JNCIA-Junos (JN0-105) Comprehensive Exam: Set 2 With A+ Graded
Solutions (2026/2027)
1. Describe the primary function of the "Forwarding Table" (FIB) and explain how it differs from
the "Routing Table" (RIB) in terms of its location and content.
==CORRECT ANSWER:== The RIB (Routing Table) resides on the Routing Engine (RE) and
contains all routes learned from all protocols. The FIB (Forwarding Table) is a subset of the
"best" routes that is pushed to the Packet Forwarding Engine (PFE) hardware to allow for
wire-speed packet switching.
2. Explain the "Shared" configuration mode in Junos and identify the primary risk associated
with using it when multiple administrators are logged in.
==CORRECT ANSWER:== The shared mode (entered via configure) allows multiple users to
edit the same candidate configuration simultaneously. The risk is that one user may
inadvertently commit changes made by another user, as a commit command applies all
pending changes in the candidate database, regardless of who made them.
3. Define the "Internal Ethernet Link" and explain its significance in the communication
between the Routing Engine and the Packet Forwarding Engine.
==CORRECT ANSWER:== The internal Ethernet link is the control plane connection between
the RE and the PFE. It is used to pass routing table updates to the PFE and to send "exception
traffic" (like protocol packets or ICMP) from the PFE back to the RE for processing.
4. Explain how Junos OS handles a "Static Route" with multiple next-hops and what the default
behavior is for traffic destined for that network.
==CORRECT ANSWER:== Junos allows for the configuration of multiple next-hops for a single
static route. By default, it will select one next-hop based on internal hashing; however, if
"per-packet load balancing" is configured in the forwarding options, it will distribute traffic
across all valid next-hops.
5. Describe the "Qualified Next-Hop" feature within a static route configuration and provide a
practical use case for it.
==CORRECT ANSWER:== A qualified next-hop allows you to assign a specific preference value
to an individual next-hop within a static route. This is commonly used to create a "floating
static route," where a primary path has a lower preference and a backup path (the qualified
next-hop) has a higher preference and only becomes active if the primary fails.
6. What is the role of the "dcd" (Device Control Daemon) process in the Junos OS architecture?
==CORRECT ANSWER:== The dcd is responsible for managing the configuration and
operational state of the router's interfaces. It acts as the interface between the configuration
database and the physical hardware drivers.
7. Explain the "Term" structure within a Routing Policy and describe what happens if a route
does not match any of the defined terms in a policy.
==CORRECT ANSWER:== A Routing Policy consists of named "terms" that act as if-then
statements. If a route does not match any term in a policy, it is passed to the next policy in
the chain; if no further policies exist, the protocol's "default policy" (such as "reject all" for
BGP export) is applied.
, 8. Define "Martian Routes" and explain why they are filtered out of the routing table by default.
==CORRECT ANSWER:== Martians are invalid or reserved IP addresses (like 127.0.0.0/8 or
0.0.0.0/8) that should never appear in a global routing table. Junos ignores these addresses
to maintain routing stability and prevent security vulnerabilities.
9. Describe the specific purpose of the "apply-groups" command and how it relates to the
"[edit groups]" hierarchy.
==CORRECT ANSWER:== The apply-groups command is used to map a pre-defined
configuration template (created under [edit groups]) to a specific part of the configuration
hierarchy, allowing for the rapid application of standardized settings across multiple
interfaces or protocols.
10. What is the "System Alarms" indicator, and how can an administrator view the specific
hardware or software issues causing an alarm from the CLI?
==CORRECT ANSWER:== System Alarms indicate hardware failures or configuration issues
(like a missing root password). An administrator can view these by issuing the show system
alarms command in operational mode.
11. Explain the difference between "Family Inet" and "Family ISO" when configuring a logical unit
on an interface.
==CORRECT ANSWER:== family inet is used to enable IPv4 packet processing on the
interface, while family iso is required if the interface will participate in the IS-IS routing
protocol, which uses ISO network entity titles (NETs).
12. Describe the "Active/Backup" relationship in a Dual-RE (Routing Engine) system and explain
what "GRES" provides during a failure.
==CORRECT ANSWER:== In a Dual-RE system, one RE is Primary (active) and the other is
Backup (standby). Graceful Routing Engine Switchover (GRES) ensures that the PFE continues
to forward traffic using the existing forwarding table during an RE failover, preventing a total
network outage.
13. What is the function of the "mgd" (Management Daemon) in Junos?
==CORRECT ANSWER:== The mgd manages all user interactions with the router, including
the CLI, the J-Web interface, and API requests. It is responsible for parsing commands and
managing the configuration database.
14. Explain the "Route Preference" value for OSPF External (Type 5) LSA routes.
==CORRECT ANSWER:== In Junos, OSPF External routes have a default preference of 150,
which is less preferred (higher value) than OSPF Internal routes (preference 10).
15. Describe the "monitor interface" command and how it differs from "show interfaces".
==CORRECT ANSWER:== show interfaces provides a static snapshot of interface
statistics. monitor interface provides a real-time, refreshing display of traffic rates (bps and
pps) and error counters for a specific interface.
16. What is the "Factory Default" configuration, and which command is used to revert a device
to this state?
==CORRECT ANSWER:== The factory default is the minimum configuration Juniper provides
out-of-the-box. It is applied by using the load factory-default command while in configuration
mode.
Solutions (2026/2027)
1. Describe the primary function of the "Forwarding Table" (FIB) and explain how it differs from
the "Routing Table" (RIB) in terms of its location and content.
==CORRECT ANSWER:== The RIB (Routing Table) resides on the Routing Engine (RE) and
contains all routes learned from all protocols. The FIB (Forwarding Table) is a subset of the
"best" routes that is pushed to the Packet Forwarding Engine (PFE) hardware to allow for
wire-speed packet switching.
2. Explain the "Shared" configuration mode in Junos and identify the primary risk associated
with using it when multiple administrators are logged in.
==CORRECT ANSWER:== The shared mode (entered via configure) allows multiple users to
edit the same candidate configuration simultaneously. The risk is that one user may
inadvertently commit changes made by another user, as a commit command applies all
pending changes in the candidate database, regardless of who made them.
3. Define the "Internal Ethernet Link" and explain its significance in the communication
between the Routing Engine and the Packet Forwarding Engine.
==CORRECT ANSWER:== The internal Ethernet link is the control plane connection between
the RE and the PFE. It is used to pass routing table updates to the PFE and to send "exception
traffic" (like protocol packets or ICMP) from the PFE back to the RE for processing.
4. Explain how Junos OS handles a "Static Route" with multiple next-hops and what the default
behavior is for traffic destined for that network.
==CORRECT ANSWER:== Junos allows for the configuration of multiple next-hops for a single
static route. By default, it will select one next-hop based on internal hashing; however, if
"per-packet load balancing" is configured in the forwarding options, it will distribute traffic
across all valid next-hops.
5. Describe the "Qualified Next-Hop" feature within a static route configuration and provide a
practical use case for it.
==CORRECT ANSWER:== A qualified next-hop allows you to assign a specific preference value
to an individual next-hop within a static route. This is commonly used to create a "floating
static route," where a primary path has a lower preference and a backup path (the qualified
next-hop) has a higher preference and only becomes active if the primary fails.
6. What is the role of the "dcd" (Device Control Daemon) process in the Junos OS architecture?
==CORRECT ANSWER:== The dcd is responsible for managing the configuration and
operational state of the router's interfaces. It acts as the interface between the configuration
database and the physical hardware drivers.
7. Explain the "Term" structure within a Routing Policy and describe what happens if a route
does not match any of the defined terms in a policy.
==CORRECT ANSWER:== A Routing Policy consists of named "terms" that act as if-then
statements. If a route does not match any term in a policy, it is passed to the next policy in
the chain; if no further policies exist, the protocol's "default policy" (such as "reject all" for
BGP export) is applied.
, 8. Define "Martian Routes" and explain why they are filtered out of the routing table by default.
==CORRECT ANSWER:== Martians are invalid or reserved IP addresses (like 127.0.0.0/8 or
0.0.0.0/8) that should never appear in a global routing table. Junos ignores these addresses
to maintain routing stability and prevent security vulnerabilities.
9. Describe the specific purpose of the "apply-groups" command and how it relates to the
"[edit groups]" hierarchy.
==CORRECT ANSWER:== The apply-groups command is used to map a pre-defined
configuration template (created under [edit groups]) to a specific part of the configuration
hierarchy, allowing for the rapid application of standardized settings across multiple
interfaces or protocols.
10. What is the "System Alarms" indicator, and how can an administrator view the specific
hardware or software issues causing an alarm from the CLI?
==CORRECT ANSWER:== System Alarms indicate hardware failures or configuration issues
(like a missing root password). An administrator can view these by issuing the show system
alarms command in operational mode.
11. Explain the difference between "Family Inet" and "Family ISO" when configuring a logical unit
on an interface.
==CORRECT ANSWER:== family inet is used to enable IPv4 packet processing on the
interface, while family iso is required if the interface will participate in the IS-IS routing
protocol, which uses ISO network entity titles (NETs).
12. Describe the "Active/Backup" relationship in a Dual-RE (Routing Engine) system and explain
what "GRES" provides during a failure.
==CORRECT ANSWER:== In a Dual-RE system, one RE is Primary (active) and the other is
Backup (standby). Graceful Routing Engine Switchover (GRES) ensures that the PFE continues
to forward traffic using the existing forwarding table during an RE failover, preventing a total
network outage.
13. What is the function of the "mgd" (Management Daemon) in Junos?
==CORRECT ANSWER:== The mgd manages all user interactions with the router, including
the CLI, the J-Web interface, and API requests. It is responsible for parsing commands and
managing the configuration database.
14. Explain the "Route Preference" value for OSPF External (Type 5) LSA routes.
==CORRECT ANSWER:== In Junos, OSPF External routes have a default preference of 150,
which is less preferred (higher value) than OSPF Internal routes (preference 10).
15. Describe the "monitor interface" command and how it differs from "show interfaces".
==CORRECT ANSWER:== show interfaces provides a static snapshot of interface
statistics. monitor interface provides a real-time, refreshing display of traffic rates (bps and
pps) and error counters for a specific interface.
16. What is the "Factory Default" configuration, and which command is used to revert a device
to this state?
==CORRECT ANSWER:== The factory default is the minimum configuration Juniper provides
out-of-the-box. It is applied by using the load factory-default command while in configuration
mode.
