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

Answer : C

The Address Resolution Protocol (ARP) is a fundamental Layer 2 utility used within the IPv4 suite to resolve a known network-layer (Layer 3) address to its corresponding physical media access control (MAC) or hardware address (Layer 2). In a typical Ethernet environment, when a Junos device needs to forward a packet to a next-hop on a local subnet, the Packet Forwarding Engine (PFE) requires the destination MAC address to properly encapsulate the frame.

The process begins with an ARP Request, which is broadcast to all hosts on the segment asking, 'Who owns this IP address?' The host assigned that specific IP responds with an ARP Reply containing its MAC address. The Junos device then stores this mapping in its ARP cache (viewable via the show arp command) to avoid repeated broadcasts for subsequent packets. This resolution is essential because while IP addresses facilitate end-to-end logical routing, the actual delivery of data across a physical wire or switch fabric relies entirely on hardware addresses. Without successful ARP resolution, the device cannot complete the Layer 2 header, and the traffic will be dropped as 'encapsulation failed.'

Answer : A

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.

Answer : A

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.

Answer : C, D

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.

Answer : B

In the Junos OS, the Command Line Interface (CLI) uses distinct prompt symbols to provide the administrator with immediate contextual awareness of their current operating environment. When the prompt ends with a hash symbol (#), it indicates that the user is in configuration mode.

This mode is the 'engine room' of the device, where you modify the candidate configuration. Here, you can add, delete, or modify statements across the various hierarchies like [edit system], [edit interfaces], or [edit protocols]. It is important to remember that changes made while the # prompt is visible are not active until a commit command is successfully executed.

Contrast this with the operational mode, which is indicated by a greater-than symbol (>). Operational mode is used for monitoring, troubleshooting, and viewing the system status (e.g., show commands). Moving between these modes is a fundamental part of the Junos workflow: you enter configuration mode by typing configure and return to operational mode by typing exit or quit. If you see a percent sign (%), you've wandered into the FreeBSD shell mode, which is a lower-level Unix environment typically reserved for advanced system maintenance. Recognizing that # means you have the power to change the system's 'brain' is a key safety check for any network architect.

Answer : D

In the IPv4 addressing scheme used within Junos OS, the /24 prefix length (representing a subnet mask of 255.255.255.0) allocates 24 bits for the network portion and 8 bits for the host portion of the 32-bit address. To determine the total number of addresses in this block, the formula $2^n$ is applied, where $n$ is the number of host bits. With 8 bits available ($2^8$), there are a total of 256 possible IP addresses.

However, the architecture of standard IP networking requires the reservation of two specific addresses within any subnet, making them unavailable for assignment to individual host interfaces. The first address (192.168.1.0) is the network address, which identifies the subnet itself. The last address (192.168.1.255) is the directed broadcast address, used to send traffic to all hosts on the segment simultaneously. Consequently, the maximum number of addresses that can be assigned to actual hosts---such as router interfaces, servers, or workstations---is calculated as $2^n - 2$. In this specific scenario, $256 - 2 = 254$. This calculation is a fundamental requirement for network architects when defining address pools and ensuring the Packet Forwarding Engine (PFE) is correctly configured with valid host-layer identifiers.

Answer : B, D

Route preference is the quantitative value Junos OS uses to rank the trustworthiness of different routing sources when multiple protocols provide a path to the same destination. A critical architectural distinction in Junos is that both direct and local routes share the same default preference value of 0. Direct routes represent subnets physically connected to an interface, while local routes represent the specific IP address assigned to the device interface itself. Because a value of 0 is the lowest possible numerical value, these routes are always preferred over any learned via dynamic protocols or static configuration.

Additionally, Junos OS treats External BGP (EBGP) and Internal BGP (IBGP) differently than other vendors by assigning both an identical default preference of 170. This means that if the same prefix is learned via both EBGP and IBGP, the preference value alone will not determine the active route; the system must instead proceed to the standard BGP path selection algorithm (evaluating attributes like Local Preference, AS Path, and Origin) to break the tie. In contrast, OSPF distinguishes between its route types, assigning a preference of 10 to internal routes and 150 to AS external routes. Understanding these default values---specifically the parity between direct/local and the unified preference for BGP variants---is foundational for predicting how the Routing Engine populates the forwarding table. Reference: Routing Fundamentals, Default Route Preference Values.