JN0-106 Junos - Associate (JNCIA-Junos) Junos OS 21.2 Questions and Answers

The clear log command is a vital operational utility within the Junos OS used to manage the size and relevance of log files without interrupting the system ' s logging processes. When a Senior Architect executes the clear log ospf-trace command, the Junos kernel truncates the specified file, effectively removing all existing text and resetting the file size to zero bytes. Crucially, the file itself is not deleted from the /var/log directory, nor is the underlying traceoptions configuration modified in any way.

Because tracing is often used for real-time debugging of protocol behaviors like OSPF, trace files can rapidly grow to several megabytes, making it difficult to find specific events. By clearing the log, the administrator ensures that any subsequent OSPF events—such as adjacency changes, LSA flooding, or SPF calculations—are recorded at the very beginning of the file, free from historical clutter. The OSPF process (rpd) continues to write to the file immediately after the truncation occurs. This operational behavior distinguishes the clear command from the file delete command, which would remove the file entirely, or the set protocols ospf traceoptions configuration command, which defines which specific events the device should record. Utilizing clear log is a standard best practice during intensive troubleshooting sessions to maintain a clean and chronologically relevant diagnostic environment.

In the Junos OS environment, precise IP addressing and subnet calculation are foundational for interface configuration. A /29 subnet mask (255.255.255.248) provides a total of 8 IP addresses. To identify the usable host range, we must first identify the network and broadcast addresses. For the subnet 172.16.40.128/29, the network address is 172.16.40.128. The broadcast address is the last address in the block, which is 172.16.40.135.

Following standard IPv4 conventions, the first and last addresses in a block are reserved for the network identity and the directed broadcast, respectively. This leaves 6 usable host addresses: 172.16.40.129 through 172.16.40.134. Counting sequentially from the beginning of the usable range: the first usable host is .129, the second is .130, the third is .131, and the fourth usable host is 172.16.40.132. Assigning this specific address ensures that the Packet Forwarding Engine can correctly route traffic within the intended segment. As a Senior Architect, it is critical to verify these boundaries when using the set interfaces unit 0 family inet address command to prevent address overlapping or the accidental assignment of reserved network/broadcast IDs.

The Routing Engine (RE) in a Junos device serves as the centralized intelligence and management hub, primarily responsible for the control and management planes of the system. In this capacity, the RE is tasked with processing routing updates, such as OSPF Link State Advertisements (LSAs) or BGP Update messages. These updates are vital for the RE to maintain the Routing Information Base (RIB), calculate the shortest paths, and subsequently populate the Forwarding Information Base (FIB) which is then pushed to the Packet Forwarding Engine (PFE).

Furthermore, the Routing Engine handles all local management traffic. This category encompasses administrative access through the Command Line Interface (CLI) via SSH or Telnet, SNMP queries from network management systems, and system logging processes. Because the RE runs the Junos OS kernel, it must directly interpret and respond to these management-level requests to ensure the device remains configurable and observable. Conversely, transit traffic—the data passing through the device from one ingress port to an egress port—is offloaded to the PFE to be handled at wire speed. While the PFE manages the heavy lifting of data forwarding and Class of Service (CoS) application, the RE remains focused on high-level protocol maintenance and system administration, ensuring that control plane stability is maintained even under heavy traffic loads. Reference: Junos OS Fundamentals, Control Plane Functions, Routing Engine Traffic.

==========

In the Junos OS architecture and general networking standards, Layer 2 switches are designed to increase network efficiency by segmenting collision domains. Unlike legacy hubs, a switch creates a separate collision domain for each of its physical ports. This micro-segmentation allows for full-duplex communication, effectively eliminating the possibility of collisions on individual links. However, while switches segment collision domains, they maintain a single broadcast domain by default.

When a switch receives a broadcast frame, such as an ARP request, it must ensure the frame reaches all possible destinations within the local segment. Consequently, the switch floods the broadcast traffic out of all ports except the one on which it was received. This flooding behavior, while necessary for protocol discovery, makes Layer 2 networks susceptible to traffic loops. If redundant physical paths exist between switches without a loop-prevention mechanism like the Spanning Tree Protocol (STP), broadcast frames can circulate endlessly, leading to a broadcast storm that consumes all available bandwidth and processor resources on the Routing Engine. Furthermore, switches are highly active learners; they populate their Media Access Control (MAC) tables by observing the source addresses of incoming frames to ensure that subsequent unicast traffic is precisely forwarded rather than flooded. Therefore, understanding the management of broadcast domains and the risks of loops is a core competency for any Junos Associate.

The exhibit illustrates the configuration of a firewall filter named mgmt_fill and its subsequent application to an interface. The first true statement is that the filter is applied to a physical interface . The configuration shows the filter attached to me0 , which in Junos nomenclature represents the Management Ethernet port—a dedicated physical port for out-of-band management traffic. This is separate from logical or virtual interfaces, as me0 provides the physical link for administrative access.

The second true statement is that the filter evaluates SSH packets egressing from the management interface . In the provided snippet, term t1 specifically matches the destination-port ssh , and the filter is applied to the interface unit. When a filter is applied to an interface, it can monitor traffic entering or leaving the device. Furthermore, the filter utilizes a count action (count c1), which is a non-terminating action used to provide telemetry on specific traffic types passing through that physical port. There is no mention of a syslog or log action in the configuration, meaning that while packets are counted, they are not being written to the system log files. This configuration is a standard method for hardening the management plane and tracking administrative session activity on the Routing Engine. Reference: Routing Policy and Firewall Filters, Firewall Filter Actions, Management Interfaces.

==========

The Junos Command Line Interface (CLI) is designed with an intuitive, context-sensitive help system that assists users in navigating the command hierarchy. When an administrator enters a question mark (?) at the prompt, the CLI provides immediate feedback based on the current location within the hierarchy. First, it lists all available commands and options that are valid at that specific point. This allows the user to see the breadth of possible next steps without needing to refer to external documentation.

Second, the ? character triggers the display of summary information for each of those commands and options. This brief descriptive text provides a clinical overview of what each command achieves, helping the user select the appropriate tool for their task. This " help-on-demand " feature is functional in both operational and configuration modes. It is important to distinguish this from the help command; while help (such as help topic or help reference) provides more exhaustive documentation and usage examples, the ? prompt is primarily a quick-reference tool for command completion and syntax discovery. This mechanism ensures that even complex configurations can be built accurately by exploring the available options and their summarized purposes directly within the terminal environment. Reference: User Interfaces, CLI Help Facilities, Command Discovery.

==========

The Junos OS operates using a candidate configuration model, where changes are made in a temporary buffer before being applied to the running system. To ensure accuracy and prevent unintended network disruptions, it is a critical best practice to review the differences between the candidate buffer and the currently active configuration. The command show | compare executed from the [edit] hierarchy level is the primary tool for this task.

When this command is issued, the CLI performs a line-by-line comparison. The output uses a " diff " format: lines prefixed with a plus sign (+) represent new statements added to the candidate configuration, while lines prefixed with a minus sign (-) indicate statements that have been removed. This visualization allows the architect to verify exactly what will change upon execution of the commit command. Unlike operational mode commands that only show the final state, show | compare highlights the delta, making it indispensable for auditing complex policy changes or interface reconfigurations. This step acts as a final manual validation gate, reducing the risk of syntax errors or logical misconfigurations entering the production environment. Using this command is a fundamental skill for passing the JNCIA-Junos exam and for professional day-to-day operations.

The exhibit illustrates the use of the apply-path feature within a prefix list configuration. In Junos OS, apply-path is a dynamic configuration utility that allows a prefix list to be automatically populated with values derived from other parts of the configuration—in this case, all IPv4 addresses assigned to interfaces with the xe-* prefix. This is particularly useful for building automated firewall filters or routing policies that stay updated as interfaces are added or modified.

When viewing the standard configuration using show configuration, the CLI only displays the literal apply-path statement. To verify the actual list of IP addresses that the system has inherited and populated into the DIRECT-IP list, the administrator must use the | display inheritance pipe filter. This command instructs the Junos OS parser to expand all inherited values and dynamic paths, showing the effective configuration as it is seen by the Packet Forwarding Engine. Based on the exhibit, this would reveal the subnets associated with xe-0/1/0, xe-0/1/1, and xe-0/1/2, while ignoring ge-0/0/0 because it does not match the xe-* wildcard. This verification is a critical troubleshooting step before applying such a prefix list to a transit firewall filter like Export-Direct.

Root password recovery in Junos OS is a critical administrative procedure designed to regain access to the management plane when the root credentials are lost. This process is fundamentally tied to the physical security of the device, as it requires two specific conditions that cannot be met over a standard network connection. First, the administrator must have direct console access to the device. Because the recovery process involves interrupting the Junos boot sequence before the operating system kernel and its associated security services (such as SSH) are fully initialized, it must be performed through the serial management port.

Second, a system reboot is mandatory. The recovery mechanism is triggered by interrupting the boot loader (typically the " Loader " or " U-Boot " stage) by pressing the Space bar or a specific key sequence during power-on. This allows the administrator to enter " single-user mode " or boot into a " recovery " state. Once in this restricted environment, the device mounts its internal storage in a way that permits the modification of the root authentication parameters without requiring the existing password. This workflow intentionally bypasses the standard authentication database to restore management control while preserving the rest of the device configuration. This procedure underscores the importance of securing physical access to the chassis, as anyone with console access and the ability to cycle power can eventually gain root-level authority over the system.

IPv6 link-local addresses, which are identified by the fe80::/10 prefix, are a mandatory component of the IPv6 architecture. The primary purpose of a link-local address is to enable immediate communication between devices located on the same local network segment (the same " link " ) without the requirement for a global unicast address or an external routing infrastructure. These addresses are non-routable, meaning they are never forwarded by a router to another network segment.

Every IPv6-enabled interface on a Junos device automatically generates a link-local address, typically derived from the interface ' s MAC address using the EUI-64 format or a stable privacy algorithm. These addresses are essential for several core functions: they are used by the Neighbor Discovery Protocol (NDP) to resolve MAC addresses, they serve as the source address for routing protocol adjacency formation (such as OSPFv3 or RIPng), and they are frequently used as the next-hop address in IPv6 routing tables. While they do not provide Layer 2 connectivity themselves (which is the role of the MAC address), they provide the necessary Layer 3 link-level presence required for nodes to discover one another and communicate across the physical medium before any global addressing is configured.

The exhibit illustrates a Junos configuration rendered as a sequence of flat, executable lines that each begin with the set keyword. This is commonly referred to as the " set format. " By default, Junos OS displays the configuration in a hierarchical, brace-delimited format (often called " staza " or " curly brace " format). While the default format is excellent for visualizing the structural relationship between different configuration objects, the set format is often preferred for documentation, scripting, or copying specific configuration snippets between different devices.

To produce the output seen in the exhibit, an administrator must append the pipe filter | display set to the show configuration command. This filter instructs the CLI parser to flatten the hierarchical structure and prepend the necessary context to every individual leaf statement. These lines are highly functional because they can be pasted directly into the CLI of another device while in configuration mode to recreate the exact settings. In contrast, display inheritance is used to reveal hidden settings applied via configuration groups, and display detail provides additional technical metadata about the objects. Neither of those options would transform the output into the discrete set command lines shown in the exhibit. Understanding how to toggle between these display formats is a fundamental skill for any architect managing Junos infrastructures through the Command Line Interface.

Analyzing the provided exhibit of the inet.0 routing table reveals that the destination network 10.0.0.0/24 currently has two viable paths: a static route with a preference of 5 and an OSPF route with a preference of 10. Both routes are configured to use 10.12.0.1 via the ge-0/0/1.0 interface as their primary next hop. The static route is currently marked with an asterisk (*), indicating it is the active path chosen by the Routing Engine due to its lower preference value.

In the event that the ge-0/0/1 interface transitions to a " down " state, the physical layer failure triggers the immediate invalidation of all routes associated with that link. Consequently, both the specific static and OSPF routes for the 10.0.0.0/24 prefix are purged from the active forwarding table. The Routing Engine must then perform a re-evaluation of the routing table to identify the next best match for any traffic destined for that range. Since there are no other more specific or equally specific routes available for the 10.0.0.0/24 network, the router falls back to the default route (0.0.0.0/0) . As shown in the exhibit, the default route points to the next-hop address 10.23.0.3 via interface ge-0/0/2.0 . Therefore, if the primary interface fails, traffic will be redirected through this secondary gateway.

The Junos Command Line Interface (CLI) is structured into two primary hierarchical modes: operational mode and configuration mode. Operational mode is the default entry point when a user logs into the device, signified by the > prompt. This mode is designed for monitoring the health of the device, troubleshooting network issues, and executing system-level maintenance tasks. Specifically, an administrator uses operational mode to verify the version of Junos OS currently active on the device using the show version command.

Additionally, system-level administrative actions that do not involve modifying the device ' s persistent configuration logic are performed within the operational mode. This includes requesting the system to reboot or halt via the request system hierarchy of commands. In contrast, tasks that involve manipulating the configuration database—such as committing proposed changes or rolling back to a previous configuration version—require the user to transition into configuration mode, indicated by the # prompt. Understanding this distinction is vital for efficient navigation of the Junos CLI; while operational mode provides the tools to observe the system and manage its physical state, configuration mode is reserved for altering the logical parameters and routing policies that govern device behavior. Reference: User Interfaces, CLI Modes, Operational Mode Commands.

==========

Junos OS firewall filters operate on a first-match basis, evaluating terms sequentially from top to bottom. In this scenario, a UDP DNS packet (destination port 53) is sent from 192.168.1.1 to 8.8.8.8. Evaluation begins with term 1 , which matches the correct source and destination IP addresses but specifies protocol tcp . Because the actual traffic uses UDP, term 1 is not a match. Evaluation then moves to term 2 . While term 2 correctly identifies protocol udp and port domain (port 53), it requires the source-address to reside within the 192.168.2.0/24 subnet. Since the source is 192.168.1.1 , term 2 also fails to match.

When a packet fails to match any explicitly defined terms in a Junos firewall filter, it is subject to the implicit deny action. This default " last term " is a hardcoded safety mechanism that automatically discards all traffic that has not been explicitly permitted. Consequently, because neither term provides a match for the specific combination of source IP, protocol, and destination port, the DNS query is silently dropped by the Packet Forwarding Engine. This behavior ensures that Junos devices maintain a " deny-by-default " security posture, requiring administrators to define precise permit statements for all required transit or management traffic. Reference: Routing Policy and Firewall Filters, Firewall Filter Evaluation, Implicit Discard.

==========

Securing the management plane is a core requirement for any Junos OS deployment. Secure Shell (SSH) is the industry-standard protocol used to provide encrypted, authenticated remote access to the Junos Command Line Interface (CLI). Unlike Telnet, which transmits both administrative credentials and command data in cleartext, SSH utilizes public-key cryptography to establish a secure tunnel, protecting the session from eavesdropping, man-in-the-middle attacks, and unauthorized interception.

In Junos OS, SSH is typically enabled within the [edit system services] hierarchy. Once active, it allows administrators to perform operational and configuration tasks with the assurance that their management traffic remains confidential. Beyond simple terminal access, SSH serves as the transport mechanism for other secure management functions, such as the NETCONF XML management protocol and Secure Copy (SCP) for file transfers. For high-security environments, Junos supports advanced SSH features including key-based authentication, strong cipher suites, and multi-factor authentication integration. Disabling insecure protocols like Telnet and FTP in favor of SSH and SFTP/SCP is a foundational best practice for hardening the Routing Engine against external threats.

In Junos OS, route preference (often referred to as administrative distance in other operating systems) is the mechanism used to select the " best " path when the routing table receives multiple advertisements for the exact same destination prefix from different routing sources. The core rule for preference is that lower values indicate a more preferred or " higher priority " route . For example, a direct route has a default preference of 0, a static route is 5, and OSPF internal routes are 10. If a router learns about the same network from both OSPF and a static entry, it will select the static route because 5 is lower than 10.

Crucially, route preference is only evaluated after the longest prefix match (LPM) has been determined. LPM is the absolute first step in packet forwarding; the router will always choose the most specific route available (e.g., a /28 over a /24). Only if there are multiple entries for that same /28 does the router look at preference to break the tie. It is a common misconception that a lower preference value can override a more specific prefix; in reality, a /32 route with a preference of 170 (BGP) will always win over a /24 route with a preference of 5 (Static). Understanding this hierarchy is vital for network architects when designing redundant paths and predictable failover behaviors.

In large-scale Junos deployments, efficiency and flexibility are achieved through the use of configuration groups . When faced with a requirement like moving all 1-gigabit interfaces to jumbo frames—with the caveat that the exact MTU value (e.g., 4000 bytes) might change after testing—manually editing every interface is both tedious and prone to error.

By creating a configuration group under the [edit groups] hierarchy, you can define the MTU parameter once. You then use the apply-groups statement at the [edit interfaces] level or on specific interface ranges. This creates a " template " effect. The flexibility comes into play during the testing phase: if results indicate that a 9000-byte MTU is preferable to 4000 bytes, the architect only needs to modify the value in the group definition. The Junos OS inheritance mechanism automatically propagates this update to every interface associated with that group.

Applying MTU at the family level (Option B) is less efficient and doesn ' t handle the physical Layer 2 MTU requirements of jumbo frames as effectively. Path-MTU discovery (Option D) is a protocol-level function for avoiding fragmentation but does not configure the local interface ' s hardware capability to accept larger frames. Thus, groups represent the best practice for centralized, scalable management.

The primary and most critical rule used by Junos OS (and routers in general) to determine the next hop for a specific packet is the Longest Prefix Match (LPM) algorithm. When the Routing Engine or Packet Forwarding Engine looks up a destination IP address in the routing or forwarding table, it may find several entries that technically encompass that address. The LPM rule dictates that the router must select the most specific route available—that is, the entry with the highest number of matching bits in its subnet mask (the longest prefix).

For example, if a router has a route for 10.1.1.0/24 and another for 10.1.1.0/28, a packet destined for 10.1.1.1 will always be forwarded according to the /28 route, as it is more specific. Longest prefix match takes precedence over all other selection criteria, including route preference (administrative distance) and metrics. Preference and metrics are only evaluated when the router has multiple entries for the exact same prefix length (e.g., two different paths to 10.1.1.0/24). This logic ensures that traffic is guided along the most precise path defined in the network topology. Routers never choose paths at random or based on the most recent update as their primary selection mechanism, as doing so would result in non-deterministic and inefficient routing behavior. Reference: Routing Fundamentals, Routing Table and Forwarding Table Selection.

==========

In Junos OS, the configuration database is designed with a robust versioning system that facilitates rapid recovery from administrative errors. Every time a commit command is successfully executed, the system automatically archives the previous active configuration and assigns it a " rollback " index. The rollback command is used within configuration mode to revert the candidate configuration to a previously saved state.

The indices are zero-based, where rollback 0 represents the currently active configuration that was just committed. To restore the configuration that was functional immediately before the most recent, problematic commit, the administrator must use rollback 1. This command replaces the current candidate configuration with the contents of the last known working state. It is important to note that executing rollback 1 only populates the candidate buffer; to make this previous configuration active and restore connectivity, the administrator must subsequently issue the commit command. This mechanism is a cornerstone of Junos OS ' s commitment to high availability and operational resilience, allowing for nearly instantaneous restoration of services without the need for manual line-by-line configuration edits during a network outage.

Junos OS routing policies are evaluated using a sequential, " first-match " logic. When a route is compared against a policy, the system evaluates the terms in the order they are defined. Once a route meets all the match criteria (the from statement) in a term, the router executes the associated action (the then statement).

If the action is a terminating action —such as accept or reject—the evaluation of that specific route for that specific policy ends immediately. In this scenario, since the route matched the first term and the action was accept, the route is successfully processed and the policy evaluation is complete. The system will not proceed to evaluate the second or third terms. This behavior is critical for network architects to understand when ordering terms; more specific " exceptions " must be placed at the top of the policy, while broader " catch-all " terms must be placed at the bottom. If the administrator wanted the evaluation to continue to the next term despite a match, they would need to explicitly include the next term action, which is a non-terminating action. Without it, a match on an accept action signifies the final decision for that route within that policy context.

Junos OS utilizes a Role-Based Access Control (RBAC) model through the use of login classes, which define the specific permissions and restrictions for different user levels. To simplify administration, Juniper provides several predefined classes that cater to common organizational roles. The requirement here is for an account that can perform basic network diagnostics (operational mode) but lacks the authority to modify the system state (configuration mode).

The operator class is specifically engineered for this purpose. It grants permissions such as clear, network, reset, trace, and view. These permissions allow the user to execute monitoring commands like show, use diagnostic tools like ping and traceroute, and clear statistics. Crucially, the operator class does not include the configure or commit permissions, preventing the user from entering configuration mode or making any permanent changes to the device.

Comparing this to other options: the read-only class is more restrictive, generally allowing only the viewing of configuration and some state data, but often restricting active diagnostic tools like ping. The super-user class provides unrestricted access, while maintenance is not a standard predefined class for general operational roles. Assigning the operator class ensures that junior staff or automated monitoring systems have the visibility they need to troubleshoot connectivity without risking the integrity of the device configuration.

The Routing Engine (RE) functions as the centralized processor and administrative core of any Junos OS-based platform. Its primary responsibility involves the execution and maintenance of the control plane, which includes running all active routing protocols such as OSPF, BGP, and IS-IS. Through these protocols, the RE exchanges topology information with neighboring routers, builds the Routing Information Base (RIB), and calculates the optimal paths for traffic. Once these paths are determined, the RE distributes the resulting Forwarding Information Base (FIB) to the Packet Forwarding Engine (PFE) for hardware-level execution.

In addition to its protocol duties, the Routing Engine manages the device configuration and the overall system environment. This includes providing the user interface (CLI or J-Web), managing the candidate and active configuration databases, and handling the commit process. While the PFE is specifically designed to forward transit traffic and evaluate that traffic against firewall filters at line rate, the RE focuses on the higher-level logic and management tasks. This architectural separation ensures that management functions—such as a complex configuration commit or a protocol re-convergence event—do not degrade the performance of the data plane, allowing the device to continue forwarding user traffic without interruption. Reference: Junos OS Fundamentals, Routing Engine Functions, Management and Control Planes.

==========

In the Junos OS architecture, the configuration environment is strictly bifurcated to ensure system stability and integrity. The candidate configuration is a specific database that serves as a staging area, storing all proposed changes made by an administrator during a configuration session before they are finalized. Unlike other network operating systems where changes might take effect immediately upon entry, Junos utilizes this " sandbox " approach where modifications reside only in the candidate database until a commit command is explicitly executed.

Once the commit process is initiated, the Junos OS kernel validates the candidate configuration for syntax errors and logical consistency. If the validation check is successful, the candidate configuration is promoted and becomes the active configuration—the operational state currently running on the device hardware. This separation allows multiple administrators to work on complex changes without impacting live transit traffic or device stability prematurely. If a mistake is identified during the editing process, the candidate configuration can be easily discarded or compared against the active state to identify discrepancies. This workflow is a cornerstone of Junos OS, facilitating a " check-before-apply " methodology that is essential for carrier-grade reliability and minimizing the risk of accidental outages during critical maintenance windows. Reference: Junos OS Fundamentals, Configuration Databases, Candidate vs. Active.

==========

In Junos OS, the Routing Information Base (RIB) selection process follows a strict hierarchy where the Longest Prefix Match (LPM) is the absolute primary tie-breaker. When a packet is destined for 10.50.10.55 , the Routing Engine searches the inet.0 table for all matching entries. In this exhibit, four routes match: the default route ( 0.0.0.0/0 ), a general static route ( 10.0.0.0/8 ), an OSPF route ( 10.50.0.0/16 ), and a BGP route ( 10.50.10.0/24 ).

The LPM rule dictates that the router must select the most specific route available, which is defined as the entry with the highest number of matching bits in the subnet mask. The 10.50.10.0/24 route matches 24 bits of the destination address, making it more specific than the 16-bit, 8-bit, or 0-bit alternatives. It is critical to understand that route preference (e.g., Static at 5, OSPF at 10, or BGP at 170) is only evaluated if there are multiple paths to the exact same prefix and length. Because these prefixes vary in length, the length takes precedence over the protocol preference. Therefore, the BGP-learned route via 192.168.1.20 is selected as the active path, ensuring traffic follows the most granular routing information provided to the device. Reference: Routing Fundamentals, Routing Table Selection, Longest Prefix Match.

==========

In networking architecture, point-to-point (P2P) links are used to interconnect two Layer 3 devices, such as a pair of Juniper MX series routers. Because these links only involve two endpoints, using a large subnet mask would result in wasted IP addresses. The standard historical prefix for a P2P link that provides exactly two usable host addresses is /30.

A /30 subnet mask (255.255.255.252) reserves 2 bits for the host portion. Applying the usable host formula ($2^2 - 2$) results in 2 usable addresses. In the case of 192.168.1.0/30:

192.168.1.0 is the Network Address.

192.168.1.1 is the first usable host address (assigned to Router A).

192.168.1.2 is the second usable host address (assigned to Router B).

192.168.1.3 is the Broadcast Address.

While modern Junos OS versions also support /31 masks for point-to-point links (which eliminate the network and broadcast address overhead to provide two hosts), the /30 mask remains the classic and most widely recognized answer for providing " only two usable host addresses " within the standard subnetting framework. Using /30 ensures compatibility across all legacy and multi-vendor environments while precisely meeting the connectivity requirements of a dedicated link between two interfaces.