View Full Juniper JN0-637 Exam Dumps and Practice Test Dumps

 

Question 301. Which BGP attribute is primarily used to prevent routing loops between autonomous systems?

1. MED
2. AS Path
3. Local Preference
4. Origin

Correct Answer: 2. AS Path

Explanation:
The BGP AS Path attribute records the sequence of autonomous systems through which a route has passed. When a BGP router receives a route advertisement containing its own autonomous system number in the AS Path, it can identify the route as a potential routing loop and reject it. This mechanism is fundamental to inter-domain BGP operation. Other BGP attributes serve different purposes. Local Preference influences the preferred exit point within an autonomous system, MED can influence the preferred entry point from a neighboring AS, and Origin indicates how a route entered BGP. AS Path is therefore the primary loop-prevention mechanism between autonomous systems.

Question 302. What does the BGP Local Preference attribute primarily influence?

1. The preferred exit path from an autonomous system
2. The number of MPLS labels pushed
3. The OSPF DR election
4. The IS-IS DIS election

Correct Answer: 1. The preferred exit path from an autonomous system

Explanation:
BGP Local Preference is used within an autonomous system to influence which exit point should be preferred for outbound traffic. A higher Local Preference value is generally preferred over a lower value. Network operators can use this attribute to control which external connection carries traffic leaving the autonomous system. Unlike MED, which can influence how neighboring autonomous systems select an entry point, Local Preference is an internal BGP attribute used to communicate preferred outbound paths among routers in the same AS. It does not affect MPLS label operations or IS-IS and OSPF election processes.

Question 303. Which BGP community prevents a route from being advertised outside the local autonomous system?

1. NO_ADVERTISE
2. NOPEER
3. NO_EXPORT
4. INTERNET

Correct Answer: 3. NO_EXPORT

Explanation:
The well-known BGP NO_EXPORT community instructs a BGP speaker not to advertise the associated route outside the local autonomous system or, in relevant environments, outside the local confederation. This makes it useful when an operator wants a route to remain within a particular routing domain. NO_ADVERTISE has a broader restriction and prevents the route from being advertised to any other BGP peer. INTERNET indicates normal advertisement behavior, while NOPEER provides more specific peer-related propagation control. Understanding these communities is important when implementing BGP route propagation policies.

Question 304. In BGP path selection, what does a lower MED value generally indicate?

1. A less-preferred path
2. A path that must be rejected
3. A higher Local Preference
4. A more-preferred entry path into the neighboring AS

Correct Answer: 4. A more-preferred entry path into the neighboring AS

Explanation:
The Multi-Exit Discriminator, or MED, is a BGP attribute that can be used to influence how a neighboring autonomous system selects among multiple entry points into an AS. When comparable routes are considered, a lower MED is generally preferred. This allows an AS to signal which connection it would prefer incoming traffic to use. MED is different from Local Preference, which influences outbound path selection within an AS. MED is also not a rejection mechanism. Its effect can depend on the BGP topology and configuration, but the standard path-selection principle is that a lower MED is preferred.

Question 305. What is the primary purpose of a BGP route reflector cluster ID?

1. To identify an MPLS label
2. To help prevent routing loops among route reflectors
3. To determine OSPF interface cost
4. To assign IPv6 link-local addresses

Correct Answer: 2. To help prevent routing loops among route reflectors

Explanation:
A BGP route reflector uses a cluster ID as part of its loop-prevention mechanism when reflecting routes. The cluster ID helps identify the route-reflector cluster so that a route-reflector can recognize routes that have already passed through the same cluster and avoid inappropriate reflection. This is particularly important in designs containing multiple route reflectors or redundant route-reflector paths. Cluster IDs are unrelated to MPLS labels, OSPF interface costs, or IPv6 address assignment. Route reflection itself reduces the requirement for a full iBGP mesh while retaining mechanisms to control route propagation and prevent routing loops.

Question 306. Which IS-IS level is primarily responsible for routing between different IS-IS areas?

1. Level 1
2. Level 0
3. Level 2
4. Level 3

Correct Answer: 3. Level 2

Explanation:
IS-IS Level 2 routing is responsible for connectivity between different IS-IS areas. Level 1 routers primarily maintain routing information within their local area, while Level 2 routers form the backbone that connects areas together. A router configured as Level 1-2 can participate in both functions. IS-IS does not define a Level 0 or Level 3 routing hierarchy for this purpose. Understanding the distinction between Level 1 and Level 2 is important when designing scalable IS-IS networks and troubleshooting routes that must cross area boundaries.

Question 307. Which IS-IS PDU is used to exchange information that describes a router’s link-state topology?

1. LSP
2. IIH
3. CSNP
4. PSNP

Correct Answer: 1. LSP

Explanation:
The IS-IS Link State PDU, or LSP, carries link-state and reachability information used by routers to construct their link-state databases. LSPs are flooded throughout the appropriate IS-IS level and contain information such as connectivity, metrics, and reachable prefixes through TLVs. IIH packets are used to establish and maintain adjacencies. CSNPs summarize database information, while PSNPs can request or acknowledge specific LSP information. By exchanging LSPs, IS-IS routers develop a consistent view of network topology and can calculate appropriate shortest paths.

Question 308. What is the primary purpose of IS-IS TLVs?

1. To replace IP addressing
2. To carry structured information within IS-IS PDUs
3. To encrypt IS-IS traffic
4. To establish TCP sessions

Correct Answer: 2. To carry structured information within IS-IS PDUs

Explanation:
Type-Length-Value, or TLV, structures allow IS-IS to carry different types of information within its protocol data units in a flexible and extensible format. Different TLVs can describe reachability, interface information, authentication, traffic-engineering information, and other routing details. This design has allowed IS-IS to evolve while maintaining a consistent protocol structure. TLVs do not replace IP addressing, provide general encryption, or establish TCP sessions. Understanding TLVs is important when examining IS-IS packets and troubleshooting why particular routing information is or is not being advertised.

Question 309. Which MPLS component performs label-based forwarding through the provider core?

1. Route Reflector
2. Label Switching Router
3. DHCP Server
4. DNS Resolver

Correct Answer: 2. Label Switching Router

Explanation:
A Label Switching Router, or LSR, performs label-based forwarding within an MPLS network. It receives an MPLS packet, examines the incoming label, determines the appropriate forwarding action, and typically swaps the label before sending the packet toward the next hop. Edge routers have additional roles at the boundaries of the MPLS network, including imposing or removing labels. MPLS forwarding therefore differs from traditional IP forwarding because the core can make forwarding decisions using labels associated with established forwarding equivalence classes. Route reflectors, DHCP servers, and DNS resolvers perform unrelated functions.

Question 310. What does an MPLS ingress Label Edge Router (LER) normally do when introducing an IP packet into an MPLS LSP?

1. Removes the IP header
2. Establishes an OSPF adjacency
3. Adds or imposes an MPLS label
4. Removes the final MPLS label

Correct Answer: 3. Adds or imposes an MPLS label

Explanation:
An ingress Label Edge Router introduces traffic into an MPLS domain and normally imposes an MPLS label based on the forwarding information associated with the packet. Once labeled, the packet can traverse the provider core using MPLS label operations. The ingress LER does not normally remove the IP header or establish OSPF adjacencies as part of the label-imposition operation. Removing an MPLS label is associated with label-pop operations, often occurring at or before the egress depending on the LSP design and PHP behavior. Understanding the ingress role helps distinguish edge processing from transit label switching.

Question 311. In an MPLS Layer 3 VPN, what is the purpose of a VRF?

1. To maintain a separate routing table for a VPN or customer
2. To elect the OSPF DR
3. To assign BGP communities automatically
4. To provide Ethernet loop prevention

Correct Answer: 1. To maintain a separate routing table for a VPN or customer

Explanation:
A Virtual Routing and Forwarding instance, or VRF, maintains a separate routing table and forwarding context for a particular VPN or customer. This allows multiple customers to use overlapping address spaces while keeping their routing information logically separated on a provider device. Route Distinguishers and Route Targets work with VPN routing information to provide uniqueness and control route distribution. VRFs are not used to elect OSPF designated routers, automatically assign BGP communities, or provide Ethernet loop prevention. They are a fundamental component of MPLS Layer 3 VPN architectures.

Question 312. Which Junos command displays the routes learned from a specific BGP neighbor?

1. show route advertising-protocol bgp <peer>
2. show bgp summary
3. show route receive-protocol bgp <peer>
4. show interfaces terse

Correct Answer: 3. show route receive-protocol bgp <peer>

Explanation:
The show route receive-protocol bgp <peer> command displays routes received from the specified BGP neighbor. This is useful when troubleshooting inbound route advertisements and determining which prefixes are being learned from a particular peer. In contrast, show route advertising-protocol bgp <peer> focuses on routes being advertised to a neighbor. show bgp summary provides a high-level view of BGP sessions and route counts, while show interfaces terse displays interface status. These commands provide different operational perspectives and should be selected according to whether the engineer is investigating received routes, advertised routes, sessions, or interfaces.

Question 313. Which Junos command is useful for viewing routes being advertised to a specific BGP peer?

1. show route advertising-protocol bgp <peer>
2. show route receive-protocol bgp <peer>
3. show chassis hardware
4. show ospf neighbor

Correct Answer: 1. show route advertising-protocol bgp <peer>

Explanation:
The show route advertising-protocol bgp <peer> command is used to examine routes that the local router is advertising to a specified BGP neighbor. It is particularly useful for troubleshooting outbound routing policy, export filters, and unexpected route advertisements. The corresponding receive-protocol command examines routes learned from a neighbor. show chassis hardware provides hardware inventory, while show ospf neighbor displays OSPF adjacency information. When troubleshooting BGP propagation, checking both received and advertised routes can help determine whether a problem exists with route learning, selection, policy, or outbound advertisement.

Question 314. What is the primary function of an OSPF Area Border Router (ABR)?

1. To provide MPLS label switching
2. To connect different OSPF areas
3. To elect the BGP route reflector
4. To replace the OSPF backbone

Correct Answer: 2. To connect different OSPF areas

Explanation:
An OSPF Area Border Router connects one or more non-backbone OSPF areas to the OSPF backbone, Area 0. An ABR maintains link-state information for the areas to which it is connected and can advertise summarized or inter-area routing information between them. This hierarchical design helps OSPF scale more efficiently than a single large area. An ABR does not replace Area 0, perform MPLS label switching, or elect BGP route reflectors. Proper ABR placement and backbone connectivity are important considerations when designing and troubleshooting multi-area OSPF networks.

Question 315. What is the purpose of OSPF Area 0?

1. It is the mandatory backbone area for inter-area OSPF connectivity
2. It is used only for IPv6 routing
3. It replaces all other OSPF areas
4. It is an MPLS forwarding domain

Correct Answer: 1. It is the mandatory backbone area for inter-area OSPF connectivity

Explanation:
OSPF Area 0 serves as the backbone area and provides the central connectivity structure for inter-area routing. Other OSPF areas are normally connected to Area 0 either directly or through appropriate virtual-link mechanisms where applicable. Area 0 is not limited to IPv6 and does not replace the other OSPF areas. MPLS forwarding is a separate technology that can coexist with OSPF as an underlying routing mechanism. Understanding the role of the backbone area is essential when designing multi-area OSPF and troubleshooting inter-area route propagation.

Question 316. Which protocol is commonly used to discover directly connected network devices at Layer 2?

1. BGP
2. LLDP
3. OSPF
4. LDP

Correct Answer: 2. LLDP

Explanation:
The Link Layer Discovery Protocol, or LLDP, is a Layer 2 neighbor-discovery protocol that allows directly connected network devices to advertise information about themselves. Depending on the implementation, this can include the device identity, interface information, system capabilities, and other management details. LLDP is particularly useful for network discovery and troubleshooting physical connectivity. BGP and OSPF are routing protocols operating at higher layers, while LDP is used to distribute MPLS labels. LLDP information can help administrators identify which device is connected to a particular interface without relying solely on manually maintained network documentation.

Question 317. What is the primary purpose of LACP in an Ethernet network?

1. To dynamically form and maintain a link aggregation bundle
2. To exchange BGP routes
3. To distribute MPLS labels
4. To elect an OSPF DR

Correct Answer: 4. To dynamically form and maintain a link aggregation bundle

Explanation:
The Link Aggregation Control Protocol, or LACP, is used to dynamically negotiate and maintain aggregated Ethernet links. Multiple physical interfaces can be combined into a logical bundle, providing increased aggregate bandwidth and redundancy. LACP helps ensure that participating interfaces meet the requirements for joining the aggregation group and can detect changes in link availability. LACP does not exchange BGP routes, distribute MPLS labels, or perform OSPF DR elections. Link aggregation is particularly useful in environments where multiple physical links need to operate as a single logical connection between network devices.

Question 318. Which IPv6 protocol mechanism performs neighbor discovery and router discovery functions?

1. ARP
2. DHCPv4
3. NDP
4. LACP

Correct Answer: 3. NDP

Explanation:
IPv6 uses Neighbor Discovery Protocol, or NDP, to provide functions that include neighbor discovery, address resolution, router discovery, and related reachability operations. NDP is based on ICMPv6 messages such as Router Solicitation, Router Advertisement, Neighbor Solicitation, and Neighbor Advertisement. IPv6 does not use ARP in the same way that IPv4 does. DHCPv4 is an IPv4 configuration mechanism, while LACP is an Ethernet link-aggregation protocol. Understanding NDP is important for troubleshooting IPv6 connectivity, default-router discovery, and neighbor reachability.

Question 319. Which Junos command compares the current candidate configuration with the active configuration?

1. show configuration
2. show | compare
3. rollback 1
4. commit check

Correct Answer: 2. show | compare

Explanation:
The Junos show | compare command is commonly used to display differences between the candidate configuration and the currently active configuration. It helps administrators review pending changes before committing them. This is particularly valuable when troubleshooting configuration changes or verifying that only the intended statements have been modified. show configuration displays configuration information but does not provide the same direct comparison. rollback loads or references previous configurations, while commit check validates configuration syntax and consistency without activating the changes. Reviewing differences before committing can help prevent unintended configuration changes.

Question 320. Which Junos command checks whether the candidate configuration is valid before committing it?

1. show route
2. commit confirmed
3. rollback 0
4. commit check

Correct Answer: 4. commit check

Explanation:
The commit check command validates the candidate configuration before it is committed as the active configuration. It checks configuration syntax and performs applicable validation without actually activating the changes. This makes it useful for identifying configuration errors before performing a normal commit. commit confirmed is instead used to temporarily commit changes that require confirmation within a specified interval. rollback 0 refers to the current active configuration state, while show route displays routing information. Using commit check is an important step when reviewing significant Junos configuration changes before activation.