is in operational mode, as opposed to configuration mode which is indicated by a prompt ending in ' # ' . Operational mode allows users to view the status of the device and execute operational commands, but does not allow for configuration changes.

In a scenario where a router has a route to a specific network (in this case, 10.1.1.0/24) with a next hop that is unreachable or incorrectly specified (e.g., " r jet " seems to be a typo or an undefined entity), the router will typically discard the traffic destined for that network. This action is taken because the router cannot determine a valid path to forward the traffic. Unlike some scenarios where the router might generate an ICMP (Internet Control Message Protocol) unreachable message, in many configurations, especially in production networks, the traffic might be silently discarded without providing feedback to the sender, as generating ICMP messages for all undeliverable packets could lead to additional network congestion and potential security concerns.

Referring to the firewall filter configuration in the exhibit, when a packet matches the specified term limit-icmp , two actions are defined in the then statement: count count-icmp and discard . The count count-icmp action means that each time a packet matches this term, a counter named count-icmp will be incremented, providing a tally of how many packets have matched the term. The discard action means that the packet will be dropped and not forwarded through the device. This effectively prevents the packet from reaching its intended destination. There is no action specified that would cause an ICMP destination unreachable message to be returned, nor is there any action that would allow the packet to be forwarded.

Junos OS supports only 1 rescue configuration file on a device. This rescue configuration is a safeguard feature that allows network administrators to revert to a known good configuration in case of a configuration error or issue, ensuring network stability.

In Junos OS, each device supports only one rescue configuration file. The rescue configuration is a specific configuration that can be saved and later retrieved if needed. This is used as a fallback configuration that you know works and can be applied in case of an emergency or if the current configuration has issues.

The Packet Forwarding Engine (PFE) is an integral component of Juniper Networks devices, responsible for the data plane operations. The data plane, also known as the forwarding plane, is where the actual processing and forwarding of packets occur based on the routing and forwarding tables. The PFE executes the forwarding decisions made by the Routing Engine (RE), handling all packet transmissions, including routing, filtering, and switching packets towards their destination. This contrasts with the control plane operations handled by the RE, which involve routing table maintenance, system management, and control protocol processing.

If you ' ve committed a configuration and then need to revert to the previous configuration, the rollback command is used. Since the incorrect IP address has not been committed, as indicated by the commit check command being successful, issuing rollback 1 will undo the changes made in the current session, which includes the accidental entry of the IP address.

The clear log ospf-trace command on a Juniper Networks router is used specifically to manage the contents of the log file named ospf-trace . Executing this command clears or deletes the existing data within the ospf-trace log file but does not stop the logging process. The router continues to log new OSPF-related events and data into this file after the command is executed. This functionality is crucial for troubleshooting and monitoring the OSPF (Open Shortest Path First) protocol ' s operation by allowing network administrators to remove old or irrelevant log data while continuously capturing new events without interruption.

https://www.juniper.net/documentation/us/en/software/junos/user-access-evo/user-access/topics/topic-map/junos-os-login-class.html#:~:text=You%20can%20define%20any%20number,to%20an%20individual%20user%20account.

In Junos OS, each user account can be assigned only one login class. Login classes in Junos OS define the permissions for users, controlling what they can access and modify within the system. This setup helps in maintaining a clear and secure access control mechanism.

A Layer 3 router forwards packets based on the destination IP address. It operates at the network layer of the OSI model and uses routing tables to determine the best path for packet delivery. Unlike Layer 2 switches, which forward packets based on MAC addresses, routers handle logical addressing, making them crucial for inter-network communication.

In Junos OS routing policy evaluation, " next policy " (A) and " next term " (C) are common actions that affect the flow of policy evaluation. " Next policy " directs the evaluation to the next policy in the sequence, whereas " next term " moves the evaluation to the next term within the current policy, allowing for granular control over routing decisions.

Junos OS supports several external authentication methods for administrative access, with TACACS+ (Terminal Access Controller Access-Control System Plus) and RADIUS (Remote Authentication Dial-In User Service) being among the most commonly used. Both TACACS+ and RADIUS are protocols that allow network devices to communicate with a central authentication server, enabling centralized control over user authentication and authorization. This centralization simplifies the management of user credentials and access policies, especially in larger networks with multiple devices.

Junos OS supports three types of configuration sessions :

Configure exclusive : allows only one user to enter configuration mode. This prevents conflicts. If uncommitted changes exist from a previous session, they are retained.

Configure private : allows multiple users to edit simultaneously, each with their own candidate copy.

Configure (default/shared): multiple users can edit the same candidate configuration at once.

Thus, configure exclusive:

✔ Retains uncommitted changes in the candidate configuration.

✔ Permits only one user to edit at a time.

From Junos OS Fundamentals :

“The configure exclusive command allows a single administrator to lock the configuration. Uncommitted changes remain intact until committed or discarded.”

Issuing the rollback 0 command in Junos OS will discard any uncommitted changes and revert to the last committed configuration. This command effectively cancels any configuration changes that have been made but not yet committed, ensuring that the device returns to its previous stable state.

The show chassis hardware command in Junos OS displays detailed information about the hardware installed in the device, including the chassis itself. This command provides a list of all hardware components, their serial numbers, part numbers, and version information. When looking for the chassis serial number specifically, this command is the most direct and comprehensive way to retrieve that information, as it includes the serial number of the chassis among the details provided.

In the default configuration of a Junos device, the primary system log file is " messages " (B). This log file contains a wide range of system messages, including operational status changes, system errors, and other critical information, making it a key resource for troubleshooting and monitoring the system ' s health.

Traceoptions in Junos OS are used for detailed debugging and troubleshooting of protocols and processes within the system. They are not enabled by default due to the potential performance impact and volume of data generated. Instead, traceoptions are enabled through specific configuration settings under the relevant protocol or process hierarchy. This allows administrators to target their troubleshooting efforts and control the scope of logging. By default, the output generated by traceoptions is stored in files located in the /var/log directory, with the file name typically specified in the traceoptions configuration. This structured approach to logging and debugging helps in diagnosing complex issues without overwhelming the system or the administrator with irrelevant data.

UDP (User Datagram Protocol) is known for being connectionless (D) and providing best-effort delivery without the reliability mechanisms present in TCP (C). This means that UDP does not establish a connection before sending data and does not guarantee delivery, order, or error checking, making it faster but less reliable than TCP.

In Junos OS, when two entries for the same destination exist in the routing table, the route with the lowest preference number is selected as the active route. This preference number, also known as the route preference or administrative distance, is used to prioritize routes received from different routing protocols.

The OSI (Open Systems Interconnection) model is a conceptual framework used to understand network interactions in seven distinct layers. IP (Internet Protocol) addresses are part of Layer 3, known as the Network Layer. This layer is responsible for packet forwarding, including routing through intermediate routers, and it handles the logical addressing scheme of the network to ensure that packets can be routed across multiple networks and reach their destination. IP addresses provide unique identifiers for network interfaces, allowing for communication between devices on a network or across different networks.

The request system zeroize media command on a Junos device securely erases all data, including configuration and log files, and resets the device to its factory default settings. This command is used when redeploying a device to ensure no residual data remains from its previous deployment. It ' s a comprehensive and secure way to clear all configurations and data, making the device as if it were new. The other commands listed do not perform a full reset to factory defaults; for example, show system storage displays storage information, and request system storage cleanup offers to delete unnecessary files without resetting the device to factory settings.

For the users in the 10.1.1.0/24 subnet and the server in the 10.1.2.0/24 subnet to communicate with each other, they need to route packets through the router that connects these two subnets. Each user and the server need to have their default gateway set to the IP address of the router interface on their respective subnet (.254). This ensures that packets destined for other subnets are sent to the router, which then routes them to the correct destination subnet.