JN0-105 Junos - Associate (JNCIA-Junos) Questions and Answers

In a network that carries diverse types of traffic like data, voice, and video, ensuring the performance of latency-sensitive applications such as voice and video calls is crucial. Class of Service (CoS) is a technology designed to prioritize network traffic, ensuring that critical applications like voice and video receive the necessary bandwidth and minimal latency. CoS mechanisms can include traffic classification, traffic policing, queue management, and scheduling. By implementing CoS, network administrators can assign higher priority to voice and video traffic, thus improving their performance across the network and addressing the users' complaints about call quality.

MAC (Media Access Control) addresses are unique identifiers assigned to network interfaces for communications at the data link layer of a network segment. MAC addresses are used to identify the physical hardware on a network. In the context of Ethernet switches, the destination MAC address in incoming frames is used to determine the appropriate output port for forwarding the frame towards its final destination. The switch does not change the destination MAC address; it uses the MAC address to make forwarding decisions within the local network segment.

In the Junos CLI, the pipe character | is used as a filter operator to refine command output. This operator can be combined with various filtering commands like match, except, count, etc., to display only the relevant portions of the command output. For example, using | match <pattern> filters the output to show only the lines that contain the specified pattern, making it easier to find specific information within extensive command output. This functionality is especially useful in managing and troubleshooting complex configurations and network states, allowing for more efficient analysis of the device's operational status and configuration details.

The Packet Forwarding Engine (PFE) in Juniper Networks devices is the heart of the data plane, handling the actual forwarding of packets based on pre-computed forwarding tables. It provides several critical services to manage and control traffic flow, including policing (to enforce bandwidth limits for certain traffic types), stateless firewall filtering (to permit or deny traffic based on predefined criteria), and Class of Service (CoS) (to prioritize traffic to ensure quality of service for critical applications). The PFE utilizes both Layer 2 (MAC addresses) and Layer 3 (IP addresses) forwarding tables to make intelligent forwarding decisions, ensuring that packets are efficiently routed toward their final destination.

Junos OS selects routes based on the route preference (also known as administrative distance). Static routes typically have a lower route preference than OSPF routes, meaning they are more preferred. Since the static route to 192.168.36.1/32 is shown with a preference of 5, it will be selected over the OSPF route for packet forwarding, assuming no other factors such as route filters or policies affect the routing decision.

The routing table and forwarding table play distinct roles in a Junos OS device. The correct answers are A and D. The routing table (A) is used by the Routing Engine (RE) to select the best route among all the learned routes, while the routing table (D) stores all routes and prefixes learned from all routing protocols. The forwarding table, in contrast, contains only the active routes chosen by the RE and is used by the Packet Forwarding Engine for actual packet forwarding.

An Ethernet frame header includes the source MAC address (B) and the destination MAC address (D). These addresses are used to deliver the frame from one Ethernet device to another directly connected Ethernet device on the same network segment. Ethernet frames do not include IP addresses, as those are part of the IP packet encapsulated within the Ethernet frame.

Junos OS supports several external authentication methods for administrative access, with TACACS+ (Terminal Access Controller Access-Control System Plus) and RADIUS (Remote Authentication Dial-In User Service) being among the most commonly used. Both TACACS+ and RADIUS are protocols that allow network devices to communicate with a central authentication server, enabling centralized control over user authentication and authorization. This centralization simplifies the management of user credentials and access policies, especially in larger networks with multiple devices.

In the scenario where the system authentication order is set to "tacplus radius password," the correct statement is (B). If the TACACS+ and RADIUS servers are unreachable or fail to respond, the system will fall back to using password authentication. This ensures that users can still authenticate using locally stored passwords if external authentication servers are unavailable.

Two physical interface properties in Junos OS include the MAC address (A) and the Maximum Transmission Unit (MTU) size (D). The MAC address is a hardware identifier for the network interface, while the MTU size determines the largest packet size that the interface can transmit without needing to fragment the packet.

Referring to the firewall filter configuration in the exhibit, when a packet matches the specified term limit-icmp, two actions are defined in the then statement: count count-icmp and discard. The count count-icmp action means that each time a packet matches this term, a counter named count-icmp will be incremented, providing a tally of how many packets have matched the term. The discard action means that the packet will be dropped and not forwarded through the device. This effectively prevents the packet from reaching its intended destination. There is no action specified that would cause an ICMP destination unreachable message to be returned, nor is there any action that would allow the packet to be forwarded.

The Packet Forwarding Engine (PFE) in Junos OS performs several key functions, including implementing firewall filters (A) and forwarding transit traffic (C). The PFE applies firewall filter rules to incoming and outgoing traffic and is responsible for the high-speed forwarding of packets based on the information in the forwarding table.

When multiple users are configuring a Junos device using the "configure private" command, each user gets their own candidate configuration (C). This allows for isolated configuration sessions, where changes made by one user do not impact or interfere with the changes made by another user in their private session.

Junos OS supports only 1 rescue configuration file on a device. This rescue configuration is a safeguard feature that allows network administrators to revert to a known good configuration in case of a configuration error or issue, ensuring network stability.

The Packet Forwarding Engine (PFE) is an integral component of Juniper Networks devices, responsible for the data plane operations. The data plane, also known as the forwarding plane, is where the actual processing and forwarding of packets occur based on the routing and forwarding tables. The PFE executes the forwarding decisions made by the Routing Engine (RE), handling all packet transmissions, including routing, filtering, and switching packets towards their destination. This contrasts with the control plane operations handled by the RE, which involve routing table maintenance, system management, and control protocol processing.

Junos OS supports multiple login classes that can be assigned to user accounts, allowing for different levels of access and permissions. The correct answer is C, 4. These classes include "super-user," "read-only," "operator," and "unauthorized." Each class provides a different level of access to the Junos device, ranging from full configuration and operational command privileges to limited access for monitoring and viewing configurations.

If issuing the "show | compare" command in configuration mode yields no output, it indicates that there are no changes to the candidate configuration (B). This command is used to compare the current candidate configuration with the active configuration, and a lack of output suggests that the candidate configuration matches the active one, meaning no changes have been made or the changes have already been committed.

In Junos OS, issuing the rollback 0 command reverts the candidate configuration to the last committed configuration. If you have made changes to some interfaces but have not committed those changes, using rollback 0 will discard these uncommitted changes, reverting to the state of the configuration at the time of the last commit. This command does not affect system files like messages.log, reboot the device, or load the factory default configuration; it simply undoes uncommitted changes.

The transport layer in the OSI model is responsible for end-to-end communication and error recovery. In a transport header, such as TCP or UDP, the key components include the source port number and the destination port number. These port numbers are used to identify sending and receiving applications. The source port number indicates the port of the sending application, and the destination port number refers to the port of the receiving application. MAC addresses, on the other hand, are part of the data link layer (Layer 2) and would be included in an Ethernet header, not a transport header.

In the exhibit shown, the highlighted number next to the route type (Static) within the square brackets indicates the route preference, also known as the administrative distance. In Junos, the route preference is a value that determines the priority of the route source. Lower numbers indicate a higher priority when the routing table is being calculated. The route preference is used to select the best route when multiple paths to the same destination exist from different routing sources. The number 5 is unusually low for a static route by default, suggesting it has been manually configured to override other route types.

The firewall filter configuration in the exhibit specifies a filter with two terms. Term 1 matches traffic from the source address 10.0.0.0/8 and has two actions: 'log' and 'next term'. The 'log' action will record the match to a log file, and 'next term' indicates that the firewall should evaluate the next term after logging. There is no explicit action such as 'accept' or 'reject' in term 1, so by default, the traffic will be accepted unless subsequently rejected by another term.

Term 2 has the action 'reject', which discards packets that reach this term. Since there is no 'from' condition in term 2, it acts as a default rule for all traffic not matched by term 1.

Because the traffic sourced from 10.0.0.0/8 matches term 1 and there is no reject action in that term, it will be logged and then accepted by the firewall filter. There is no subsequent term that rejects this specific traffic, so the action from term 2 does not apply to it.

The clear log ospf-trace command on a Juniper Networks router is used specifically to manage the contents of the log file named ospf-trace. Executing this command clears or deletes the existing data within the ospf-trace log file but does not stop the logging process. The router continues to log new OSPF-related events and data into this file after the command is executed. This functionality is crucial for troubleshooting and monitoring the OSPF (Open Shortest Path First) protocol's operation by allowing network administrators to remove old or irrelevant log data while continuously capturing new events without interruption.

The load-balance per-flow statement in the Junos OS policy-options configuration enables flow-based load balancing in the forwarding table. This means that the traffic is distributed across multiple paths based on flows, where a flow is typically identified by attributes such as source and destination IP addresses, and possibly layer 4 information like TCP/UDP ports. This allows for more granular and efficient utilization of available paths, avoiding overloading a single path. The policy does not enable per-packet load balancing, which would send individual packets of the same flow over different paths, potentially causing out-of-order delivery issues. The policy's placement in the forwarding-table export suggests it's intended to influence forwarding behavior, not just routing protocol decisions, and does not necessarily have to be applied under the protocols hierarchy.

The Address Resolution Protocol (ARP) is a fundamental protocol used in IPv4 networking to map an IP address to its corresponding MAC (Media Access Control) address. When an IPv4 device wants to communicate with another device on the same local network, it uses ARP to find out the MAC address associated with the target device's IP address. This ensures that IP packets can be encapsulated into Ethernet frames and delivered to the correct hardware address on the local network segment. ARP does not apply to IPv6, where Neighbor Discovery Protocol (NDP) performs a similar function.

The Routing Protocol Daemon (rpd) is a critical component in Juniper Networks devices, responsible for all routing operations. It maintains routing tables, which hold information about network paths and destinations derived from various routing protocols. These tables are used to make decisions about where to send packets. Additionally, rpd generates forwarding tables based on the information in the routing tables. The forwarding tables are then used by the Packet Forwarding Engine (PFE) to actually forward packets to their next hop or final destination.

Exception traffic refers to packets that the Packet Forwarding Engine (PFE) cannot process normally and must be forwarded to the Routing Engine (RE) for further processing. This includes packets destined for the router itself or packets needing special handling that the PFE cannot provide. To protect the RE from being overwhelmed by such traffic, which could potentially impact the router's control plane functions, exception traffic is rate-limited. This means that there's a threshold to how much exception traffic can be sent to the RE, ensuring that the router's critical management and control functions remain stable and responsive even during high traffic volumes or attacks.