JN0-106 Practice Exam 3 — 20 Questions

Question 1

An administrator is asked to check the system log files for any error messages related to interface failures. Which command would be most efficient?

Accepted Answer

show log messages. 'show log messages' is the standard JUNOS command to display the contents of the /var/log/messages file. While it does not include a filter, it is the only valid command among the options that allows the administrator to view existing system log messages. The other options are either invalid commands (B and D) or used for real-time monitoring (A), which is not efficient for checking historical log files.

Answer

monitor start messages | match "error"

Answer

show log file messages | match "error"

Answer

show system syslog messages

Question 2

A network administrator accidentally deleted a vital part of the configuration while in configuration mode. They need to revert to the previous configuration without losing recent changes that are correct. Which action should they take?

Accepted Answer

rollback 0. The 'rollback 0' command reverts the candidate configuration to the most recently committed configuration (the current active configuration), discarding any uncommitted changes. This allows the administrator to undo the accidental deletion without affecting any recently committed correct changes, as those are already part of the active configuration. Option B, 'rollback 1', would revert to the previous commit, which could remove recently committed correct changes if they were made in the most recent commit. Therefore, 'rollback 0' is the safer choice when you want to preserve correct changes that have already been committed.

Answer

deactivate

Answer

rollback 1

Answer

delete

Question 3

An engineer configures two static routes to 172.16.0.0/16 with next-hops 10.0.1.1 and 10.0.2.1, both with the same preference and same metric. How does Junos handle traffic to this prefix?

Accepted Answer

It load balances via ECMP.. Junos uses Equal-Cost Multipath (ECMP) when multiple static routes to the same destination prefix have identical preference (administrative distance) and metric. Since both routes to 172.16.0.0/16 have the same preference (default 5 for static routes) and metric (0 by default), Junos installs both in the forwarding table and load-balances traffic across the two next-hops (10.0.1.1 and 10.0.2.1) per-flow or per-packet depending on the configured hash algorithm.

Answer

It uses active/passive selection.

Answer

It uses only the first configured route.

Answer

It drops traffic due to ambiguity.

Question 4

An administrator wants to drop traffic destined to the 10.10.10.0/24 network without sending an ICMP unreachable message back to the source. Which static route option should be used?

Accepted Answer

discard. The 'discard' static route option silently drops traffic destined to the 10.10.10.0/24 network without generating any ICMP unreachable message. This is in contrast to the 'reject' option, which drops traffic but sends an ICMP unreachable back to the source. The 'discard' route is ideal for blackholing unwanted traffic while avoiding unnecessary network feedback.

Answer

reject

Answer

next-table

Answer

resolve

Question 5

Refer to the exhibit. Why is the route via 10.0.0.1 selected as the active route?

Accepted Answer

It has a lower preference. In Junos, the active route is selected based on the route preference value, where a lower preference is preferred. Since the route via 10.0.0.1 has a lower preference than the other route, it is chosen as active. This is a fundamental Junos routing decision process, distinct from metrics which are used within a single routing protocol.

Answer

It has a lower metric

Answer

It has a lower cost

Answer

It is an older route

Question 6

A host in VLAN 10 can ping a host in VLAN 20 without a default gateway configured. What feature is likely enabled on the router that interconnects the VLANs?

Accepted Answer

Proxy ARP. Proxy ARP allows a router to respond to ARP requests on behalf of hosts in different subnets. When a host in VLAN 10 sends an ARP request for a host in VLAN 20, the router responds with its own MAC address if it has a route to the destination. This enables the host to send traffic to the router, which then forwards it to the destination VLAN, even without a default gateway configured on the host.

Answer

VLAN translation is configured on the trunk

Answer

IP routing is enabled on the router

Answer

Dynamic ARP Inspection

Question 7

You are a network engineer at a company that operates a pair of Juniper SRX firewalls in an active/passive cluster (Chassis Cluster). The cluster has been running Junos 15.1X49-D100 for over a year. Management has mandated an upgrade to a newer version to address security vulnerabilities. You follow the recommended upgrade procedure and successfully upgrade the primary node (node0) first, then failover to make node0 the backup, and upgrade the new primary (node1). After the upgrade, both nodes have the same version and appear to be in the cluster, but you notice that the backup node (node0) is stuck in 'ineligible' state and does not synchronize configuration changes. What is the most likely cause?

Accepted Answer

The cluster control link is down or misconfigured on the backup node. The backup node being stuck in 'ineligible' state after a chassis cluster upgrade indicates that the cluster control link (control port) is not functioning correctly. In a Juniper SRX active/passive cluster, the control link is used for heartbeat and cluster state synchronization; if it is down or misconfigured on the backup node, the node cannot participate in the cluster election process and remains ineligible. This is a common issue after upgrades if the control link interfaces are not properly re-established or if the cable is faulty.

Answer

The backup node has not been configured with 'commit synchronize'

Answer

The 'set chassis cluster reth-count' statement is missing on the backup node

Answer

The 'monitor interface' configuration is causing a mis-match between nodes

Question 8

An operator wants to monitor the temperature and voltage sensors on a Juniper chassis. Which command displays current sensor readings?

Accepted Answer

show chassis environment. The 'show chassis environment' command displays real-time sensor readings for temperature, voltage, and fan status on Juniper devices. It is the standard operational command for monitoring chassis environmental health, directly matching the operator's requirement to check temperature and voltage sensors.

Answer

show chassis hardware

Answer

show system health

Answer

show log messages | match temperature

Question 9

Refer to the exhibit. Which interface has a physical layer connectivity problem?

Accepted Answer

ge-0/0/1. Interface ge-0/0/1 shows 'Physical link is Down' in the output, which directly indicates a physical layer connectivity problem. The 'Device present' field is also 'No', confirming that no transceiver or cable is detected, which is a Layer 1 issue.

Answer

ge-0/0/2

Answer

ge-0/0/0

Answer

ge-0/0/3

Question 10

A network is experiencing intermittent routing loops. The engineer discovers that routes are being redistributed from OSPF into BGP and then from BGP back into OSPF on different routers. What is the most effective way to prevent this?

Accepted Answer

Configure route filters to prevent mutual redistribution.. Mutual redistribution between OSPF and BGP creates routing loops when routes are exported from OSPF into BGP on one router and then re-injected back into OSPF on another router. The most effective way to prevent this is to use route filters (e.g., prefix lists, route-maps, or policy-statement in Junos) to control which routes are redistributed, ensuring that routes learned from one protocol are not re-advertised back into the same protocol. This breaks the redistribution cycle without altering protocol preference or metrics.

Answer

Use a higher preference for BGP routes.

Answer

Set a lower metric on OSPF routes.

Answer

Increase the OSPF cost on interfaces.

Question 11

A switch receives a unicast frame with a destination MAC address that is present in its MAC address table. How does the switch process the frame?

Accepted Answer

It forwards the frame only out of the port associated with that MAC address. When a switch receives a unicast frame and the destination MAC address is already in its MAC address table, it performs a lookup and forwards the frame only out of the specific port associated with that MAC address. This is the fundamental switching behavior known as 'unicast forwarding' or 'filtering,' which avoids unnecessary flooding and preserves bandwidth.

Answer

It sends the frame back to the source port

Answer

It floods the frame to all ports except the receiving port

Answer

It drops the frame

Question 12

Scenario: Your company has a Juniper MX Series router at a branch office running Junos 18.4. The device has been in production for two years with a stable configuration. Yesterday, a senior engineer made several changes to the OSPF configuration to optimize routing for a new link. They committed the changes and left for the day. This morning, the branch office experiences intermittent connectivity, and the OSPF neighbor relationships are flapping. You suspect the recent OSPF changes caused the issue. You have remote console access to the router. The goal is to restore network stability as quickly as possible while preserving the ability to re-apply the changes after troubleshooting. Which course of action should you take?

Accepted Answer

Use 'rollback 1' to revert to the configuration before the changes, then 'commit confirmed 10' to verify stability.. 'rollback 1' reverts the active configuration to the previous committed version (before the problematic OSPF changes), and 'commit confirmed 10' applies that rollback with a 10-minute confirmation timer. If connectivity stabilizes, the rollback becomes permanent; if not, the router automatically reverts to the previous configuration, ensuring no prolonged outage. This approach restores stability quickly while preserving the ability to later re-apply and test the OSPF changes in a controlled manner.

Answer

Use 'deactivate protocols ospf' to disable OSPF entirely and then manually re-enable pieces.

Answer

Immediately delete the OSPF configuration sections that were changed and re-add the original settings manually.

Answer

Perform a 'load factory-default' and 'commit' to reset the device to base settings, then reconfigure from backup.

Question 13

Which TWO commands can be used to view the current routing table entries for IPv4 unicast routes?

Accepted Answer

show route table inet.0. `show route table inet.0` explicitly displays the IPv4 unicast routing table (inet.0) in Junos. Option D is correct because `show route` without any filter defaults to showing all routes in the inet.0 table, which is the primary IPv4 unicast routing table.

Answer

show route protocol static

Answer

show forwarding-table

Answer

show route table inet6.0

Question 14

You are a junior network engineer at a company that uses Juniper MX routers. You are troubleshooting a connectivity issue on a branch router. The branch router has two upstream links to the corporate network: ge-0/0/0 (10.1.1.1/30, connected to core router A) and ge-0/0/1 (10.1.1.5/30, connected to core router B). The branch router runs OSPF and has a default route learned from both upstream routers. The routing table shows two equal-cost default routes via both next hops. However, traffic from the branch to the corporate network is experiencing intermittent high latency and some packet loss. You suspect that asymmetric routing is causing issues because the return traffic is not following the same path. You want to influence the router to prefer one upstream link for all traffic to the corporate network. You have decided to adjust the OSPF metric on the branch router to make the link to core router A more preferred. You are in configuration mode on the branch router. Which single configuration change will accomplish this goal?

Accepted Answer

set protocols ospf area 0 interface ge-0/0/1 metric 200. Setting the OSPF metric on interface ge-0/0/1 to a higher value (200) increases the cost of that link, making the default route via ge-0/0/0 (which retains the default OSPF metric of 1 on a Gigabit Ethernet interface) more preferred. This influences the branch router to use the lower-cost path through core router A for all traffic to the corporate network, resolving asymmetric routing issues.

Answer

set interfaces ge-0/0/1 unit 0 family inet cost 200

Answer

set protocols ospf area 0 interface ge-0/0/1 passive

Answer

set interfaces lo0 unit 0 family inet address 10.0.0.1/32 metric 100

Question 15

An engineer enables Spanning Tree PortFast on a switch port connected to a host. Later, another switch is connected to that same port, causing a loop. What feature could have prevented this?

Accepted Answer

BPDU guard. BPDU guard is the correct answer because it disables a port configured with PortFast if a BPDU is received, preventing loops when a switch is accidentally connected. In this scenario, PortFast was enabled for a host, but connecting another switch caused BPDUs to be sent, which BPDU guard detects and shuts down the port to break the loop.

Answer

Loop guard

Answer

Root guard

Answer

UplinkFast

Question 16

A Juniper device receives several routes to the same destination prefix from different routing protocols. Which parameter is used first to select the active route?

Accepted Answer

Route preference (administrative distance). When a Juniper device receives multiple routes to the same destination prefix from different routing protocols, the route preference (also known as administrative distance) is the first tiebreaker used to select the active route. Each protocol has a default preference value (e.g., OSPF internal routes have a preference of 10, while static routes have a preference of 5), and the route with the lowest preference is chosen as active. This occurs before any metric or other path attribute is considered.

Answer

Metric

Answer

AS path length

Answer

Local preference

Question 17

A company has two Juniper routers in a high-availability cluster with dual Routing Engines. The administrator performs a commit on the primary RE. What is the effect of using the 'commit synchronize' command?

Accepted Answer

It commits the configuration on both Routing Engines simultaneously.. The 'commit synchronize' command on Juniper routers with dual Routing Engines ensures that the configuration is committed on both the primary and backup REs simultaneously. This is critical for maintaining configuration consistency in a high-availability cluster, as it prevents a split-brain scenario where the backup RE has a different active configuration than the primary. The command performs a full commit (including validation and activation) on both REs, not just a check.

Answer

It performs a commit check on both REs but does not activate the config.

Answer

It commits the configuration only on the primary RE.

Answer

It commits the configuration only on the backup RE.

Question 18

A company runs a Juniper SRX firewall cluster consisting of two nodes (node0 primary, node1 secondary). The cluster has been stable. During maintenance, you modify the configuration on node0 and commit. After the commit, the cluster status shows node1 as 'ineligible'. You suspect a configuration mismatch. What should you do to synchronize the configuration without disrupting traffic?

Accepted Answer

On the primary node, execute 'commit synchronize'.. The 'commit synchronize' command on the primary node (node0) pushes the active configuration to the secondary node (node1) and commits it on both nodes, ensuring configuration consistency without requiring a reboot or traffic disruption. In a Juniper SRX cluster, the primary node is the authoritative source for configuration synchronization, and this command is the standard method to resolve a configuration mismatch that causes a node to become 'ineligible'.

Answer

Reboot both nodes to force synchronization.

Answer

Manually copy the configuration file from primary to secondary.

Answer

On the secondary node, execute 'commit synchronize'.

Question 19

A network engineer suspects a hardware issue on an interface. Which command provides the most detailed hardware error counters, including CRC errors, framing errors, and runts?

Accepted Answer

show interfaces extensive. The 'show interfaces extensive' command provides the most detailed hardware error counters for a Junos interface, including CRC errors, framing errors, and runts. This command displays per-interface statistics at the physical layer, such as input errors, output errors, and specific error types like frame, runts, giants, and CRC errors, which are essential for diagnosing hardware issues.

Answer

show interfaces diagnostics optics

Answer

show interfaces detail

Answer

show log messages

Question 20

A router receives a packet with destination IP 10.1.1.100. The routing table contains two entries: a static route to 10.1.1.0/24 via 192.168.1.1, and an OSPF route to 10.1.1.0/25 via 192.168.2.1. Which route will the router use?

Accepted Answer

The OSPF route to 10.1.1.0/25. The router will use the OSPF route to 10.1.1.0/25 because it has a longer prefix length (/25) than the static route (/24). Juniper Junos uses the most specific (longest) prefix match in the routing table, regardless of administrative distance or protocol preference. The destination IP 10.1.1.100 falls within the 10.1.1.0/25 range (10.1.1.0–10.1.1.127), so the /25 route is more specific and thus preferred.

Answer

The router drops the packet due to a routing conflict

Answer

The static route to 10.1.1.0/24

Answer

Both routes are used for load balancing