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

Explanation:
The Packet Forwarding Engine (PFE) handles all data plane operations on a Junos device. It performs high-speed packet forwarding, applies firewall filters, and handles CoS processing. The PFE receives forwarding tables from the Routing Engine (RE) but does not maintain routing tables or select routes.

Correct Option:

B: It implements firewall filters.
Firewall filters configured on interfaces are pushed from the RE to the PFE. The PFE evaluates each packet against these filters at wire speed, performing actions such as accept, discard, reject, or counting without involving the RE.

D: It forwards transit traffic.
The PFE's primary function is to forward transit packets from an ingress interface to the appropriate egress interface based on the forwarding table. It performs this at high speed using specialized hardware (ASICs) or optimized software.

Incorrect Option:

A: It maintains the routing table.
The routing table (RIB) is maintained exclusively by the Routing Engine (RE). The RE runs routing protocols, learns routes, and selects active routes. The PFE only receives a subset of active routes as the forwarding table (FIB).

C: It selects active routes.
Active route selection is performed by the RE based on route preference and other tie-breakers. The PFE does not participate in routing decisions; it simply uses the forwarding table provided by the RE.

Reference:
Juniper TechLibrary – "Packet Forwarding Engine (PFE)": The PFE performs firewall filtering, CoS handling, and high-speed transit packet forwarding. Route selection and RIB maintenance are control plane functions of the Routing Engine.

Explanation:
After a problematic commit, the previous working configuration is stored as rollback 1. The rollback 1 command replaces the current candidate configuration with rollback 1. You must then issue commit to activate it. This is the quickest way to revert without manual editing.

Correct Option:

C: rollback 1
rollback 1 loads the configuration prior to the most recent commit into the candidate configuration. After this, a commit will activate the reverted configuration, restoring connectivity. No manual editing is required.

Incorrect Option:

A: rollback 0
rollback 0 loads the currently active (committed) configuration into the candidate configuration. Since the problematic configuration is already active, this does nothing to revert it; it only discards any uncommitted changes.

B: show | compare rollback
This command displays differences between the candidate configuration and a specified rollback file. It is a comparison tool, not a method for loading or reverting configurations.

D: load override
load override replaces the entire candidate configuration with the contents of a specified file. While it can be used to revert, it requires you to specify a file path (e.g., /config/rollback/1). rollback 1 is the simpler, preferred method.

Reference:
Juniper TechLibrary – "Reverting to a Previous Configuration": To restore the most recent working configuration after a bad commit, use rollback 1 in configuration mode followed by commit.

Explanation:
Address Resolution Protocol (ARP) operates at the boundary between Layer 2 (data link) and Layer 3 (network) in the TCP/IP stack. Its primary function is to resolve an IPv4 address into its corresponding MAC address, enabling communication on a local Ethernet network segment.

Correct Option:

C: to determine the MAC address of a given IP address
When a device knows the IPv4 address of a destination on the same network but does not know its MAC address, it broadcasts an ARP request. The target device responds with its MAC address, allowing Layer 2 frame encapsulation.

Incorrect Option:

A: to determine the MPLS label of a given IP address
MPLS label determination is performed by label distribution protocols (LDP, RSVP, BGP) or static configuration, not ARP. ARP is unrelated to MPLS.

B: to determine the IP address of a given URL
Resolving a URL to an IP address is the function of DNS (Domain Name System), not ARP. DNS is a higher-layer application that translates domain names to IP addresses.

D: to determine the IP address of a given MAC address
The reverse process (determining IP from MAC) is performed by Reverse ARP (RARP) or DHCP, not standard ARP. Standard ARP resolves MAC addresses from known IP addresses.

Reference:
Juniper TechLibrary – "Address Resolution Protocol (ARP)": ARP is used to map an IPv4 address to a media access control (MAC) address, enabling communication on a local area network.

Explanation:
The Junos CLI help command provides context-sensitive assistance directly within the CLI. When used with system log messages (e.g., help syslog ), it displays a detailed explanation of the error or event, including possible causes and recommended actions.

Correct Option:

D: an explanation for specific system log error messages
The help syslog command (or help log in some versions) provides descriptions for specific system log message IDs (e.g., help syslog UI_COMMIT_FAILED). This is invaluable for troubleshooting and understanding Junos error messages.

Incorrect Option:

A: a URL for accessing the technical documentation
The help command does not output URLs. It provides local textual help. Technical documentation URLs must be obtained separately from Juniper's support website.

B: message of the day
The Message of the Day (MOTD) is configured under system login message and appears upon login, not via the help command.

C: hyperlinks for remediation actions
Junos CLI help output is plain text, not hyperlinked. While it may suggest remediation actions, it does not provide clickable hyperlinks.

Reference:
Juniper TechLibrary – "Using the Help System": The help syslog command displays a description and recommended action for each system log message ID, aiding in operational troubleshooting.

Explanation:
Junos OS uses standard interface naming conventions where the prefix indicates the interface speed. xe stands for 10-Gigabit Ethernet. This naming applies across most Juniper platforms, including MX, EX, QFX, and SRX series.

Correct Option:

B: xe
The xe prefix denotes a 10-Gigabit Ethernet interface (e.g., xe-0/0/0). It follows the format type-pic/port/port-number. 10-Gigabit interfaces are common for uplinks and high-bandwidth connections.

Incorrect Option:

A: ge
ge stands for Gigabit Ethernet (1 Gbps), not 10-Gigabit. Example: ge-0/0/1 indicates a 1-Gigabit Ethernet interface.

C: et
et stands for 100-Gigabit Ethernet (also used for 40-Gigabit on some platforms). Example: et-0/0/0 indicates a 100-Gigabit Ethernet interface.

D: fe
fe stands for Fast Ethernet (100 Mbps). This is an older standard, typically found on legacy Juniper devices. Example: fe-0/0/0.

Reference:
Juniper TechLibrary – "Interface Naming Overview": Junos interface naming uses prefixes to indicate media type and speed: fe (Fast Ethernet 100M), ge (Gigabit Ethernet 1G), xe (10-Gigabit Ethernet), et (100-Gigabit Ethernet).

Explanation:
The Routing Engine (RE) is the control plane of a Junos device. It handles all management and control functions, including running routing protocols to learn routes, maintaining the routing table, managing device configurations, and communicating with the Packet Forwarding Engine (PFE).

Correct Option:

A: The Routing Engine runs routing protocols.
Routing protocols such as OSPF, BGP, IS-IS, and RIP run as daemons on the RE. The RE processes protocol updates, calculates best paths, and populates the routing table (RIB).

C: The Routing Engine manages the device configuration.
The RE stores the active and candidate configurations, processes commit operations, enforces permission checks, and handles all configuration management tasks (rollback, save, load, etc.).

Incorrect Option:

B: The Routing Engine evaluates transit traffic against firewall filters.
Firewall filter evaluation for transit traffic is performed by the PFE in hardware or optimized software forwarding path. Only exception traffic (e.g., packets destined to the RE) is evaluated by the RE.

D: The Routing Engine forwards transit traffic.
Transit packet forwarding is the exclusive function of the PFE. The RE does not forward transit traffic; it would be too slow. The RE only sends and receives its own control-plane traffic.

Reference:
Juniper TechLibrary – "Routing Engine (RE) Functions": The RE runs routing protocols, maintains routing tables, manages device configuration, and controls the PFE. Packet forwarding is handled by the PFE.

Explanation:
The load override command replaces the entire existing candidate configuration with the contents of a specified source (file, terminal, or URL). It completely discards the previous candidate configuration and loads the new one, making it ideal for deploying a full new hierarchical configuration.

Correct Option:

B: load override terminal
load override terminal allows you to paste a complete configuration into the CLI. This replaces the entire candidate configuration with the pasted content. All existing configuration statements are removed before the new configuration is loaded.

Incorrect Option:

A: load merge terminal
load merge terminal adds the pasted configuration to the existing candidate configuration without removing existing statements. If conflicts occur (same hierarchy with different values), you may get errors or unintended combinations.

C: load factory default
load factory default replaces the candidate configuration with the factory-default configuration, not with your custom large hierarchical configuration. This is used to reset a device, not to deploy a specific new configuration.

D: load set terminal
load set terminal accepts a series of set commands (operational mode format) and converts them into configuration statements. It does not replace the entire configuration by default; it merges unless preceded by a configuration-mode delete command.

Reference:
Juniper TechLibrary – "Loading Configurations": Use load override to completely replace the candidate configuration with a new configuration from a file, terminal, or URL, discarding any prior candidate changes.

Explanation:
The clear log command in Junos operational mode empties the contents of a specified log file without deleting the file itself or disabling logging. Trace or logging operations continue to write new entries to the now-empty file. This is useful for resetting log data before troubleshooting.

Correct Option:

D: Data in the ospf-trace file is removed and logging continues.
clear log ospf-trace truncates (empties) the ospf-trace file. The file remains present on the system, and any active OSPF tracing or logging configured under edit protocols ospf traceoptions will continue to append new entries to this file.

Incorrect Option:

A: The ospf-trace file is deleted.
The file is not deleted; only its contents are cleared. To delete the file, you would use file delete or request system storage cleanup. clear log does not remove files.

B: Trace parameters are removed from the OSPF protocol configuration.
The clear log command does not modify configuration. Trace parameters remain in the configuration under traceoptions. This command only affects the log file content, not the configuration.

C: Logging data into ospf-trace is stopped.
Logging or tracing continues uninterrupted. The command clears historical data but does not stop the logging process. To stop logging, you must remove or deactivate the traceoptions configuration and commit.

Reference:
Juniper TechLibrary – "clear log": The clear log command removes all existing entries from a specified log file without deleting the file. New log messages continue to be written to the file as configured.