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

In the OSI model, the Data Link Layer is responsible for node-to-node delivery of data. It frames the packets received from the Network Layer and prepares them for physical transmission. The Protocol Data Unit (PDU) for the Data Link Layer is called a "frame." Frames encapsulate the network layer packets, adding a header and a trailer that include the hardware addresses of the source and destination, among other things, facilitating the data link layer services like frame synchronization, flow control, and error checking.

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.

To view real-time usage statistics for the busiest interfaces on a device running Junos OS, the correct command is B, "monitor interface traffic." This command provides a dynamic, real-time view of the traffic flowing through the interfaces, allowing administrators to quickly identify and monitor the busiest interfaces on the device.

Firewall filters in Junos OS are stateless, meaning they process each packet individually without regard to the state of a connection or sequence of packets. These filters can match various packet attributes, including those at Layer 4, such as TCP and UDP port numbers. This allows for granular control over traffic based on the type of service or application. Unlike stateless filters, stateful firewalls keep track of the state of active connections and make decisions based on the context of the traffic flow, which is not a capability of Junos firewall filters. Additionally, Junos firewall filters primarily operate up to Layer 4 and do not natively inspect Layer 7 parameters, which involve application-level data.

Routing policies in Junos OS are crucial for controlling route advertisements and path selection. The correct answers are B and C. An import routing policy for BGP determines which received prefix advertisements are placed in the routing information base (RIB), and policy terms are evaluated from top to bottom, with action taken on the first match found. This sequential evaluation allows for precise control over routing decisions.

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 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 candidate configuration in Junos OS is a temporary configuration that allows network administrators to make and stage multiple configuration changes before applying them to the device. This approach enables the deployment of multiple changes in a single operation, ensuring that all configurations work together as intended before making them active. Additionally, the candidate configuration can be discarded if the administrator decides not to apply the staged changes, allowing for a "trial and error" approach without affecting the currently active configuration. This feature provides flexibility and reduces the risk of disruptive changes to the network.

The Routing Engine (RE) in Junos OS has several critical functions, including processing all management traffic (A) and running the Junos operating system (B). The RE handles system management tasks, user interfaces, system services, and routing protocol processes. It does not directly process transit traffic or evaluate firewall filters for transit traffic, as these tasks are handled by the Packet Forwarding Engine (PFE).

Implementing Class of Service (CoS) in a network provides numerous benefits, particularly in managing traffic based on its importance, source, or type. CoS enables network administrators to manage traffic congestion by applying various queuing techniques and policies to ensure that critical services remain unaffected during high congestion periods. Additionally, CoS allows for the prioritization of latency-sensitive traffic such as voice and video, ensuring that these services maintain quality despite varying network conditions.

The Routing Engine (RE) in Juniper Networks devices plays a pivotal role in the control plane, handling tasks that are critical for the operation and management of the network. One of its key functions is processing management traffic, which includes user commands, system configuration, and monitoring operations. The RE also maintains routing tables, which are essential for network routing decisions. These tables contain network topology information and routing paths, which the RE uses to update the Packet Forwarding Engine (PFE) so that it can forward packets appropriately. The RE does not forward transit traffic or process Class of Service (CoS) marked traffic, as these tasks are handled by the PFE.

To recover the root password on a Junos router without losing the configuration, you should (A) enter and commit the new root password once you have gained access to the system, (D) hit the space bar to interrupt the boot process and enter recovery mode when prompted during the boot process, and (E) use a console connection to reboot the device and access the bootloader prompt. These steps allow you to reset the root password while preserving the existing configuration.

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.

At the operational mode prompt on a Junos device, you can use various commands to view the device's status and request system operations. The show interface ge-0/0/0 command is used to display information about a specific interface, while the request system shutdown command is used to properly shut down the device. The set command is used in configuration mode, not operational mode, and the run command is used to execute operational mode commands from configuration mode.

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.

In Juniper firewall filter configurations, "discard" and "accept" are two valid terminating actions for a term within a filter. The "discard" action drops the packet, preventing it from reaching its intended destination, while the "accept" action allows the packet to pass through the filter, proceeding to its next hop or destination. "Count" is a non-terminating action that increments a counter every time a packet matches the term but does not inherently determine the packet's fate. "Next term" directs the evaluation to proceed to the next term in the filter for further processing, also a non-terminating action.

In Junos OS, the archival configuration under [edit system] allows for the automatic backup of configuration files to designated locations upon commit. When multiple archive-sites are specified, as shown in the exhibit with both SCP and FTP sites listed, the device does not choose between them or use them in a round-robin manner. Instead, it attempts to transfer the configuration backup files to all specified sites simultaneously upon each commit. This ensures redundancy and increases the likelihood that a backup will be successfully stored even if one of the transfer methods or destinations fails.

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.

Exception traffic includes traffic that is not simply forwarded by the router but requires special handling, such as routing updates (B) and log messages (C). These types of traffic are processed by the router's control plane rather than just being forwarded through the data plane.

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.

References:

Juniper official documentation: Configuring Basic Routing.

General networking principles.

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.

The show route table inet6.0 command displays all IPv6 routes in the default routing instance. In Junos OS, the routing table for IPv6 addresses is referred to as inet6.0, whereas inet.0 is used for IPv4 unicast routes. The other options do not correspond to the correct IPv6 routing table.

References:

Juniper official documentation: Junos OS Routing Tables Overview.

Layer 2 network switches are crucial components in local area networks (LANs), providing multiple functions for data packet forwarding and network segmentation. One inherent characteristic of switches is their susceptibility to traffic loops, especially in networks with redundant paths. Without proper loop prevention protocols like Spanning Tree Protocol (STP), loops can cause broadcast storms and network instability. Additionally, switches inherently flood broadcast traffic to all ports within the broadcast domain, except the port on which the broadcast was received. This is because broadcast frames are meant to be delivered to all devices within the VLAN, and the switch ensures this by flooding these frames to all ports in the VLAN, except the source port.

The Routing Protocol Daemon (rpd) in Junos OS is responsible for maintaining routing protocols and tables. It handles all routing information, including the calculation of routes and the population of the routing table, making it crucial for dynamic routing operations.

The Packet Forwarding Engine (PFE) in Junos OS performs several key functions, including implementing firewall filters (A) and forwarding transit traffic (C). The PFE applies firewall filter rules to incoming and outgoing traffic and is responsible for the high-speed forwarding of packets based on the information in the forwarding table.

The monitor interface ge-0/0/3 command is used in Junos OS to view real-time statistics for a specific interface. This command helps in diagnosing issues like packet loss by displaying real-time updates of traffic and error statistics for the specified interface.

The routing table and forwarding table play distinct roles in a Junos OS device. The correct answers are A and D. The routing table (A) is used by the Routing Engine (RE) to select the best route among all the learned routes, while the routing table (D) stores all routes and prefixes learned from all routing protocols. The forwarding table, in contrast, contains only the active routes chosen by the RE and is used by the Packet Forwarding Engine for actual packet forwarding.