Juniper Junos, Associate (OS 21.2) JN0-106 JNCIA-Junos Exam Questions

Answer : A

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.

Answer : B

To effectively troubleshoot packet forwarding in a Junos environment, an architect must distinguish between the control plane's Routing Information Base (RIB) and the data plane's Forwarding Information Base (FIB). While the command show route displays the RIB (the master routing table maintained by the Routing Engine), it does not necessarily reflect the actual instructions being executed by the hardware. The definitive command for viewing the data plane's active path selection is show route forwarding-table.

Executing this command reveals the contents of the FIB as it has been pushed from the Routing Engine to the Packet Forwarding Engine (PFE). The output provides critical diagnostic data, including the destination prefix, the specific next-hop IP address, the interface through which the packet will egress, and the type of route (such as unicast or broadcast). This is the 'ground truth' for packet movement; if a route exists in the RIB but is missing from the forwarding table, it indicates a failure in the communication between the RE and PFE. Utilizing this command is the primary method for identifying black holes, incorrect next-hop resolution, or issues with hardware-level filter applications that might be impacting transit traffic flow at wire speed.

Answer : C

When a network architect needs to completely sanitize a Junos device for redeployment or decommissioning, the request system zeroize command is the most thorough tool available. Unlike simply loading the factory default configuration, which only resets the configuration database, the zeroize command performs a deep wipe of the system.

Specifically, this command removes all user-created data, including all configuration files (active and rollback), log files, license keys, and any locally stored files in the /var/tmp or /home directories. It essentially restores the device to its 'pristine' out-of-box state. Once the command is initiated, the device reboots and goes through a sanitization process. This is a critical security measure to ensure that sensitive information---such as encrypted passwords, proprietary routing policies, or SNMP community strings---does not leave the organization when hardware is transferred.

In contrast, the request system storage cleanup command is a maintenance utility used to delete temporary files and old log entries to free up disk space, but it does not affect the running configuration or user accounts. Therefore, for a total system reset that includes configuration deletion, zeroize is the mandatory operational command.

Answer : C

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.

Answer : D

Junos OS firewall filters operate on a sequential, 'first-match' logic, but their behavior is significantly influenced by the use of terminating versus non-terminating actions. In this exhibit, a packet with a source address of 10.0.0.0/8 is evaluated against the filter named test.

Evaluation begins with term 1. The packet matches the source-address criteria, triggering the actions defined in the then statement. The first action is log, which sends the packet header information to the firewall task buffer for logging. The second action is next term. This is a critical non-terminating action; it instructs the Packet Forwarding Engine (PFE) to continue the evaluation process using the subsequent term in the filter rather than stopping after the match.

Evaluation then moves to term 2. Because term 2 contains no from match conditions, it acts as a 'catch-all' for any traffic that reaches it. The action in this term is reject. This is a terminating action that discards the packet and sends an ICMP 'destination unreachable' message back to the source. Therefore, the packet is first recorded by the logging process and is subsequently dropped by the rejection mechanism. If next term had not been present in term 1, the packet would have been implicitly accepted (as any matched term without a terminating action like discard, reject, or accept defaults to an implicit accept in that specific term). However, the explicit instruction to move forward ensures the packet hits the reject statement.

Answer : A

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.

Answer : B, C

The Routing Engine (RE) is the 'brains' of a Juniper device, representing the control plane in the Junos dual-plane architecture. One of its primary functions is that it runs the Junos OS. The RE is essentially a high-performance workstation running a specialized version of FreeBSD that hosts the various software daemons responsible for the device's logic, such as the Routing Protocol Process (rpd) and the Device Control Process (dcd).

The second core function is that the RE processes all management traffic. Any packet destined for the router itself---such as an SSH session, an SNMP poll, a NETCONF request, or an NTP update---is handled by the Routing Engine's CPU. The RE also maintains the master routing table, manages configuration commits, and provides the CLI environment for the administrator.

It is critical to distinguish these control-plane duties from the duties of the Packet Forwarding Engine (PFE). The PFE is responsible for the 'heavy lifting,' such as processing transit traffic (data passing through the router) and evaluating firewall filters at wire speed. While the RE defines the policies and routing tables, it hands off a streamlined version of this information to the PFE so the RE isn't bogged down by millions of individual packets. This separation ensures that a busy management session or a complex routing calculation on the RE doesn't impact the device's ability to forward traffic at maximum capacity.