JNCIA-JUNOS Networking Fundamentals — 10 Questions

Question 1

A network administrator notices that traffic between two VLANs is not reaching its destination. The switch has an IRB interface configured with an IP address in each VLAN's subnet. What is the most likely missing configuration?

Accepted Answer

The VLANs are not defined on the switch.. The IRB interface provides Layer 3 routing between VLANs, but it requires the VLANs themselves to be defined on the switch. If the VLANs are not defined, the switch cannot associate the IRB interface with the correct broadcast domains, and traffic will not be forwarded between them. Option C correctly identifies this missing configuration.

Answer

The IRB interface does not have an IP address configured.

Answer

The VLANs are on different switches.

Answer

The switch ports are not configured for VLAN tagging.

Question 2

A network engineer is troubleshooting OSPF adjacencies between two Juniper routers. The routers are directly connected and have matching OSPF configurations except for the router IDs. Router A has router-id 10.0.0.1, Router B has router-id 10.0.0.2. The adjacency remains in the EXSTART state. What is the most likely cause?

Accepted Answer

The MTU on the interface is mismatched.. The EXSTART state indicates that the routers have progressed past the 2-Way state and are attempting to exchange Database Description (DBD) packets. A common cause for getting stuck in EXSTART is an MTU mismatch, because OSPF uses the interface MTU to determine the maximum size of DBD packets. If Router A's MTU is larger than Router B's, Router B will drop the oversized DBD packet and the adjacency will remain in EXSTART.

Answer

The router IDs are not the same.

Answer

The area ID is not configured.

Answer

The hello and dead intervals are mismatched.

Question 3

An administrator needs to ensure that traffic from the 192.168.1.0/24 subnet is allowed to reach the internet through a Juniper SRX firewall. The SRX is configured with security policies. Which policy element is required to permit this traffic?

Accepted Answer

Source zone. In Juniper SRX security policies, the source zone is a mandatory element that defines the origin of the traffic. Since the traffic originates from the 192.168.1.0/24 subnet, the policy must specify the source zone (e.g., 'trust' or 'internal') to match the incoming traffic and permit it toward the internet (destination zone, e.g., 'untrust'). Without a source zone, the policy cannot be applied to the correct traffic flow.

Answer

Source address

Answer

Application

Answer

Destination zone

Question 4

A network engineer is designing a redundant network with two Juniper routers running VRRP. The virtual IP address is 10.0.0.1. Both routers are configured as VRRP group 1. What is the purpose of the 'priority' parameter in VRRP configuration?

Accepted Answer

It determines the master router election.. In VRRP, the 'priority' parameter (range 1-254, default 100) is used to elect the master router. The router with the highest priority becomes the master for the virtual IP address 10.0.0.1, ensuring redundancy by taking over traffic forwarding if the current master fails.

Answer

It sets the interval for VRRP advertisements.

Answer

It determines the virtual MAC address.

Answer

It changes the virtual IP address.

Question 5

Which TWO statements about Juniper's implementation of static routes are correct? (Choose two.)

Accepted Answer

The default preference for static routes is 5.. Option A is correct because in Juniper's Junos OS, the default preference (administrative distance) for static routes is 5. This value indicates that static routes are highly trusted, only surpassed by directly connected routes (preference 0). This is a Juniper-specific default, differing from Cisco's default of 1 for static routes.

Answer

Static routes always have a metric of 1.

Answer

Static routes are automatically redistributed into OSPF.

Answer

The default preference for static routes is 170.

Question 6

You are responsible for a Juniper MX router that connects two customer sites over a Layer 3 VPN. The router is configured with BGP for VPN routes and uses MPLS to forward traffic. Recently, the customer reported that traffic from Site A (10.0.1.0/24) to Site B (10.0.2.0/24) is intermittently failing. You check the routing table on the router and see that both routes are present with valid next-hops. However, when you ping from the router's loopback to the remote site's loopback, the ping succeeds. MPLS labels are being assigned and the LSP is up. You also notice that when the failure occurs, the router's BGP session to the remote PE is still established. The failure seems random and lasts a few seconds before recovering. Which troubleshooting step is most likely to identify the root cause?

Accepted Answer

Review the forwarding table for the affected prefixes and check for any discrepancies.. Option B is correct because the issue is intermittent packet loss with valid routes in the routing table but successful pings from the loopback. This points to a forwarding table (FIB) inconsistency, where the control plane (routing table) has the correct next-hop, but the forwarding plane (PFE) may have a stale or incorrect entry for the specific prefixes. Checking the forwarding table with 'show route forwarding-table' will reveal if the next-hop or label information differs from the routing table, which is a classic symptom of a hardware programming issue or a transient PFE problem.

Answer

Check the interface error counters for CRC errors or drops.

Answer

Verify the BGP session state and check for route flapping.

Answer

Check the MPLS label switching table to ensure labels are correctly assigned.

Question 7

A network engineer is troubleshooting connectivity between two directly connected Juniper routers. The interface on Router A shows 'up' but no packets are being received from Router B. Which command should the engineer use on Router A to check if the interface is expecting to receive a specific encapsulation type?

Accepted Answer

show interfaces terse. Option B is correct because the 'show interfaces terse' command displays a concise summary of interface status, including the encapsulation type configured on each interface. If Router A expects a specific encapsulation (e.g., PPP, HDLC, or Ethernet) that does not match what Router B is sending, the interface will show 'up' but will not receive packets. This command allows the engineer to quickly verify the configured encapsulation type without extraneous details.

Answer

show interfaces extensive

Answer

show configuration interfaces

Answer

monitor traffic interface

Question 8

A network administrator is configuring a new subnet for servers that must not communicate with each other directly but must be able to reach a default gateway. Which type of route should be configured on the servers?

Accepted Answer

Default route pointing to the gateway. A default route (0.0.0.0/0) pointing to the gateway is correct because it allows servers to reach any external destination (including the gateway) without requiring explicit routes to each other. Since the servers must not communicate directly, they should not have routes to each other's subnets; the default route ensures all non-local traffic is forwarded to the gateway, which can then enforce isolation policies.

Answer

Host route for the gateway

Answer

Dynamic route via OSPF

Answer

Static route to each other server

Question 9

An engineer is designing a network with two routers connected via a serial link. The link should support multiple logical subinterfaces for different VLANs. Which encapsulation type must be used on the serial interface?

Accepted Answer

Frame Relay. Frame Relay is correct because it supports multiple logical subinterfaces (DLCIs) on a single serial link, allowing different VLANs to be mapped to separate virtual circuits. Unlike PPP or HDLC, Frame Relay inherently provides the ability to create point-to-point or multipoint subinterfaces for Layer 2 segmentation.

Answer

Ethernet

Answer

PPP

Answer

HDLC

Question 10

A Juniper router is configured with two static routes to the same destination network, both with the same preference. One route has a metric of 5, the other a metric of 10. Which route will be installed in the routing table?

Accepted Answer

The route with metric 5 is installed. In JUNOS, when multiple static routes to the same destination have the same preference (administrative distance), the route with the lower metric is selected for installation in the routing table. Here, metric 5 is lower than metric 10, so the route with metric 5 is installed. This behavior follows the standard route selection process where preference is evaluated first, then metric (also called cost or next-hop metric) as a tiebreaker.

Answer

Neither route is installed because they conflict

Answer

The route with metric 10 is installed

Answer

Both routes are installed with equal preference