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Question: 161. Which Junos command displays OSPF link-state database information?

1. show ospf database
   2. show bgp summary
   3. show route forwarding-table
   4. show ethernet-switching table

Correct Answer: 1. show ospf database

Explanation:

The show ospf database command displays the OSPF link-state database, including information about LSAs learned and originated by the router. The database represents the topology information that OSPF routers use to calculate shortest paths. Examining the database is useful when troubleshooting missing routes, unexpected topology information, or LSA propagation problems. Commands such as show bgp summary and show ethernet-switching table focus on different protocols or functions and do not display the OSPF link-state database.

Question: 162. Which BGP attribute is generally preferred when its value is higher during path selection?

1. MED
   2. Local Preference
   3. AS_PATH length
   4. Origin

Correct Answer: 2. Local Preference

Explanation:

Local Preference is a BGP attribute for influencing outbound path selection within an autonomous system, and a higher value is generally preferred. Administrators can assign different Local Preference values to routes learned through different external connections to control which exit point is selected. This attribute is propagated through iBGP so that multiple routers can follow a consistent outbound routing preference. MED differs because a lower MED is generally preferred when the relevant comparison applies.

Question: 163. Which IPv4 prefix represents the private address range 192.168.0.0 through 192.168.255.255?

1. 172.16.0.0/12
   2. 10.0.0.0/8
   3. 192.168.0.0/16
   4. 169.254.0.0/16

Correct Answer: 3. 192.168.0.0/16

Explanation:

The 192.168.0.0/16 prefix is an RFC 1918 private IPv4 address range. It includes addresses from 192.168.0.0 through 192.168.255.255 and is commonly used in residential, enterprise, and laboratory networks. Private addresses are not intended to be directly routed across the public Internet. The other RFC 1918 private ranges are 10.0.0.0/8 and 172.16.0.0/12. The 169.254.0.0/16 range is used for IPv4 link-local addressing rather than RFC 1918 private addressing.

Question: 164. What is the primary purpose of BGP route filtering through routing policy?

1. To control which routes are accepted or advertised
   2. To calculate Ethernet frame checksums
   3. To assign IPv6 link-local addresses
   4. To elect an OSPF Designated Router

Correct Answer: 1. To control which routes are accepted or advertised

Explanation:

Routing policies can control which BGP routes are accepted, rejected, modified, or advertised to other peers. Policies can match prefixes, communities, AS paths, protocol attributes, and other route characteristics and then apply appropriate actions. This allows administrators to implement routing controls without relying solely on the default behavior of BGP. Routing policy is therefore an important tool for managing route propagation, traffic engineering, and routing security.

Question: 165. Which OSPF neighbor state occurs after successful database synchronization?

1. Init
   2. Exchange
   3. Full
   4. Down

Correct Answer: 3. Full

Explanation:

The OSPF Full state indicates that neighboring routers have completed the required database synchronization and have matching link-state information for the adjacency. This is the normal fully established state for routers that need a full adjacency. Earlier states include Down, Init, 2-Way, ExStart, and Exchange. If an adjacency remains stuck in a state such as ExStart or Exchange, troubleshooting may involve checking MTU values, network types, timers, router IDs, and other configuration parameters.

Question: 166. Which Junos command provides a concise view of interface operational and administrative status?

1. show interfaces terse
   2. show route detail
   3. show ospf database
   4. show system alarms

Correct Answer: 1. show interfaces terse

Explanation:

The show interfaces terse command provides a concise summary of interface information, including administrative and operational states and addressing information where applicable. It is especially useful as a quick first step when checking whether an interface is physically and logically operational. The detailed show interfaces command provides substantially more information, while routing and system commands focus on other areas. Checking interface state is often essential before troubleshooting higher-level routing protocols.

Question: 167. Which protocol uses a hop-count metric and considers 15 hops the maximum reachable distance?

1. OSPF
   2. RIP
   3. BGP
   4. IS-IS

Correct Answer: 2. RIP

Explanation:

RIP is a distance-vector routing protocol that uses hop count as its routing metric. A route with 1 through 15 hops can be considered reachable, while 16 represents an unreachable destination. This relatively small maximum makes RIP unsuitable for many large modern networks. OSPF and IS-IS use link-state calculations, while BGP uses a path-vector approach with multiple attributes for route selection. RIP’s simple metric is one of its defining characteristics.

Question: 168. Which BGP mechanism helps prevent routing loops between autonomous systems?

1. AS_PATH loop detection
   2. OSPF DR election
   3. LACP negotiation
   4. IPv6 SLAAC

Correct Answer: 1. AS_PATH loop detection

Explanation:

BGP uses the AS_PATH attribute as an important mechanism for preventing routing loops. When a route is advertised through eBGP, the advertising router normally adds its autonomous system number to the AS_PATH. If a router receives a route advertisement containing its own AS number, it can recognize that the route has already traversed its autonomous system and reject it as a loop. This mechanism is fundamental to the scalability and stability of inter-domain BGP routing.

Question: 169. Which IPv6 feature automatically detects whether an address is already in use on a local link?

1. MED
   2. Duplicate Address Detection
   3. Route reflection
   4. LACP

Correct Answer: 2. Duplicate Address Detection

Explanation:

IPv6 Duplicate Address Detection, or DAD, is part of Neighbor Discovery and is used to determine whether an address is already in use before an interface begins using it. The process uses ICMPv6 Neighbor Solicitation and Neighbor Advertisement behavior. DAD helps prevent two interfaces on the same link from unintentionally using the same IPv6 address. It is an IPv6 host and interface function rather than a BGP, OSPF, or Ethernet link-aggregation feature.

Question: 170. Which Junos configuration command is commonly used to verify that a candidate configuration has no syntax or structural errors before committing?

1. commit check
   2. show route
   3. show interfaces terse
   4. rollback 0

Correct Answer: 1. commit check

Explanation:

The commit check command validates the candidate configuration without activating it as the committed configuration. It is useful before committing changes because it can identify configuration syntax, hierarchy, or other validation problems. A successful commit check does not itself make the candidate configuration active. An administrator can then use commit to activate the validated configuration. This workflow provides a useful safeguard when making significant Junos configuration changes.

Question: 171. Which OSPF LSA is generated by an ABR to advertise networks from one OSPF area into another?

1. Type 5
   2. Type 2
   3. Type 3
   4. Type 1

Correct Answer: 3. Type 3

Explanation:

OSPF Type 3 Summary LSAs are generated by Area Border Routers to advertise inter-area network information. They allow routers in one OSPF area to learn about networks located in another area. Type 1 Router LSAs describe individual routers within an area, Type 2 Network LSAs describe multi-access networks through the DR, and Type 5 LSAs advertise external routes. Understanding the LSA types is important for troubleshooting inter-area and external OSPF routing behavior.

Question: 172. Which BGP attribute identifies the next-hop IP address that should be used to reach a route?

1. Origin
   2. NEXT_HOP
   3. Local Preference
   4. MED

Correct Answer: 2. NEXT_HOP

Explanation:

The BGP NEXT_HOP attribute identifies the IP address that a router should use as the next hop when forwarding traffic toward the advertised destination. A BGP route generally needs a reachable next hop to become usable in the routing table. If the next-hop address cannot be resolved through the local routing information, the BGP route may not be installed as an active route. NEXT_HOP therefore plays an important role in connecting BGP control-plane information with actual packet forwarding.

Question: 173. Which Junos command displays information about the routing protocol that installed a route?

1. show route protocol
   2. show system uptime
   3. show chassis hardware
   4. show ethernet-switching table

Correct Answer: 1. show route protocol

Explanation:

The show route protocol command can be used to examine routes based on the protocol that contributed them, such as BGP, OSPF, or static routing. This is useful for determining where routing information originated and for troubleshooting route selection. An administrator can use the protocol-specific form to focus on routes learned through a particular routing source. This differs from interface, hardware, and switching-table commands, which provide information about other parts of the device.

Question: 174. Which routing protocol uses TCP port 179 for its sessions?

1. OSPF
   2. RIP
   3. BGP
   4. IS-IS

Correct Answer: 3. BGP

Explanation:

BGP establishes its transport sessions using TCP port 179. TCP provides reliable and ordered delivery for BGP messages between peers. When troubleshooting BGP session establishment, connectivity to TCP port 179 is therefore an important consideration. OSPF operates directly over IP, RIP uses UDP port 520 for IPv4, and IS-IS operates directly at the network-access layer rather than using TCP port 179.

Question: 175. What is the main benefit of Equal-Cost Multipath routing?

1. It allows multiple equal-cost paths to be used for forwarding
   2. It prevents all dynamic routing updates
   3. It replaces the need for routing tables
   4. It converts IPv4 traffic into IPv6 traffic

Correct Answer: 1. It allows multiple equal-cost paths to be used for forwarding

Explanation:

Equal-Cost Multipath, or ECMP, allows a router to use multiple paths that have equivalent routing costs toward the same destination. Instead of installing and using only one equal-cost route, the device can maintain multiple forwarding paths and distribute traffic according to its forwarding behavior. ECMP can improve utilization and provide additional redundancy. It does not eliminate routing tables or perform protocol translation between IPv4 and IPv6.

Question: 176. Which IPv6 address type is intended only for communication on the local link?

1. Global unicast
   2. Unique local
   3. Link-local
   4. Multicast

Correct Answer: 3. Link-local

Explanation:

IPv6 link-local addresses are designed for communication on the local network segment and are not routed between links. They use the FE80::/10 prefix and are automatically associated with IPv6-enabled interfaces in many configurations. Link-local addresses are important for Neighbor Discovery and can also be used by routing protocols. Global unicast addresses are intended for broader routed communication, while unique local addresses are used for private internal addressing and multicast addresses identify groups of interfaces.

Question: 177. Which Junos command displays information about the current system alarms?

1. show system alarms
   2. show route
   3. show bgp summary
   4. show ospf neighbor

Correct Answer: 1. show system alarms

Explanation:

The show system alarms command provides information about active alarms on a Junos device. These alarms can indicate hardware, environmental, or other system conditions requiring attention. Checking alarms can be useful when investigating device instability or unexpected behavior because some network problems may be related to an underlying hardware or environmental condition. Routing and protocol commands do not provide the same system-level alarm information.

Question: 178. Which BGP path attribute is normally preferred when the AS_PATH is shorter, assuming other relevant selection factors do not override it?

1. AS_PATH
   2. NEXT_HOP
   3. MED
   4. Community

Correct Answer: 1. AS_PATH

Explanation:

AS_PATH length is one of the factors considered during BGP route selection. When the relevant preceding attributes are equal and the comparison reaches AS_PATH length, a shorter AS_PATH is generally preferred. The attribute also serves as an important loop-prevention mechanism because it records the autonomous systems through which the route has passed. NEXT_HOP identifies the forwarding next hop, MED can influence inbound path selection, and communities provide policy signaling rather than directly representing AS-path length.

Question: 179. Which protocol is primarily used to provide first-hop gateway redundancy rather than exchanging routing information?

1. BGP
   2. VRRP
   3. OSPF
   4. IS-IS

Correct Answer: 2. VRRP

Explanation:

VRRP is a first-hop redundancy protocol designed to provide highly available default-gateway functionality. Multiple routers can participate in a virtual router, allowing hosts to use a shared virtual gateway address. If the active router becomes unavailable, another participating router can assume the forwarding role. Unlike BGP, OSPF, and IS-IS, VRRP is not primarily used to exchange network reachability information and calculate routes throughout a network.

Question: 180. Which Junos command can be used to display the current routing table?

1. show route
   2. show configuration
   3. show chassis hardware
   4. show system uptime

Correct Answer: 1. show route

Explanation:

The show route command displays routing-table information on a Junos device. The output can include destination prefixes, route protocol, preference, next-hop information, and active or inactive route details. It is one of the fundamental operational commands for verifying Layer 3 connectivity and troubleshooting route selection. More specific forms of the command can be used to inspect particular protocols, prefixes, or routing tables. The other commands provide configuration, hardware, and system-uptime information rather than routing-table contents.