Juniper  JN0-363 Exam Dumps & Practice Test Questions

Question 1:

Which three of the following are considered well-known mandatory BGP attributes that must appear in every BGP update? (Choose three.)

A. MED
B. Origin
C. Community
D. AS Path
E. Next Hop

Correct answer: B, D, E

Explanation:

In the Border Gateway Protocol (BGP), attributes play a crucial role in determining the most optimal route among multiple available paths. These attributes are grouped into several categories: well-known mandatory, well-known discretionary, optional transitive, and optional non-transitive. Among these, well-known mandatory attributes are the most critical because they are required to be present in every BGP update. If any of these are missing, the BGP update is considered invalid and will be rejected by the receiving router.

Let’s evaluate the answer choices in detail:

A. MED (Multi-Exit Discriminator): This attribute is optional and non-transitive. It is primarily used to influence inbound traffic into an AS when multiple entry points exist. Although it can affect route preference, it is not mandatory and doesn’t need to appear in every BGP update. Therefore, it does not fall into the well-known mandatory category.

B. Origin: The origin attribute reveals how a particular route was introduced into BGP — whether through IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), or as an incomplete (e.g., redistributed from another routing protocol). It is a well-known mandatory attribute, meaning it must be present in every update and helps routers evaluate the trustworthiness of the route.

C. Community: The community attribute is optional transitive, used to group routes and apply routing policies. It is extremely useful in complex policy-based routing environments, but it is neither mandatory nor required in every BGP message.

D. AS Path: This attribute provides a list of ASes that the route has passed through. It is one of the most important attributes in BGP because it is used to prevent routing loops and assess route origin. Since it is well-known mandatory, every BGP update must contain this attribute.

E. Next Hop: The next hop attribute specifies the IP address of the router that should be used to forward the packet toward its destination. It is essential for routing decisions, especially in external BGP (eBGP) scenarios. As a well-known mandatory attribute, its presence is non-negotiable in every BGP route update.

In conclusion, Origin (B), AS Path (D), and Next Hop (E) are all required components in BGP updates and fall under the well-known mandatory category. MED and Community, while useful, are not mandatory and are therefore not part of the correct set.

Question 2:

In what sequence does BGP evaluate attributes to determine the best route among multiple options?

A. Next hop → Local preference → AS path → MED → Origin
B. Next hop → Local preference → AS path → Origin → MED
C. Next hop → Origin → Local preference → AS path → MED
D. Next hop → AS path → Local preference → Origin → MED

Correct answer: B

Explanation:

When the Border Gateway Protocol (BGP) is tasked with choosing the best route from several available paths to a destination, it follows a specific sequence of evaluating various path attributes. These attributes offer insights into the path's efficiency, reliability, and policy-based preferences. BGP doesn’t rely solely on metrics like distance but uses a combination of logical evaluations based on these attributes.

Here’s a breakdown of how BGP selects the best path:

1. Next Hop Reachability: Before any comparison of attributes begins, BGP ensures that the route’s next-hop IP address is reachable. If the next hop is unreachable, the route is discarded immediately, regardless of how favorable its other attributes may be.

2. Local Preference: After reachability is confirmed, BGP evaluates the local preference. This is a policy-driven value used within an autonomous system to choose among multiple exit points. The route with the higher local preference is always selected. This attribute is particularly significant in enterprise and ISP networks for enforcing routing policies.

3. AS Path Length: If two routes have the same local preference, the AS path is examined. The route that has traversed fewer ASes (i.e., a shorter AS path) is preferred. This is based on the assumption that fewer hops typically correlate with faster and more direct connectivity.

4. Origin Type: If the AS path lengths are also equal, BGP looks at the origin of the route. Routes learned via IGP are preferred over those from EGP, and incomplete origins are considered the least preferred. This gives insight into the source’s reliability.

5. Multi-Exit Discriminator (MED): Finally, if the routes are still considered equal, BGP evaluates the MED. A lower MED value is preferred. This attribute allows neighboring ASes to influence inbound traffic selection when there are multiple entry points. MED is only considered when comparing routes from the same neighboring AS.

Let’s compare the options:

• A places MED before Origin, which violates BGP’s standard evaluation order.

• B correctly follows the BGP decision process: next hop, local preference, AS path, origin, and then MED.

• C incorrectly places Origin before Local Preference.

• D wrongly evaluates AS Path before Local Preference, which is not correct.

Hence, the proper evaluation order and correct answer is B.

Question 3:

Which two core mechanisms are involved in how Ethernet switches build and update their MAC address tables? (Choose two.)

A. Learning
B. Exporting
C. Aging
D. Timing

Answer: A, C

Explanation:

Ethernet switches rely on dynamic mechanisms to keep track of which devices are connected to which ports. This is managed through a data structure known as the MAC address table (or bridging table), which stores the relationship between MAC addresses and the switch ports they are learned from. Two essential concepts govern the ongoing accuracy of this table: learning and aging.

Learning is the process by which a switch updates its MAC address table. Every time a switch receives a frame, it looks at the source MAC address. If that MAC address isn’t already in the table, the switch records it along with the port it came from. This way, over time, the switch becomes aware of which devices are reachable through which ports. When a frame destined for a known MAC address is received, the switch can efficiently forward it only to the relevant port, avoiding unnecessary traffic on other ports. This greatly improves network performance and reduces broadcast traffic.

Aging, on the other hand, ensures the MAC address table remains current and doesn’t become cluttered with stale information. If a MAC address hasn’t been seen on the network for a certain period (typically 300 seconds), the switch assumes the device is no longer present or has moved. The entry is then removed from the table to make room for new devices. This is crucial in dynamic environments where devices frequently join and leave the network or change ports. Without aging, the switch would retain outdated entries indefinitely, leading to inefficient switching and potential forwarding errors.

Let’s evaluate the incorrect options:

Exporting does not influence the maintenance of a MAC address table. While exporting might relate to network monitoring tools that collect switch data, it doesn’t impact the core switching function of MAC address management.

Timing is a general term in networking and may relate to time-based operations. However, it is not a specific Ethernet bridging concept and doesn’t directly describe how MAC addresses are learned or expired.

In summary, learning and aging are the two bridging concepts critical for accurate and up-to-date MAC address tables. Learning ensures the switch knows where devices are located, and aging prevents obsolete information from lingering.

Question 4:

Which BGP attribute tracks the list of autonomous systems a route has passed through to prevent routing loops?

A. AS path
B. MED
C. Local preference
D. Next hop

Answer: A

Explanation:

In Border Gateway Protocol (BGP), routing decisions across autonomous systems (ASes) rely on a series of attributes attached to each route advertisement. Among these, the AS path attribute is particularly vital for maintaining a loop-free inter-domain routing environment. It records the sequence of AS numbers that a given route has traversed before reaching a router.

When a route is advertised from one AS to another, the advertising router appends its own AS number to the beginning of the existing AS path. Over time, as the route is passed through additional ASes, the AS path grows by collecting these AS numbers in sequence. This record allows any BGP router to immediately identify if a route has already been seen by checking for its own AS number in the AS path. If it finds its number, the router recognizes a potential loop and discards the route.

This mechanism is fundamental for loop prevention in BGP. Unlike Interior Gateway Protocols (IGPs) like OSPF or EIGRP that use metrics such as hop count or cost, BGP relies on policy and attributes. The AS path not only helps with loop detection but also enables policy-based routing—administrators can configure route preferences based on the AS path length or specific AS numbers within it.

Let’s clarify why the other options are incorrect:

MED (Multi-Exit Discriminator) is used to suggest preferred entry points into an AS when multiple entry links exist. It influences inbound traffic decisions but has no impact on loop prevention.

Local preference is an attribute used within a single AS to influence which exit path is preferred when multiple options exist. It’s important for outbound traffic policies but doesn’t provide visibility into how many ASes a route has passed through.

Next hop indicates the IP address of the next-hop router for a route. It helps in forwarding traffic but offers no historical context and thus cannot assist in identifying loops.

To conclude, AS path is the only attribute among the options that explicitly records the route’s journey through various ASes and enables loop prevention. It’s a core safeguard in BGP’s design that keeps inter-domain routing stable and predictable.

Question 5:

When a Junos device is newly connected to an Ethernet network, how does it discover and store MAC address information?

A. It sends a multicast message to the network requesting all MAC addresses and records the MAC and interface.
B. It broadcasts a request for all devices and MAC addresses, then logs the responses along with their interfaces.
C. It observes the destination MAC address in the traffic and records that along with the receiving interface.
D. It learns the source MAC address from incoming frames and associates it with the interface that received the frame.

Correct Answer: D

Explanation:

When a Junos device connects to an Ethernet network, it builds its MAC address table by passively listening to network traffic. This process is known as MAC learning and is common across all Ethernet switching devices. The goal is to map each MAC address to the correct interface on the switch, so frames can be forwarded only where needed instead of being flooded across all ports.

The mechanism works by examining the source MAC address of every incoming Ethernet frame. Whenever a frame enters a Junos device through one of its interfaces, the device extracts the source MAC address from the frame header and notes the port it came through. This pairing (MAC address and interface) is then stored in the MAC address table.

Let’s analyze why option D is correct and the others are not:

• Option A (Multicast-based discovery): This is inaccurate. Ethernet switches do not use multicast messages to actively request MAC addresses. Multicast in Ethernet is used for specific group communications like video streams or updates, not for MAC address discovery.

• Option B (Broadcast request for MACs): This also doesn’t reflect how switches operate. While hosts might use broadcast protocols like ARP to discover IP-to-MAC mappings, switches themselves do not broadcast to collect MAC addresses. Instead, they observe normal frame traffic.

• Option C (Learning from destination MAC): This is a common misconception. The destination MAC address tells the switch where the frame is going—not who sent it. If the destination MAC isn’t already known, the switch floods the frame to all ports except the ingress port, but it doesn’t use the destination MAC to update its table.

• Option D (Learning from source MAC): This is the correct and standard behavior. When a switch receives a frame, it uses the source MAC address to update its internal MAC address table. This allows it to learn which devices are reachable through which interfaces.

In practice, this learning process is dynamic. If a device moves to a different interface, the MAC address table will be updated the next time the switch receives a frame from that source MAC on the new interface. Entries may also time out if a device stops communicating, helping the switch maintain an accurate view of the network.

Thus, option D correctly describes the passive, frame-driven method by which Junos and all Ethernet switches learn MAC addresses.

Question 6:

Which two statements accurately describe the default and extended metric capabilities of IS-IS interfaces? (Choose two.)

A. IS-IS interfaces default to a maximum metric value of 1023.
B. Wide metrics allow an IS-IS interface to support metric values up to around 16 million.
C. Wide metrics limit the maximum interface metric to 1023.
D. The default maximum metric value for an IS-IS interface is 63.

Correct Answers: B, D

Explanation:

IS-IS (Intermediate System to Intermediate System) is a robust link-state routing protocol used widely in enterprise and service provider networks. Like other link-state protocols, IS-IS uses metrics assigned to interfaces to calculate the cost of paths in the network. Understanding the default metric behavior and how wide metrics change the capabilities of IS-IS is essential for proper routing design and optimization.

Under default IS-IS configurations, interfaces operate with narrow metrics. This legacy mode, defined in the original IS-IS standard (ISO/IEC 10589), supports a metric range from 1 to 63. So, the maximum value that can be assigned to any IS-IS interface by default is 63, making option D accurate.

However, this limited metric range can be problematic in modern networks with high-speed links and the need for granular control over route preference. To solve this, wide metrics were introduced through extensions like RFC 5305. These wide metrics allow for 24-bit values, drastically increasing the possible range of interface metrics up to 16,777,215 (or 2²⁴ – 1). This makes option B correct.

Let’s now analyze why the other options are incorrect:

• Option A claims that the default maximum metric is 1023. This is incorrect under both narrow and wide metric modes. In default narrow mode, the cap is 63—not 1023. The 1023 figure is occasionally confused with older non-standard implementations but is not accurate as per official IS-IS specifications.

• Option C incorrectly asserts that wide metrics limit the value to 1023. This is a misunderstanding. Wide metrics expand the capability dramatically, and while 1023 is a valid value within the wide metric range, it’s far from the maximum. The purpose of wide metrics is to increase the available metric range—not reduce it.

In real-world environments, wide metrics are almost always enabled on IS-IS networks, especially when deploying traffic engineering, MPLS, or handling complex topology changes. However, to ensure compatibility, all routers participating in the IS-IS area must support wide metrics; otherwise, inconsistencies and routing failures can occur.

In conclusion, the two correct statements are:

• B: Wide metrics enable a much larger range, supporting interface metrics up to approximately 16 million.

• D: In narrow metric mode (the default), the maximum allowable value for an IS-IS interface metric is 63.

Question 7:

Which two statements accurately describe characteristics of BGP? (Choose two.)

A. IBGP peers must belong to the same Autonomous System (AS).
B. The default TTL value for packets between EBGP peers is 1.
C. EBGP routers must reside in the same AS.
D. The TTL for IBGP protocol packets is set to 1 by default.

Correct Answers: A, B

Explanation:

Border Gateway Protocol (BGP) is the foundational routing protocol used for exchanging routes between different autonomous systems (ASes). There are two primary forms of BGP: Internal BGP (IBGP) and External BGP (EBGP). Each has distinct behaviors regarding AS numbers and TTL (Time-To-Live) values, both of which impact neighbor relationships and packet forwarding.

Let’s examine the provided options in context:

A. IBGP peers must belong to the same Autonomous System (AS).
This is correct. IBGP is explicitly designed for routing information exchange within a single AS. When two routers within the same AS run BGP, their peering relationship is defined as IBGP. As a result, they must share the same AS number. IBGP helps propagate routes learned from external sources throughout the AS without altering the AS path, which is crucial for preventing routing loops.

B. The default TTL value for packets between EBGP peers is 1.
This statement is also correct. When forming EBGP sessions, BGP expects peers to be directly connected. To enforce this, the default TTL value in the IP header is set to 1, meaning the packet must reach its destination in a single hop. If the EBGP neighbor is not directly reachable, the packet will expire, preventing the session from forming. This behavior enhances security and control. For non-directly connected EBGP peers, features like EBGP multihop can be configured to increase the TTL value.

C. EBGP routers must reside in the same AS.
This is incorrect. EBGP is used specifically to connect routers that are in different ASes. The distinction between IBGP and EBGP is primarily based on whether the AS numbers of the peers are the same or different. If they’re the same, it’s IBGP; if different, it’s EBGP.

D. The TTL for IBGP protocol packets is set to 1 by default.
This is false. IBGP assumes that routers might not be directly connected, so it sets the default TTL to 255. This high value allows packets to traverse multiple hops across the internal network without expiring. This is essential for large networks where BGP routers may not have direct connections to all other BGP-speaking routers.

In summary: IBGP neighbors must use the same AS number; EBGP packets have a default TTL of 1 to enforce direct connectivity. EBGP routers must be in different ASes, and IBGP uses a TTL of 255.

Question 8:

In Junos OS, what method is used to determine the OSPF router ID if none is manually configured?

A. Select the IP address of the interface with the lowest MAC address.
B. Use the IP address assigned to the loopback interface.
C. No router ID is assigned unless it is explicitly configured.
D. Choose the IP address of the interface with the highest priority.

Correct Answer: B

Explanation:

In OSPF (Open Shortest Path First), the router ID (RID) serves as a unique identifier for each router in an OSPF domain. This ID is crucial for differentiating routers when they exchange link-state advertisements and build the OSPF topology. When using Junos OS, the process of selecting a router ID is well-defined and predictable if it is not set manually.

The selection logic in Junos OS proceeds as follows:

1. Manual Configuration: If a router ID is explicitly configured under the OSPF hierarchy, it takes precedence and is used immediately.

2. Loopback Interface IP Address: If no manual ID is provided, Junos OS will check for loopback interfaces. It then selects the highest IP address assigned to any loopback interface as the router ID. This is a preferred method because loopbacks are virtual interfaces that remain up as long as the router is operational, making them reliable and stable sources for the RID.

3. Physical Interface IP Address: If no loopback interfaces are configured or have IPs, Junos will choose the highest IP address among all physical interfaces. However, this is a fallback option and not ideal, since physical interfaces may go down due to hardware issues or link failures, causing potential OSPF instability.

Now let’s assess the options:

A. The IP address of the interface with the lowest MAC address is not used in Junos for determining router ID. MAC addresses are Layer 2 identifiers and irrelevant in OSPF’s RID selection process.

B. This is correct. If no router ID is manually defined, Junos will use the IP from a loopback interface, prioritizing stability.

C. This is false. Junos does not require a router ID to be manually configured. It will automatically assign one using the above hierarchy.

D. Interface priority is used during DR/BDR election, not for router ID selection. So this is unrelated and incorrect.

In conclusion, Junos OS intelligently chooses the OSPF router ID using loopback interface IPs when not manually defined, making B the accurate answer.

Question 9:

Which protocol is primarily responsible for advertising label bindings between routers in an MPLS-enabled Juniper service provider network?

A. OSPF
B. RSVP
C. LDP
D. BGP

Correct Answer: C

Explanation:

In a Juniper MPLS (Multiprotocol Label Switching) network, Label Distribution Protocol (LDP) is the protocol primarily responsible for distributing label bindings between routers. LDP enables routers to establish MPLS label-switched paths (LSPs) by mapping IP prefixes to labels and distributing this information across the network.

While OSPF (Open Shortest Path First) is used for routing information and calculating the shortest paths in the IGP (Interior Gateway Protocol), it does not distribute labels. OSPF simply provides the routing table and network topology knowledge that LDP uses to set up label bindings.

RSVP (Resource Reservation Protocol), on the other hand, is also used in MPLS networks—but primarily for traffic engineering (RSVP-TE). RSVP is more resource-aware and is used when explicit LSP paths and bandwidth reservations are required. However, it is not the standard protocol for label distribution in a basic LDP/MPLS configuration.

BGP (Border Gateway Protocol) is used for routing between autonomous systems and can distribute MPLS labels in specific cases like BGP-LU (BGP Label Unicast) or VPNv4 routes in MPLS VPNs, but it is not the primary method in standard LDP-based MPLS deployments.

Therefore, when configuring a basic MPLS network on Juniper devices, LDP is the protocol that advertises and manages label bindings between routers. It operates over TCP and UDP port 646 and requires IGP reachability between LDP peers for label distribution to function effectively. Juniper devices use LDP by default in most basic MPLS configurations unless explicitly set up to use RSVP or BGP for label distribution.

Question 10:

In a Layer 3 VPN implemented with Junos OS, which of the following components is responsible for maintaining customer route separation?

A. GRE Tunnel
B. Route Distinguisher (RD)
C. IGP Metric
D. LSP

Correct Answer: B

Explanation:

In a Layer 3 VPN environment using Junos OS (and based on MPLS), maintaining customer route separation is a critical function. The Route Distinguisher (RD) is the key mechanism used to accomplish this.

When multiple customers use overlapping IP address spaces (e.g., 10.0.0.0/8), Juniper routers must distinguish between these routes to avoid mixing them. The Route Distinguisher (RD) is a unique identifier prepended to the customer's IP prefix to create a globally unique VPN route known as a VPN-IPv4 address. This makes 10.0.0.0/8 from Customer A different from the same prefix used by Customer B.

The RD does not influence routing decisions—it simply ensures uniqueness in the global BGP table by adding a distinguishing attribute. This separation is crucial for multi-tenant service provider environments where scalability and security are paramount.

GRE tunnels (Generic Routing Encapsulation) are tunneling mechanisms that encapsulate packets but do not provide route separation at the control plane level.

IGP metrics such as OSPF cost or IS-IS metric are used within the service provider’s core network to determine the best path for routing traffic, but they don’t differentiate between customers' routes.

Label Switched Paths (LSPs) provide the transport path across the MPLS backbone. While essential for forwarding data, they also do not separate routing information between customers at the control plane.

Thus, the Route Distinguisher (RD) is the component that ensures customer routes are kept logically distinct even if they share the same IP prefix, enabling scalable and isolated routing tables for each VPN on a Juniper platform.