Free JN0-106 Practice Test Questions | Know You're Ready for Junos - Associate (JNCIA-Junos) Junos OS 21.2

Explanation:
In Junos OS, the routing table (RIB) is control-plane based, selects the best routes, and then populates the forwarding table (FIB) in the data plane for fast packet switching. The FIB is a derivative of the RIB, optimized for high-speed lookups.

Correct Option:

B: The routing table selects the best route based on routing information.
The routing table runs routing protocols (OSPF, BGP, static) and applies route preferences to select the single active, best path to each destination. This occurs in the control plane before any forwarding decision.

C: The forwarding table is built from the routing table.
Once the routing table determines the active routes, it exports them to the forwarding table (FIB). The FIB resides in the Packet Forwarding Engine (PFE) for efficient, high-speed packet forwarding.

Incorrect Option:

A: The routing table resides in the data plane for fast lookups.
The routing table resides in the control plane (Routing Engine), not the data plane. The data plane uses the forwarding table for fast lookups, while the routing table focuses on route learning and selection.

D: The routing table is built from the forwarding table.
This is backward. The routing table is built from routing protocols and static configurations; the forwarding table is a subset of the routing table’s active routes, optimized for forwarding. The forwarding table does not build the routing table.

Reference:
Juniper TechLibrary – “Routing Tables and Forwarding Tables Overview” (Junos OS Evolved or Junos OS documentation): The routing table (RIB) selects active routes; the forwarding table (FIB) is derived from the RIB and used by the PFE for packet forwarding.

Explanation:
The forwarding table (FIB) resides in the Packet Forwarding Engine (PFE) and contains only the active, best routes selected by the routing table. Its main function is to enable high‑speed packet forwarding without involving the control plane.

Correct Option:

A: It contains only active routes used to forward packets through the PFE.
The forwarding table is a subset of the routing table, holding only the optimal active routes. The PFE uses this table to make fast forwarding decisions for every packet, without consulting the Routing Engine.

Incorrect Option:

B: It advertises routes to neighboring routers.
Route advertisement is a function of routing protocols (OSPF, BGP, etc.) managed by the routing table and protocol daemons, not the forwarding table. The forwarding table has no role in control‑plane operations like advertisements.

C: It stores all learned routes from routing protocols.
All learned routes are stored in the routing table, not the forwarding table. The forwarding table receives only the active, selected routes from the routing table, not every learned route.

D: It determines the best route based on route preference.
Route selection and preference calculation are performed by the routing table in the control plane. The forwarding table simply uses the already‑selected best routes; it does not perform any route selection logic.

Reference:
Juniper TechLibrary – “Understanding the Forwarding Table”: The forwarding table is installed in the PFE and contains only active routes from the routing table, used for actual packet forwarding.

Explanation:
In Junos OS CLI, typing ? (question mark) at the command-line prompt activates context-sensitive help. It lists all available commands, statements, or options at the current hierarchy level and provides a summary of their functions without executing any command.

Correct Option:

A: It lists the available commands and options.
The ? displays all possible completions for the current input context, including commands, options, and variables. This helps users discover syntax without consulting external documentation.

D: It displays summary information about the commands and options.
Alongside listing available commands and options, the ? output includes a brief one-line description or summary for each item, explaining what the command or option does.

Incorrect Option:

B: It lists tips for the help menu.
Junos ? does not provide meta‑tips about the help system itself. Tips or help usage instructions are typically obtained through commands like help or help reference, not through the ? operator.

C: It displays help about a text string contained in a statement.
Searching for a text string within statements is done using help grep or help reference . The ? only shows completions for the current word or position, not searching across statement contents.

Reference:
Juniper TechLibrary – "CLI Tips and Tools" (Junos OS Operational Mode): Using ? at the CLI prompt displays a list of available commands, options, and a brief description for each, providing context‑sensitive help.

Explanation:
Route preference (also called administrative distance) in Junos OS is a numeric value where lower values indicate higher priority. It is used to select the active route when multiple protocols provide routes to the same destination. Preference is evaluated before the longest prefix match.

Correct Option:

B: Route preference is considered after evaluating the longest prefix match.
This statement requires careful interpretation. In Junos route selection, the longest prefix match is evaluated first to find candidate routes. Among those with the same prefix length, the lowest route preference (highest priority) determines the active route. So preference is considered after prefix length is matched.

C: Lower route preference values indicate higher priority.
This is the core rule of route preference in Junos. Direct routes have preference 0, OSPF internal 10, Static 5, BGP 170 (or 20 for local preference). Lower number = more trusted = higher priority.

Incorrect Option:

A: Higher route preference values indicate higher priority.
This is the opposite of how Junos works. Higher preference values indicate lower priority (less trusted). For example, BGP (170) is less preferred than OSPF (10) unless overridden.

D: Route preference determines the forwarding table size.
The forwarding table size is determined by the number of active routes and hardware capacity, not by route preference values. Preference only affects which route becomes active but does not change the table size.

Reference:
Juniper TechLibrary – "Routing Policy, Route Preference, and Administrative Distance": Junos selects the route with the lowest preference value among routes to the same destination with the same prefix length. Preference is evaluated after longest prefix match.

Explanation:
In Junos OS, user permissions are controlled by class flags. The view permission flag allows read-only access to operational commands, including viewing the routing table and interface statistics, while completely blocking any configuration changes.

Correct Option:

C: view
The view flag provides read-only access to show, monitor, and help commands. Users can display routing tables (show route), interface statistics (show interfaces), and other operational data but cannot enter configuration mode or modify any device settings.

Incorrect Option:

A: configure
The configure flag explicitly grants access to configuration mode and the ability to modify the device's configuration. This would violate the requirement of preventing configuration changes.

B: all
The all flag grants superuser access with full read and write privileges, including configuration changes, file operations, and all operational commands. This exceeds the required permissions.

D: network
The network flag controls access to routing protocols and network-level operations. While it may allow viewing some routing information, it is not designed as a read-only permission and could potentially allow certain network-level changes depending on other flags assigned.

Reference:
Juniper TechLibrary – "Login Classes and Permission Flags": The view permission flag restricts users to operational mode commands without configuration or maintenance capabilities, ideal for monitoring roles.

Explanation:
Junos OS provides an automated configuration archival feature. The transfer-on-commit statement under set system archival configuration triggers an automatic backup of the candidate configuration to a specified archive site every time a commit operation is performed.

Correct Option:

C: set system archival configuration transfer-on-commit
This command enables automatic configuration backup upon every commit. When configured together with archive-sites, the device will transfer the active configuration to the external server each time a configuration change is committed.

Incorrect Option:

A: set system commit synchronize
This command is used only on chassis with dual Routing Engines. It forces both REs to commit simultaneously. It has no relation to configuration backup or archival.

B: set system archival configuration transfer-interval
This sets periodic time‑based backups (e.g., daily or hourly), not backup triggered by commit events. It does not satisfy the "every time you commit" requirement.

D: set system archival configuration archive-sites
This defines the destination (e.g., FTP, SCP, or HTTP URL) for archived configurations. However, without transfer-on-commit, no automatic backup occurs on commit. This command alone is insufficient.

Reference:
Juniper TechLibrary – "Automatically Archiving Configurations": Use transfer-on-commit under system archival configuration to back up configuration to archive-sites after each commit operation.

Explanation:
Junos OS provides predefined login classes with specific permission sets. The operator class allows users to execute operational mode commands (show, ping, monitor, etc.) for monitoring and troubleshooting but explicitly denies access to configuration mode and commit operations.

Correct Option:

D: operator
The operator class is designed for network monitoring staff. It grants permissions for operational commands (show, ping, traceroute, monitor) and troubleshooting tools, but has no configuration or commit privileges. This matches the requirement exactly.

Incorrect Option:

A: maintenance
The maintenance class allows users to perform system maintenance tasks including clearing logs, restarting processes, and some diagnostic commands. However, it may also permit certain configuration-level changes and file manipulations.

B: read-only
There is no predefined read-only class in standard Junos. The closest is the view permission flag or custom class. The default class for read‑only access is typically operator or a custom view class.

C: super-user
The super-user class provides full, unrestricted access including configuration mode, commit operations, file system access, and all operational commands. This would violate the requirement of preventing configuration changes.

Reference:
Juniper TechLibrary – "Understanding Predefined Login Classes": The operator class provides permissions to run operational commands for monitoring and diagnosis but cannot change the configuration.

Explanation:
In Junos devices, the Routing Engine (RE) and Packet Forwarding Engine (PFE) communicate over an internal 100 Mbps or 1 Gbps Ethernet link. This control plane to data plane connection is used for exchanging routing tables, forwarding tables, statistics, and exception packets.

Correct Option:

D: They use an internal Ethernet link.
The RE and PFE are connected via an internal Ethernet interface (often called fxp0, em0, or internal fabric). The RE sends forwarding tables to the PFE and receives exception packets, monitoring data, and statistics over this dedicated internal link.

Incorrect Option:

A: They use shared memory access.
Shared memory is not used between RE and PFE because they are independent processing units. Shared memory typically exists within a single processor or system, not across separate components.

B: They use external breakout cables.
No external cables connect the RE to the PFE. Both components reside inside the chassis and communicate via internal backplane or dedicated Ethernet links, not external physical cables.

C: They use an MPLS connection.
MPLS is a forwarding mechanism for data packets, not a control plane interconnect. The RE and PFE do not use MPLS to communicate with each other; MPLS operates on data traffic passing through the PFE.

Reference:
Juniper TechLibrary – "Routing Engine and Packet Forwarding Engine Communication": The RE and PFE communicate over an internal Ethernet link that carries control traffic, forwarding tables, and monitoring statistics.