In today’s rapidly evolving digital landscape, the demand for highly skilled networking professionals has never been greater. Among the many certifications that validate the technical expertise of network engineers and administrators, the Juniper JN0-105 exam stands out for its clarity, focus, and practical relevance. The JN0-105 exam, also known as the Junos, Associate exam, is the required test for earning the JNCIA-Junos certification, which is a critical stepping stone for anyone looking to build a career in networking using Juniper Networks technologies.

What Is the Juniper JN0-105 Exam?

The JN0-105 exam is designed for networking professionals who are at the beginning to intermediate level in their careers. It evaluates a candidate’s understanding of Juniper Networks’ Junos operating system as well as general networking concepts. The exam consists of multiple-choice questions and is administered in a controlled environment with a 90-minute time limit. It is based on Junos OS version 21.2, which is widely used across various Juniper routers and switches.

Unlike vendor-neutral certifications, the JN0-105 specifically tests knowledge and hands-on capabilities related to the Junos operating system, offering a more focused learning path for professionals who plan to work in Juniper-powered infrastructures.

Once passed, the JN0-105 grants candidates the JNCIA-Junos certification—one of the core credentials in the Juniper certification pathway. This credential acts as a foundational milestone and enables candidates to pursue more advanced certifications such as JNCIS and JNCIP.

But more than just a test, the JN0-105 is a structured experience in learning how modern networks operate, how to manage them efficiently, and how to build a career around real-world, enterprise-level infrastructure.

Why the JNCIA-Junos Certification Matters

In a world powered by connectivity, businesses depend on stable and secure networks. From cloud-based applications to hybrid infrastructures and beyond, the entire digital ecosystem relies on the smooth and continuous operation of networks. As such, companies are constantly on the lookout for professionals who not only understand the theory of networking but also have proven expertise in managing live environments.

Earning the JNCIA-Junos certification demonstrates that you possess this level of proficiency. It’s more than a line on your résumé; it shows that you’ve invested time and effort to understand how Juniper’s technology works. For hiring managers, that kind of commitment is a signal that you’re serious about your craft.

Moreover, Juniper Networks’ equipment is widely used in service provider, enterprise, and data center networks. By mastering Junos OS, you are aligning your career path with some of the most reliable and performance-optimized network platforms available. The JNCIA-Junos opens doors to jobs in network administration, support engineering, infrastructure monitoring, and even cloud operations.

The certification is particularly valuable because it gives you a vendor-specific foundation that also overlaps with industry-standard protocols and architectures. That blend of deep and wide knowledge is what makes the certification so relevant and respected.

Key Areas of the JN0-105 Exam

The exam objectives are divided into several key domains, each reflecting a critical area of understanding in network operations and Junos device management. These domains include:

• Networking Fundamentals
  
• Junos OS Fundamentals
  
• User Interfaces
  
• Configuration Basics
  
• Operational Monitoring and Maintenance
  
• Routing Fundamentals
  
• Routing Policy and Firewall Filters

Exploring Networking Fundamentals

The Networking Fundamentals section of the JN0-105 exam focuses on essential concepts that every network professional must understand. These include the basic structure of networks, common protocols, the OSI model, IPv4 and IPv6 addressing, subnetting, and key technologies such as Ethernet and ARP.

Let’s explore the core components of this domain.

The Structure of Modern Networks

At the heart of any network is the idea of communication—between devices, across distances, and under protocols that ensure reliability and speed. Whether it’s a simple local area network in a small office or a sprawling global enterprise backbone, all networks share some common components:

• Hosts: These are endpoints such as computers, smartphones, or servers that generate and consume data.
  
• Switches: Devices that connect hosts within the same local area network.
  
• Routers: Gateways that connect different networks and manage traffic based on IP addresses.
  
• Firewalls: Security devices that monitor and control traffic entering or leaving the network.
  

Understanding the function of each device and how they work together is fundamental. For instance, a router’s ability to determine the best path for packet delivery is what ensures efficiency, while a firewall’s ruleset protects the network from malicious traffic.

The OSI Model: A Layered Approach

One of the first things you’ll study in networking is the OSI model. This seven-layer framework helps explain how data travels from one device to another across a network. Each layer has a specific function, and understanding how they interact is critical for both troubleshooting and design.

1. Physical: Deals with the actual hardware—cables, switches, network interface cards.
   
2. Data Link: Manages how data is formatted for transmission and how devices on the same local network communicate (includes MAC addressing).
   
3. Network: Handles logical addressing (IP addresses) and determines the path data takes through the network.
   
4. Transport: Manages end-to-end communication, error checking, and flow control (uses protocols like TCP and UDP).
   
5. Session: Establishes and maintains connections between devices.
   
6. Presentation: Ensures that data is readable by translating and encrypting as necessary.
   
7. Application: The interface for the user’s applications to access network services.
   

Although real-world networking stacks don’t always follow the OSI model strictly, it remains a valuable conceptual tool.

IPv4 and IPv6 Addressing

Another key element of networking fundamentals is IP addressing. IPv4, the older version, uses 32-bit addresses and offers around 4.3 billion unique addresses. With the expansion of the internet and the rise of connected devices, IPv4 address exhaustion became a significant problem.

Enter IPv6. This newer protocol uses 128-bit addresses, offering a nearly unlimited pool of unique IP addresses. The JN0-105 exam tests your understanding of how both formats work, how they’re structured, and when each is used.

Understanding IP addresses also involves knowing how to break them down. This includes topics like subnetting, which allows you to divide a network into smaller, manageable segments. You’ll need to know how to calculate subnet masks, identify network and host portions of an IP address, and configure routing tables appropriately.

Ethernet, ARP, and MAC Addresses

Ethernet remains the most common data link layer protocol in networking today. It operates mostly in wired LANs and is known for its reliability, simplicity, and scalability. Understanding Ethernet frames, including how headers and trailers work, is crucial for interpreting packet data.

Equally important is the Address Resolution Protocol (ARP), which maps IP addresses to MAC addresses on local networks. MAC addresses are unique identifiers burned into network interface cards. Without ARP, hosts wouldn’t be able to communicate with each other effectively on the same LAN.

The exam may ask questions that involve understanding ARP tables, interpreting MAC addresses, or troubleshooting Ethernet-based connectivity issues.

Switching and Routing

Even at the fundamental level, knowing the difference between switching and routing is essential. Switching occurs at the data link layer, managing traffic within a single network. Routing happens at the network layer and enables communication between different networks.

A switch uses MAC addresses to forward frames to the correct port. It maintains a MAC address table that it uses to remember which port corresponds to which device.

Routers, on the other hand, use IP addresses and routing tables to determine where to send packets. They often run routing protocols like OSPF or BGP (though these are covered in greater depth in later exams).

The JN0-105 ensures that candidates can distinguish between these roles and understand when and how to apply them in network scenarios.

 Deep Dive into Junos OS Fundamentals – Powering Network Simplicity and Reliability

For candidates preparing for the JN0-105 exam, understanding the Junos OS is a crucial skill. This domain assesses your ability to work with Junos software at a structural and functional level. Knowing how the OS operates, how configurations are handled, and how Junos differs from other networking systems provides you with a strong technical edge.

Let’s unpack the Junos OS in detail, beginning with its architecture and moving through its key interfaces, configuration style, and operational characteristics.

What Makes Junos OS Unique?

Junos OS is designed with modularity and stability in mind. Unlike some monolithic network operating systems, Junos separates its control and forwarding planes. This architectural decision offers a major advantage—if something crashes or misbehaves in the control plane, it does not interfere with the traffic-forwarding operations of the device. That means better uptime, more predictable behavior, and easier troubleshooting.

Another distinguishing feature is Junos OS’s single software train. This means that the same operating system runs across Juniper’s routing, switching, and security platforms. Whether you’re working on an entry-level router or a high-capacity core switch, the commands, interface, and logic remain consistent. This greatly reduces the learning curve as you scale from one platform to another.

Junos OS is built on a FreeBSD-based Unix kernel, offering a solid, secure, and stable foundation. With shell access, administrators can perform advanced operations and scripting when needed, but for most users, interaction happens through the dedicated Junos CLI and configuration modes.

The Junos CLI – Intuitive and Structured

A major part of the JN0-105 exam involves knowing how to interact with Junos devices through the command-line interface (CLI). The CLI is not just a text-based interaction point—it’s a structured and hierarchical system that guides you through device configuration, operational monitoring, and troubleshooting.

There are two main modes within the Junos CLI:

• Operational Mode: Used for monitoring and operational commands. You can view interface statistics, check routing tables, perform pings and traceroutes, and monitor system status.
  
• Configuration Mode: Used for modifying the device configuration. You enter this mode to create or edit configurations, which are committed only when you’re ready.
  

To move between modes, you use the command configure to enter configuration mode and exit to return to operational mode. This separation keeps actions intentional and minimizes accidental disruptions to the running system.

Commands in operational mode include:

• show interfaces
  
• show configuration
  
• ping
  
• traceroute
  
• request system reboot
  

These commands allow administrators to check the health and behavior of the device without changing its state.

Configuration Hierarchy in Junos OS

One of the most elegant features of Junos OS is its hierarchical configuration structure. Instead of flat configuration files, Junos uses a tree-like format where settings are logically grouped. This makes the configuration easier to read, maintain, and scale.

For example, the root of the configuration might include:

• system
  
• interfaces
  
• protocols
  
• routing-options
  
• firewall
  

Each of these branches can contain sub-branches. For instance, under system, you may have host-name, services, and login. This layered design ensures that even large and complex configurations can be navigated with clarity and precision.

When you’re configuring a device, you can navigate to any part of the hierarchy and add or change values. For example:

pgsql

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edit system host-name

set host-name CoreRouter01

The changes aren’t applied until you explicitly commit them. This deliberate two-step model (edit then commit) helps prevent misconfigurations and gives administrators a chance to review pending changes.

Committing and Rolling Back Configurations

Another standout feature of Junos OS is the ability to commit and roll back configurations. This adds a safety net to the administrative process. You don’t have to worry about locking yourself out with a bad change—you can revert easily.

After making changes in configuration mode, the command commit applies the changes to the running configuration. If you decide to backtrack or notice a mistake, you can use rollback commands.

Junos OS stores up to 50 previous versions of the configuration. This historical record allows you to roll back to a known good state using:This capability reduces the risk of operational errors and makes it easy to experiment, learn, and recover. It’s a favorite feature among new engineers learning Junos for the first time.

Interface Naming and Structure

In the Junos OS ecosystem, interfaces are labeled in a consistent, hierarchical format. This might differ from what you’re used to in other networking platforms, so understanding Juniper’s naming convention is essential for both configuration and troubleshooting.

An interface name like ge-0/0/0 can be broken down as follows:

• ge indicates a Gigabit Ethernet interface.
  
• The first digit (0) indicates the Flexible PIC Concentrator (FPC) number.
  
• The second digit (0) indicates the PIC (Physical Interface Card).
  
• The third digit (0) indicates the port number on that PIC.
  

This structure is especially useful on modular platforms where hardware may be distributed across multiple slots and cards. The naming format is consistent across all Juniper platforms, whether you’re configuring an access switch or a core router.

Applying and Viewing Interface Configurations

Within the configuration mode, setting up an interface typically involves specifying the IP address, encapsulation type, and unit number. Junos OS uses logical units for interface configurations. Even if there’s only one IP address or protocol associated with the interface, you’ll configure it under a unit.This command provides a summarized view of all interfaces, including status indicators and protocol families assigned. It’s incredibly useful for a quick system check.

User Accounts and Authentication

Security and access control are baked into the Junos OS from the ground up. One of the first tasks in device setup is configuring administrative access. This involves creating user accounts, defining permissions, and enabling authentication methods.

Junos OS supports multiple user classes, each with specific permission levels. For instance:

• super-user has full control over the system.
  
• operator has monitoring capabilities but cannot change configurations.
  
• read-only provides access to read configuration data only.

Junos OS also supports remote authentication using protocols such as RADIUS and TACACS+, which are common in enterprise deployments where centralized user management is a priority.

System Services and Management Access

Enabling and securing management access is another important area covered in the Junos OS Fundamentals domain. Junos devices support several remote access protocols, including:

• SSH
  
• Telnet
  
• FTP
  
• HTTP/HTTPS
  
• NETCONF
  

These services must be explicitly enabled and configured for access to be possible. SSH, for example, is often preferred for secure command-line access.

In addition to service access, Junos OS provides tools for system logging, event tracing, and alarm management, helping administrators maintain awareness of the device’s health and performance at all times.

Managing Junos Configuration Files

Junos OS supports both local and remote configuration file storage. You can back up your current configuration to a USB drive, an FTP server, or a local storage partition. Managing these configuration files is part of routine network maintenance and disaster recovery

You can also compare changes using the show | compare command, which displays the difference between the candidate and committed configurations. This ability to preview before committing is an essential tool for avoiding unexpected results.

Junos OS as a Platform of Mastery

The Junos OS isn’t just a means to configure a network device. It’s a disciplined, intelligent platform designed to make network management scalable, secure, and efficient. From the structured CLI to hierarchical configurations, commit-based changes, and rollback features, Junos gives network engineers both control and safety.

For those pursuing the JNCIA-Junos certification, learning Junos OS fundamentals is a core requirement. The JN0-105 exam does not expect you to master every edge case or command. Instead, it expects you to demonstrate operational familiarity with navigating, configuring, and monitoring Junos-based systems confidently.

As your knowledge of Junos deepens, so too does your readiness to take on advanced Juniper certifications. But even within this foundational tier, the OS equips you with tools that are directly applicable to real-world networking challenges.

Mastering Junos Interfaces, Configuration Workflows, and Operational Monitoring

These are critical areas of the Juniper JN0-105 exam and core components of the JNCIA-Junos certification framework.

As you work in a production or lab environment, knowing how to configure, verify, and troubleshoot Junos-based devices becomes a day-to-day necessity. Whether through command-line tools or graphical interfaces, the ability to manage configurations with precision and maintain a healthy network state defines the professional value of a Junos-certified engineer.

Interacting with Junos: Multiple User Interfaces

Junos OS provides multiple user interface options for interacting with its networking platforms. While the command-line interface (CLI) is the most widely used among network engineers, Junos also includes web-based and API-driven options that expand usability and allow integration into modern automated environments.

The three primary user interfaces you should understand are:

• The Command-Line Interface (CLI)
  
• J-Web (Graphical User Interface)
  
• NETCONF/XML API for automation
  

Each interface has its strengths and is designed to suit different tasks and user preferences.

The Command-Line Interface (CLI)

The CLI is the most powerful and commonly used interface for working with Junos OS. It offers full access to configuration and operational commands and follows a structured, intuitive hierarchy. This interface is preferred by network engineers for tasks such as interface configuration, routing protocol setup, troubleshooting, and monitoring.

CLI access can be established through:

• Console cable (direct connection)
  
• Secure Shell (SSH) for remote login
  
• Telnet (if enabled, though less secure)
  

Once logged in, you are placed in operational mode, where you can run diagnostics, show commands, or transition to configuration mode.

A key strength of the Junos CLI is its consistency. Whether you’re working on a switch, router, or firewall, the command structure remains identical. This consistency greatly reduces the learning curve and increases efficiency when managing multiple device types.

J-Web: The Graphical Interface

For users more comfortable with graphical interfaces or those performing quick administrative tasks, J-Web offers a user-friendly web-based GUI. It allows you to access configuration options, interface summaries, routing tables, system logs, and performance graphs.

J-Web is particularly helpful for small-scale deployments or environments where full CLI expertise is not yet developed. It can also serve as a visual tool for confirming configurations created through the CLI or automation scripts.

To access J-Web:

1. Connect a laptop or computer to the management port of the device.
   
2. Enter the management IP address into a web browser.
   
3. Log in with your admin credentials.
   

J-Web includes dashboard panels for system health, interface status, and configuration sections that allow changes to be made and committed similarly to the CLI. While it doesn’t provide the same depth or flexibility as the CLI, it is still a valuable tool for many use cases.

NETCONF and XML API

For environments moving toward automation and network orchestration, Junos OS supports NETCONF, a standards-based network management protocol that uses XML for data representation. This allows administrators and automation tools to retrieve configurations, apply changes, and query system information in structured formats.

NETCONF forms the foundation for integrating Junos with automation platforms like Ansible, SaltStack, and Juniper’s own automation framework, Junos PyEZ. While the JN0-105 exam does not require deep programming knowledge, awareness of NETCONF and its role in modern networking is helpful for context and career growth.

Configuration Basics: Best Practices and Execution

Understanding configuration basics in Junos OS is about more than just syntax. It’s about using a deliberate, reliable approach to managing changes. The hierarchical structure, rollback features, and candidate configuration model all contribute to safe and efficient workflows.

Configuration Mode

You enter configuration mode from the CLI using the configure command. Once in this mode, you can make changes to the candidate configuration, which exists as a working copy that does not affect the live system until it is committed.

This model is ideal for building configurations incrementally, reviewing them, and committing them only when ready. It encourages safety and reduces the chance of making unintended changes.

Example:

pgsql

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configure

edit interfaces ge-0/0/1

set unit 0 family inet address 10.1.1.1/24

commit

Navigating Configuration Hierarchy

As discussed in Part 2, Junos configurations are organized into a hierarchy. You can navigate this structure using commands such as edit, top, and up. This allows you to move fluidly through configuration sections without losing context.

For example:

• edit protocols ospf area 0.0.0.0
  
• top returns to the root level of the hierarchy
  
• up moves one level higher
  

The JN0-105 exam will test your ability to understand where configurations belong and how to modify them accurately.

Committing and Reviewing Changes

Once your configuration changes are complete, use the commit command to apply them to the running system. It’s good practice to use the show | compare command beforehand to view pending changes relative to the last committed configuration.

If needed, you can schedule a commit to be confirmed:

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commit confirmed 5

This command commits changes but will automatically roll back after 5 minutes unless you explicitly confirm them with another commit command. It’s useful when making remote changes that could result in connectivity loss.

Viewing and Saving Configurations

You can view the current active configuration using the show configuration command. This will display the entire hierarchical structure or a specific section if you specify a path.

You can also save configurations to external storage or a local file for backup purposes. The save command is used to export a configuration:

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save /var/tmp/myconfig.conf

To restore a saved configuration:

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load override /var/tmp/myconfig.conf

commit

This is useful for template deployments or disaster recovery scenarios.

Operational Monitoring and Maintenance: Ensuring Network Health

A Junos-certified network professional must not only configure but also continuously monitor and maintain the health of the system. This includes checking hardware status, monitoring interfaces, examining routing tables, and handling alarms or system logs.

Monitoring Interfaces

You can monitor interfaces using:

• show interfaces for detailed information
  
• show interfaces terse for a summary view
  
• monitor interface ge-0/0/0 for real-time traffic analysis
  

These commands reveal link status, packet statistics, errors, and bandwidth utilization. This information is critical for identifying bottlenecks or troubleshooting connectivity problems.

Monitoring Routing Tables

The command show route displays the routing table, including routes learned from static entries, OSPF, BGP, and connected interfaces. You can filter the output by destination or protocol to narrow your view.

Example:

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show route protocol ospf

This helps ensure that routing protocols are working correctly and that the device is receiving accurate route advertisements.

System Logs and Alarms

Junos OS maintains a variety of logs, including:

• System messages
  
• Security events
  
• Configuration changes
  

These can be viewed using:

• show log messages
  
• show system alarms
  
• show system uptime
  

The show log messages command is one of the most important tools for identifying configuration problems, failed services, or unexpected reboots. Alarms indicate hardware or software issues that need attention, such as fan failures or high CPU usage

Device Reboots and Software Updates

While Junos OS is known for its stability, occasional reboots or updates are necessary. A controlled reboot is initiated using:

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request system reboot

To update the Junos image, the system must first download the new version, verify it, and apply the update through:

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request system software add <image-name.tgz>

Reboots after a software update ensure the new image is activated. Managing this process safely, especially in production environments, is part of the operational responsibility covered in the JN0-105 curriculum.

Storage and Memory Monitoring

Monitoring system resources such as disk space and memory usage helps prevent performance degradation. Commands such as:

• show system storage
  
• show system processes extensive
  

reveal usage patterns and highlight resource-intensive processes. Keeping these under control ensures long-term device reliability.

Proactive Maintenance Practices

Preventive maintenance is an often-overlooked part of network administration. It includes regularly checking logs, scheduling configuration backups, documenting changes, and testing redundancy mechanisms like link failover or routing convergence times.

A well-maintained Junos environment is resilient, easier to troubleshoot, and less likely to suffer downtime from preventable issues. Even simple practices like checking interface counters or CPU load during quiet hours can uncover trends that lead to smarter decisions.

Becoming Fluent in Junos Device Management

Configuration and operational monitoring are more than just exam topics—they are essential tools of your trade as a network engineer. Junos OS offers a clean, structured, and intelligent interface for interacting with and managing network devices at scale.

From configuring interfaces and routing options to performing system diagnostics and proactive maintenance, the knowledge you gain here is not just academic. It directly impacts your ability to keep networks running smoothly, troubleshoot issues rapidly, and evolve systems without disruption.

The Juniper JN0-105 exam emphasizes this practical competency. By mastering Junos user interfaces, configuration best practices, and operational monitoring commands, you are preparing not just to pass an exam—but to excel in the real world of networking.

Routing Fundamentals, Policies, and Firewall Filters in Junos OS

As we arrive at the final chapter of our deep dive into the Juniper JN0-105 exam and the JNCIA-Junos certification, we now move toward the dynamic and powerful mechanics of routing. A robust understanding of routing—how packets travel between networks, how decisions are made, and how policies shape that flow—is not only essential for passing the JN0-105 exam, but it’s also the backbone of every scalable network infrastructure.

Routing Fundamentals and Routing Policy with Firewall Filters are two of the most advanced sections of the JN0-105 exam. These topics represent the intelligence behind network communication and the tools to control it.

What Is Routing in Junos OS?

Routing is the process of determining the best path for data to travel across interconnected networks. Routers inspect packet headers, evaluate destination addresses, consult routing tables, and forward data accordingly. In Junos OS, this process is highly structured, efficient, and transparent.

Routing in Junos OS is based on protocols, preferences, and table lookups. Devices make forwarding decisions using their routing table, which contains information about network prefixes and the next-hop addresses used to reach them.

There are two main types of routes:

• Static routes: Manually configured paths to specific networks.
  
• Dynamic routes: Learned and maintained through routing protocols.
  

Understanding how these routes are calculated, stored, and applied is essential to mastering network behavior.

Routing Tables and Forwarding Tables

In Junos OS, multiple routing tables exist to handle different types of traffic. The most commonly used is the inet.0 table, which contains IPv4 unicast routes. Each routing instance maintains its own routing table, and protocols populate these tables based on their own decisions and rules.

After a route is selected, it is placed into the forwarding table. The forwarding table is what the device uses to send packets on their way. This separation of responsibility helps Junos OS maintain high reliability and performance.

You can inspect the routing table with:

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show route

And for a more protocol-specific view:

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show route protocol ospf

Static Routing in Junos

Static routes are manually configured and do not change unless an administrator updates or removes them. They are ideal for simple networks or for routes that must always point to a specific next-hop, regardless of network changes.

Configuring a static route in Junos is straightforward:

vbnet

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set routing-options static route 192.168.5.0/24 next-hop 10.1.1.2

This command tells the router that to reach the 192.168.5.0/24 network, it should send packets to the next-hop IP 10.1.1.2.

Static routes are stored in the routing table with the keyword static and have a preference of 5 by default. Preference is Junos’s way of determining which route to use when multiple paths to the same destination exist. Lower values are preferred.

You can view all static routes by filtering the routing table:

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show route protocol static

Dynamic Routing Protocols

Dynamic routing enables routers to automatically discover and maintain routes. This is critical in medium to large networks where changes are frequent and manual updates are impractical.

The JN0-105 exam focuses on foundational knowledge of common dynamic routing protocols. These include:

• RIP (Routing Information Protocol)
  
• OSPF (Open Shortest Path First)
  
• BGP (Border Gateway Protocol)
  

Of these, RIP and OSPF are most relevant at the associate level.

RIP Overview

RIP is a distance-vector protocol that uses hop count as its metric. It is easy to configure but lacks scalability and convergence speed. Junos supports RIP and allows you to assign interfaces to RIP and announce connected routes.

Basic configuration includes:

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set protocols rip group rip-group neighbor ge-0/0/0.0

This enables RIP on the specified interface. RIP updates are sent periodically, and the maximum hop count is 15. While it’s largely used in lab environments today, understanding RIP’s limitations helps build context for more modern protocols.

OSPF Overview

OSPF is a link-state routing protocol designed for scalability and fast convergence. It uses areas to segment large networks and relies on the Dijkstra algorithm to compute the shortest path.

OSPF is commonly used in enterprise networks and is far more powerful than RIP. To enable OSPF on a Junos device:

kotlin

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set protocols ospf area 0.0.0.0 interface ge-0/0/1.0

Here, area 0.0.0.0 is the backbone area. OSPF routers exchange link-state advertisements (LSAs) to build a complete map of the network, then use that information to populate the routing table.

Route Preference and Metric

Juniper devices decide which route to install based on administrative preference and metric. Each routing protocol has a default preference value, which can be adjusted.

For example:

• Direct routes: preference 0
  
• Static routes: preference 5
  
• RIP: preference 100
  
• OSPF internal: preference 10
  
• BGP: preference 170
  

The route with the lowest preference value is preferred. Metrics, on the other hand, are used within the protocol to determine the best path. For instance, OSPF uses cost, while RIP uses hop count.

These values help Junos OS make intelligent decisions when multiple paths are available.

Routing Instances and Virtual Routing

Junos OS supports routing instances, which are like virtual routers within a single device. They allow multiple routing tables to exist independently, each with its own protocols and routes.

Common types of routing instances include:

• Virtual-router
  
• Forwarding
  
• VPN
  

You can create a virtual router instance to separate different routing domains:

kotlin

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set routing-instances vr1 instance-type virtual-router

set routing-instances vr1 interface ge-0/0/2.0

This allows complete separation of routing logic, ideal for service provider or multitenant environments.

Routing Policies in Junos OS

Routing policies control the flow of routing information between protocols and the routing table. They allow you to accept, reject, modify, or set attributes for routes based on specific criteria.

Policies are essential for manipulating route behavior and implementing business logic in the network. For instance, you might prefer OSPF routes over RIP routes even when both exist.

Structure of Routing Policy

Junos policies are created using three main elements:

1. Terms: Conditions applied to route attributes like destination prefix or next-hop.
   
2. Match Conditions: Define which routes the term applies to.
   
3. Actions: What happens if a route matches the term (accept, reject, modify).
   

Example of a basic policy that rejects a specific route:

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set policy-options policy-statement block-net term 1 from route-filter 192.168.10.0/24 exact

set policy-options policy-statement block-net term 1 then reject

This policy checks for a specific prefix and rejects it. Policies can be applied during import (routes coming into the routing table) or export (routes being advertised to neighbors).

Applying Policies

After defining a policy, it must be applied to have any effect. This is done under the appropriate protocol configuration.

Example:

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set protocols ospf export block-net

This applies the policy to OSPF routes that the device advertises. Junos offers significant flexibility in applying policies, allowing granular control over routing decisions.

Firewall Filters in Junos OS

Firewall filters are Junos OS’s version of access control lists. They inspect packet headers and determine whether to permit, deny, or count traffic based on matching criteria.

Unlike routing policies, which affect route entries, firewall filters work directly on traffic entering or leaving interfaces.

Structure of a Firewall Filter

A firewall filter is built using terms, each with match conditions and actions. Common match criteria include:

• Source and destination addresses
  
• Protocol types (TCP, UDP, ICMP)
  
• Port numbers
  
• Packet length
  

Common actions include:

• Accept
  
• Discard
  
• Count (log statistics)
  

Here’s an example of a basic filter:

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set firewall family inet filter block-telnet term deny-telnet from protocol tcp

set firewall family inet filter block-telnet term deny-telnet from destination-port telnet

set firewall family inet filter block-telnet term deny-telnet then discard

This filter discards Telnet traffic, enhancing security by preventing insecure access.

Applying a Firewall Filter

Filters must be applied to specific interfaces and directions (input, output, or both). To apply the above filter:

pgsql

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set interfaces ge-0/0/1 unit 0 family inet filter input block-telnet

This ensures that Telnet traffic arriving on this interface will be discarded.

Firewall filters are an essential tool in traffic management, DoS protection, and securing network perimeters.

Monitoring Firewall Filter Statistics

To verify the operation of a firewall filter, use:

sql

CopyEdit

show firewall

This command shows packet matches per term, helping administrators determine whether traffic is being filtered as intended. It’s useful for validating rules and measuring traffic types in real-time.

Routing Fundamentals and Policy Filtering bring the JN0-105 series full circle. They represent the network’s ability to make decisions, adapt to changes, and enforce security and design policies through structured rules.

Mastering these areas positions you as more than just a network technician—you become a strategist who understands how to shape traffic flows, secure paths, and optimize performance across distributed systems.

The JNCIA-Junos certification verifies this understanding. It shows that you can interpret and influence the movement of data with purpose and control. As you advance toward this credential, know that the knowledge you’ve built will continue to expand through more specialized certifications like JNCIS-ENT and JNCIP-SP.

But more importantly, this foundation will shape your ability to contribute to business continuity, user experience, and operational efficiency in any organization that depends on a stable and intelligent network.

Conclusion

Earning the JNCIA-Junos certification through the JN0-105 exam is more than just an academic milestone—it’s a powerful validation of your ability to understand, configure, and manage modern network infrastructure using Juniper’s robust platform. From networking fundamentals and Junos OS architecture to configuration best practices, routing intelligence, and policy enforcement, this certification equips you with the tools to excel in real-world environments. Whether you’re starting your career or expanding your skill set, the knowledge gained through this journey empowers you to build resilient, efficient, and secure networks. It marks the beginning of a professional path defined by clarity, capability, and confidence.