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JN0-363 Juniper Practice Test Questions and Exam Dumps

Question 1:

Which three of the following are well-known mandatory BGP attributes? (Choose three.)

A. MED
B. origin
C. community
D. AS path
E. next hop

Answer: B, D, E

Explanation:

In Border Gateway Protocol (BGP), attributes are used to determine the best path to a destination. These attributes are categorized into different types:

• Well-known mandatory

• Well-known discretionary

• Optional transitive

• Optional non-transitive

Well-known mandatory attributes are those that must be present in every BGP update message. If they are missing, the receiving router will reject the update as invalid. The three well-known mandatory BGP attributes are:

1. Origin

2. AS path

3. Next hop

Let’s examine each answer option in light of this.

A. MED (Multi-Exit Discriminator)
This is an optional non-transitive attribute. It is used to indicate to external neighbors the preferred path into an AS when multiple entry points exist. Because it is optional and not required in every update, it is not a well-known mandatory attribute.

B. Origin
This attribute indicates how BGP learned about the route—either via IGP, EGP, or incomplete (i.e., redistributed from another routing protocol). It is a well-known mandatory attribute, meaning it must be included in all BGP updates. This helps routers determine the reliability of the route source.

C. Community
The community attribute is optional transitive. It is widely used for route tagging and policy control, but it is not mandatory, nor is it well-known in the mandatory sense. Therefore, it does not qualify as one of the required attributes in every BGP message.

D. AS path
This is a well-known mandatory attribute. It contains the list of Autonomous Systems (ASes) that the route has traversed. It’s essential for loop prevention in BGP and is one of the most critical attributes used for route selection.

E. Next hop
This attribute specifies the IP address of the next-hop router that should be used to reach the destination. It is another well-known mandatory attribute, especially important in External BGP (eBGP) communications. Every update must include this attribute, which is used by the receiving router to determine the forwarding path.

In summary:

• Origin (B), AS path (D), and Next hop (E) are all well-known mandatory attributes that must be included in every BGP update.

• MED (A) and Community (C) are optional attributes and do not fall under this category.

The correct answers are B, D, and E.

Question 2:

What is the correct order in which BGP (Border Gateway Protocol) attributes are evaluated during the active route selection process?

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:

Border Gateway Protocol (BGP) is a path vector routing protocol that is primarily used to exchange routing information across different autonomous systems (ASes) on the internet. When multiple routes to the same destination exist, BGP uses a well-defined set of attributes to select the best path. These attributes are evaluated in a specific order during the BGP best path selection process.

Let’s break down the correct order of operations BGP follows in choosing the best route and how the attributes from option B fit into that logic:

1. Next Hop Reachability
   The first check is whether the next hop specified in the BGP route is reachable. If it is not, the route is immediately disqualified. This step is a prerequisite and is essential for considering a path.

2. Local Preference
   Once a route is considered viable based on next-hop reachability, the local preference attribute is evaluated. This is an attribute used within a single AS to indicate preference among multiple exit points. The higher the local preference, the more preferred the route. This is commonly set manually by network administrators and plays a crucial role in policy-based routing.

3. AS Path Length
   If the local preference is equal, the BGP process looks at the AS path. This is a list of AS numbers the route has traversed. Shorter AS paths are preferred over longer ones, under the assumption that fewer hops typically mean a faster or more direct route.

4. Origin Type
   If the AS path lengths are equal, BGP compares the origin of the route. Origin can be IGP, EGP, or incomplete, with IGP being most preferred and incomplete being least preferred. This attribute provides insight into how the route was introduced into BGP.

5. Multi-Exit Discriminator (MED)
   Finally, if all the above attributes are equal, BGP evaluates the MED, which is used to influence inbound routing between ASes. A lower MED value is preferred. MED is only considered when routes are being received from the same neighboring AS.

Let’s compare the provided options:

• A (next hop –> local preference –> AS path –> MED –> origin): This incorrectly places MED before origin, which is not the correct order.

• B (next hop –> local preference –> AS path –> origin –> MED): This is the correct order of BGP path selection attributes, up to and including the origin and MED steps.

• C (next hop –> origin –> local preference –> AS path –> MED): This places origin before local preference, which is incorrect.

• D (next hop –> AS path –> local preference –> origin –> MED): This incorrectly places AS path before local preference, which is not how BGP processes decisions.

It’s worth noting that after these primary steps, BGP can continue to consider other factors like eBGP over iBGP, lowest IGP metric to the next hop, oldest path, router ID, and cluster list length. However, for the scope of this question, which focuses on the core attributes, the order specified in option B aligns perfectly with standard BGP behavior.

Thus, the correct answer is B.

Question 3:

Which two bridging concepts are used to maintain an Ethernet switching table? (Choose two.)

A. learning
B. exporting
C. aging
D. timing

Answer: A, C

Explanation:

Ethernet switches use a bridging table, also known as a MAC address table, to determine where to forward frames within a local area network. This table is dynamically maintained using specific bridging concepts that control how MAC addresses are added, updated, and removed over time. The two main concepts that support this process are learning and aging.

Let’s explore each option:

A. Learning
Learning is a fundamental bridging concept. It refers to the process where a switch examines the source MAC address of incoming Ethernet frames and associates that address with the specific port it arrived on. This information is stored in the MAC address table. Over time, as more frames arrive from different devices, the switch "learns" the location of each MAC address on the network. This allows it to forward future frames to the correct port without broadcasting to all ports. Therefore, learning is essential for efficient switching and is one of the two correct answers.

B. Exporting
Exporting is not a bridging concept in the context of maintaining an Ethernet switching table. While exporting may be a term used in network management or monitoring (such as exporting logs or data to a remote system), it plays no role in the dynamic operation of a switch’s MAC address table. It is not involved in either populating or maintaining that table. Thus, it is not a correct answer.

C. Aging
Aging is the second key concept for maintaining the MAC address table. It refers to the mechanism by which entries in the table are removed after a certain period of inactivity. If no frames are received from a particular MAC address for a pre-defined time (often 300 seconds by default), the switch will consider the MAC address stale and delete the entry. This prevents the table from filling up with outdated information and ensures it reflects the current network topology. Aging is especially important in dynamic environments where devices may move or disconnect. Therefore, aging is the second correct answer.

D. Timing
Timing, while relevant in many networking contexts, is not a bridging concept in the strict sense. It might refer to how long certain operations take or the intervals at which processes run, but it is not directly involved in the management of the MAC address table in Ethernet switching. It is a more general term and does not describe a specific function of switch operation like learning or aging.

• Learning is used to populate the MAC address table.

• Aging is used to remove stale entries from the table.

The correct answers are A and C.

Question 4:

Which BGP attribute helps identify and prevent routing loops by recording the sequence of autonomous systems a route has passed through?

A. AS path
B. MED
C. local preference
D. next hop

Answer: A – AS path

Explanation:

The Border Gateway Protocol (BGP) uses a set of attributes to make routing decisions and to ensure stability and reliability in routing between different autonomous systems (ASes). One critical aspect of BGP's operation is the prevention of routing loops, which can cause data to circulate endlessly between networks, leading to congestion, delays, or loss of service. To handle this, BGP includes the AS path attribute, which is specifically designed to detect and prevent such loops.

The AS path attribute is a sequence of AS numbers that a route advertisement has passed through. Each time a BGP router sends an advertisement to a neighboring AS, it appends its own AS number to the beginning of the AS path. This way, when a route advertisement returns to an AS that has already handled it, the router will notice its own AS number in the AS path and automatically reject the route. This mechanism serves as a straightforward and efficient loop-prevention tool.

For example, imagine that AS65010 originates a route to network 10.10.10.0/24 and advertises it to AS65020. AS65020 adds its own AS number and forwards it to AS65030. If AS65030 somehow sends the route back toward AS65010, the AS path would show “65010 65020 65030.” When AS65010 receives the route again, it sees that its own AS number is already present in the path, recognizes the potential loop, and discards the route.

Now let’s look at the other options and why they are incorrect:

• B. MED (Multi-Exit Discriminator) is an attribute used to influence inbound traffic from neighboring ASes when multiple entry points exist. It serves as a suggestion for which link should be preferred but plays no role in loop detection.

• C. Local preference is used within a single AS to select the preferred route when multiple paths are available. It affects routing policy internally but does not relate to loop prevention or detection.

• D. Next hop is the IP address of the router that should be used as the next step toward the destination network. It is vital for forwarding traffic correctly but has no involvement in preventing loops.

In summary, the AS path is the only BGP attribute that directly helps prevent routing loops by keeping a history of all the ASes a route has passed through. This is a core feature of BGP and is critical for maintaining the protocol’s loop-free nature. Without this attribute, BGP would require far more complex mechanisms to achieve the same result.

Therefore, the correct answer is A – AS path.

Question 5:

How does a Junos device learn about MAC addresses when it is first connected to an Ethernet LAN?

A. The device sends out a network multicast message asking for all devices and MAC addresses on the network and stores this information in addition to the interface from which the response was received.
B. The device sends out a network broadcast message asking for all devices and MAC addresses on the network and stores this information in addition to the interface from which the response was received.
C. The device learns the destination MAC addresses from traffic in the network and stores this MAC address in addition to the interface from which the traffic was received.
D. The device learns the source MAC addresses from traffic in the network and stores this MAC address in addition to the interface from which the traffic was received.

Answer: D

Explanation:

When a Junos device, or any Ethernet switch, is first connected to an Ethernet LAN, it begins building its MAC address table (also known as a forwarding table or bridge table). This table maps MAC addresses to the switch’s interfaces, allowing it to make forwarding decisions efficiently. The method used for building this table is a process called MAC learning.

The fundamental mechanism behind MAC learning is based on observing the source MAC address of incoming Ethernet frames. Let’s break down the options to understand which one aligns with how Junos devices operate.

A. Multicast discovery
This option is incorrect. Junos devices do not use multicast messages to request MAC address information from other devices. Multicast is typically used for specific group communications (e.g., multimedia streams), not for MAC learning.

B. Broadcast discovery
This is also incorrect. While broadcast messages like ARP requests are common in Ethernet networks, switches do not initiate such broadcasts to populate their MAC address tables. They passively learn MAC addresses from the traffic flowing through them.

C. Destination MAC learning
This is a common misconception. Switches do not learn MAC addresses from the destination field of a frame. The destination MAC address indicates where the frame is going, not who sent it. If the switch doesn't already know the destination MAC, it may flood the frame out all ports (except the source port), but it does not use the destination address to populate its MAC table.

D. Source MAC learning
This is the correct answer. A Junos device (and all standard Ethernet switches) learns MAC addresses by examining the source MAC address of incoming Ethernet frames. When a frame arrives at a specific interface, the device records the source MAC address and the interface it came in on. This information is then stored in the MAC address table. When subsequent frames are destined for that MAC address, the switch knows exactly which interface to use.

This learning process is passive and efficient. For instance, if a host sends a frame to another device, the switch learns that host’s MAC address from the frame’s source field and associates it with the ingress port. Over time, as more devices communicate, the switch’s MAC table grows to represent the network topology.

Therefore, the correct answer is D because it accurately describes the way Junos devices and Ethernet switches in general learn MAC addresses: by examining the source MAC address in received Ethernet frames and associating it with the ingress interface.

Question 6:

Which two statements accurately describe IS-IS interface metrics and their default or maximum values under different configurations? (Choose two.)

A. By default, an IS-IS interface has a maximum metric value of 1023.
B. Wide metrics enable IS-IS interface to have a maximum metric value of around 16 million.
C. Wide metrics enable an IS-IS interface to have a maximum metric value of 1023.
D. By default, an IS-IS interface has a maximum metric value of 63.

Answer: 

B – Wide metrics enable IS-IS interface to have a maximum metric value of around 16 million
D – By default, an IS-IS interface has a maximum metric value of 63

Explanation:

Intermediate System to Intermediate System (IS-IS) is a link-state interior gateway protocol used primarily within large service provider and enterprise networks. One of the important aspects of IS-IS routing is how metrics are assigned to interfaces, which determines the cost of paths through the network. These interface metrics are crucial for the shortest-path-first (SPF) algorithm to compute optimal routes. Understanding the default and extended capabilities of IS-IS metrics helps network engineers accurately tune routing behavior.

By default, IS-IS uses what’s known as narrow metrics, which are defined in the original IS-IS specification (ISO/IEC 10589). Under this default (narrow) metric system, each IS-IS interface can be assigned a metric value ranging from 1 to 63, making 63 the maximum default metric value. Therefore, D is a correct statement.

As networks have grown more complex and bandwidth variability has increased, this limited metric range became insufficient. To address this, the concept of wide metrics was introduced via extensions (such as RFC 5305 and others). With wide metrics enabled, IS-IS supports 24-bit interface metrics. This expands the range dramatically, allowing values up to 16,777,215 (2²⁴ - 1). This provides greater granularity and flexibility in defining path costs. Therefore, B is also a correct statement.

Now, let’s look at the incorrect statements:

• A says “By default, an IS-IS interface has a maximum metric value of 1023.” This is incorrect. In default narrow mode, the maximum is 63, not 1023. The 1023 value is sometimes mistakenly associated with older extended implementations, but it is not the default per ISO standards.

• C says “Wide metrics enable an IS-IS interface to have a maximum metric value of 1023.” This is also incorrect. Wide metrics expand the metric range significantly beyond 1023, up to approximately 16 million. In fact, 1023 would be a valid number within the wide metric range but is far from the upper limit. The statement misrepresents the purpose and capability of wide metrics.

In practice, enabling wide metrics is recommended in modern networks using IS-IS, especially when implementing features like traffic engineering or when needing finer control over path selection across high-speed core links. However, enabling wide metrics requires all participating routers to support and be configured for it, to avoid incompatibilities.

To summarize, the correct statements are:

B – Wide metrics enable IS-IS interface to have a maximum metric value of around 16 million
D – By default, an IS-IS interface has a maximum metric value of 63

Question 7:

Which two statements about BGP are correct? (Choose two.)

A. IBGP neighbors must use the same AS number.
B. By default, the TTL on product-related packets for external neighbors is set to 1.
C. EBGP neighbors must use the same AS number.
D. By default, the TTL on protocol-related packets for internal neighbors is set to 1.

Answer: A, B

Explanation:

Border Gateway Protocol (BGP) is a path-vector routing protocol used to exchange routing and reachability information between autonomous systems (ASes) on the internet. It supports both external BGP (EBGP) and internal BGP (IBGP) operations, and these operate under specific rules regarding AS numbers and Time-To-Live (TTL) values.

Let’s analyze each option to identify which are correct:

A. IBGP neighbors must use the same AS number.
This is correct. Internal BGP (IBGP) is used to exchange routing information between BGP peers that reside within the same Autonomous System (AS). Therefore, IBGP peers must share the same AS number. IBGP is essential for propagating external routing information across an AS without altering the AS path attribute, which is crucial for maintaining loop prevention in EBGP.

B. By default, the TTL on product-related packets for external neighbors is set to 1.
This is correct, though there's a bit of ambiguity in the wording. If we interpret "product-related packets" to mean "protocol packets" used for BGP communication (which is likely intended), this refers to how EBGP sessions function by default. BGP uses TCP port 179 to establish connections, and when forming EBGP peerings, the default TTL is set to 1. This ensures that the EBGP neighbor is directly connected, as the packet would otherwise expire before reaching the destination. This default can be overridden using features like EBGP multihop, which increases the TTL to allow peerings over non-direct links.

C. EBGP neighbors must use the same AS number.
This is incorrect. The core function of EBGP is to allow routing information exchange between different ASes. So, each EBGP neighbor must have a different AS number. If both neighbors used the same AS number, BGP would treat the session as IBGP, not EBGP. Therefore, this statement is false.

D. By default, the TTL on protocol-related packets for internal neighbors is set to 1.
This is incorrect. For IBGP sessions, BGP assumes that neighbors are not necessarily directly connected. Therefore, the TTL is set to 255 by default, allowing packets to traverse multiple hops within the same AS. This is in contrast to EBGP’s default TTL of 1. As a result, this statement is also false.

In summary:

• IBGP neighbors must share the same AS number.

• EBGP uses a default TTL of 1 to enforce direct connections.

• IBGP uses a TTL of 255 to allow multi-hop peerings.

• EBGP neighbors must have different AS numbers.

The correct answers are A and B.

Question 8:

If an OSPF router has no manually configured router ID, how does Junos OS determine the router ID in this situation?

A. The Junos OS will use the IP address assigned to the interface with the lowest MAC address.
B. The Junos OS will use the IP address assigned to the loopback interface for the router ID.
C. A router ID will not be assigned until it is manually configured.
D. The Junos OS will use the IP address assigned to the interface with the highest priority.

Answer: B – The Junos OS will use the IP address assigned to the loopback interface for the router ID

Explanation:

The OSPF (Open Shortest Path First) protocol uses a router ID (RID) as a unique identifier for each router in the OSPF domain. This router ID is essential because it distinguishes one router from another in LSAs (Link-State Advertisements) and other OSPF operations, especially in database exchanges and adjacency formation.

Under Junos OS, the process of selecting the OSPF router ID follows a specific and predictable order if the router ID is not explicitly configured. Understanding this hierarchy is essential for network engineers to anticipate the behavior of OSPF in various configurations and to troubleshoot OSPF-related issues effectively.

Here’s the order that Junos OS follows to determine the router ID:

1. Manually configured router ID: If a router ID is explicitly set under the OSPF configuration, Junos will use that and ignore all other options. This is the most deterministic and recommended approach.

2. Highest IP address on a loopback interface: If no manual router ID is specified, Junos OS will look for loopback interfaces and select the highest IP address assigned to any loopback interface on the router. This is done because loopback interfaces are logical and do not go down unless administratively shut down, making them ideal for stable router ID selection.

3. Highest IP address on a physical interface: If no loopback interface has an IP address, Junos will fall back to the highest IP address assigned to any physical interface on the router. This is less desirable since physical interfaces may go up or down, which could affect OSPF stability.

Therefore, B is the correct answer: “The Junos OS will use the IP address assigned to the loopback interface for the router ID.” This aligns with the second step in Junos' router ID selection process.

Now let’s examine the incorrect options:

• A, “The Junos OS will use the IP address assigned to the interface with the lowest MAC address,” is incorrect. Junos does not use MAC addresses for router ID selection. MAC addresses are more relevant in Layer 2 technologies like Ethernet or in determining bridge IDs, not OSPF router IDs.

• C, “A router ID will not be assigned until it is manually configured,” is false. Junos OS will automatically assign a router ID using the process described above if one is not manually configured. The router does not remain without a router ID unless there are no IP addresses configured at all.

• D, “The Junos OS will use the IP address assigned to the interface with the highest priority,” is also incorrect. Interface priority in OSPF is used for DR (Designated Router) election, not router ID selection. Router ID selection has nothing to do with OSPF interface priorities.

In conclusion, Junos OS follows a logical and reliable procedure for selecting the OSPF router ID when one is not manually defined. It first checks for a configured router ID, then defaults to the highest IP address on a loopback interface, and finally, the highest IP address on a physical interface. This behavior ensures that the router ID remains stable and consistent, which is critical for the integrity of the OSPF routing process.

Thus, the correct answer is B – The Junos OS will use the IP address assigned to the loopback interface for the router ID.