JNCIA-JUNOS Networking Fundamentals Practice Test 3 — 15 Questions

Question 1

A company uses a Juniper MX router to connect two branch offices via a point-to-point link. The link is flapping. Which command would you use to check the interface errors on the link?

Accepted Answer

show interfaces ge-0/0/0 extensive. The 'show interfaces ge-0/0/0 extensive' command provides detailed interface statistics, including error counters such as CRC errors, frame errors, and link state transitions. This is the most comprehensive command to diagnose why a point-to-point link is flapping, as it reveals physical-layer issues that cause the interface to go up/down repeatedly.

Answer

show log messages

Answer

show configuration interfaces ge-0/0/0

Answer

show interfaces ge-0/0/0 terse

Question 2

An engineer is implementing VLAN trunking between two Juniper EX switches. Which statement about native VLANs on a trunk is correct?

Accepted Answer

Untagged frames arriving on a trunk port are assigned to the native VLAN.. On Juniper EX switches, a trunk port can accept both tagged and untagged frames. Untagged frames received on a trunk port are automatically assigned to the native VLAN, which by default is VLAN 1. This behavior is defined by the IEEE 802.1Q standard, where the native VLAN is the only VLAN that can carry untagged traffic on a trunk link.

Answer

The native VLAN cannot be changed after initial configuration.

Answer

Frames in the native VLAN are always tagged with the VLAN ID on the trunk.

Answer

The native VLAN must be the same as the management VLAN.

Question 3

A network engineer sees an unknown unicast flood on a Juniper EX switch. Which mechanism is most effective at reducing such floods?

Accepted Answer

Enable storm control for unknown unicast traffic.. Unknown unicast flooding occurs when a switch does not have a MAC address entry for a destination, causing it to flood the frame out of all ports in the VLAN except the ingress port. Storm control for unknown unicast traffic (set using `set ethernet-switching-options storm-control interface <interface> unknown-unicast`) directly limits the rate of such flooded traffic, preventing excessive bandwidth consumption. This is the most effective mechanism because it specifically targets and rate-limits unknown unicast floods without altering MAC learning or aging behavior.

Answer

Configure static MAC addresses for all endpoints.

Answer

Increase the MAC address table aging time.

Answer

Disable MAC learning on trunk interfaces.

Question 4

A network administrator is implementing OSPF on a Juniper MX router. During verification, the OSPF adjacency does not come up. The interfaces have correct IP addresses and are up. What is the most likely cause?

Accepted Answer

The interface MTU does not match between the two routers.. In OSPF, the interface MTU must match between neighbors for the adjacency to form. When a router receives a Database Description (DBD) packet with the 'More' bit set but the packet size exceeds the receiving interface's MTU, the packet is silently dropped, preventing the adjacency from progressing beyond the ExStart state. This is a common issue on Juniper MX routers when connecting to devices with different MTU configurations.

Answer

The router ID is not in the same subnet as the interface.

Answer

The VLAN tag is missing on the interface.

Answer

The max-lsa limit has been reached.

Question 5

A company is deploying an EVPN-VXLAN fabric with Juniper QFX switches. To provide inter-subnet routing, which interface type must be configured?

Accepted Answer

IRB interface. In an EVPN-VXLAN fabric, Integrated Routing and Bridging (IRB) interfaces are required to provide inter-subnet routing. IRB interfaces act as Layer 3 gateways within the VXLAN overlay, enabling routing between different VLANs/VXLANs by terminating both the bridge domain and the routing instance. Without IRB, traffic cannot be routed between subnets in the EVPN-VXLAN fabric.

Answer

Physical interface with vlan-tagging

Answer

VXLAN tunnel endpoint (VTEP)

Answer

Loopback interface

Question 6

Which TWO statements about VLANs are correct? (Choose two.)

Accepted Answer

Trunk links can carry traffic for multiple VLANs.. Option C is correct because trunk links, such as 802.1Q trunk ports, are designed to carry traffic for multiple VLANs by adding VLAN tags to Ethernet frames. This allows a single physical link between switches or between a switch and a router to transport frames from different VLANs simultaneously.

Answer

MAC addresses are shared across VLANs.

Answer

Each VLAN must be assigned a unique IP subnet.

Answer

A single switch port can belong to only one VLAN.

Question 7

Which THREE factors influence OSPF neighbor adjacency formation? (Choose three.)

Accepted Answer

Area ID. OSPF neighbor adjacency formation requires that both routers agree on the Area ID, because the Area ID defines the logical segment of the OSPF domain and is included in OSPF Hello packets. If the Area ID does not match, the routers will not form an adjacency, as they would consider themselves in different OSPF areas.

Answer

Router ID

Answer

MTU mismatch

Question 8

Which THREE are valid methods to reduce spanning-tree convergence time? (Choose three.)

Accepted Answer

Enabling loop guard on root ports. Option A is correct because loop guard prevents alternate or root ports from becoming designated in the event of a BPDU loss, which avoids loops and the need for STP reconvergence. By stabilizing the port state, it indirectly reduces convergence time by preventing unnecessary topology changes.

Answer

Increasing the forward-delay timer

Answer

Enabling MAC address notification

Question 9

Refer to the exhibit. A host connected to ge-0/0/1 cannot reach a host connected to ge-0/0/2 even though both are in VLAN10. What is the most likely cause?

Accepted Answer

The hosts are configured with IP addresses on different subnets.. Option B is correct because for two hosts in the same VLAN to communicate at Layer 2, they must be in the same IP subnet. If the hosts are configured with IP addresses on different subnets, they will attempt to route traffic through a default gateway rather than sending ARP requests directly, causing communication failure even though they are in the same broadcast domain.

Answer

VLAN10 is not defined in the global VLAN configuration.

Answer

An IRB interface for VLAN10 is not configured.

Answer

The interface ge-0/0/2 is configured as a trunk port.

Question 10

Refer to the exhibit. An engineer adds a new static route to 10.10.10.0/24 via next-hop 172.16.1.2. The new route does not appear in the route table. What is the most likely reason?

Accepted Answer

The next-hop 172.16.1.2 is not directly reachable.. Option C is correct because in Junos, a static route's next-hop must be directly reachable (i.e., on a connected subnet) for the route to be installed in the inet.0 route table. If the next-hop 172.16.1.2 is not directly connected, the route remains hidden (inactive) and does not appear in the forwarding table. Junos does not perform recursive next-hop resolution for static routes by default unless configured with 'resolve'.

Answer

The route requires an export policy to be installed.

Answer

The route has a different AS path.

Answer

Static routes cannot have the same preference as an existing route.

Question 11

You are a network engineer for a large enterprise deploying a new data center using a spine-and-leaf architecture with Juniper QFX5100 switches. The underlay network uses OSPF for loopback reachability, and the overlay uses EBGP for EVPN. The leaf switches are configured as VTEPs (Virtual Tunnel Endpoints). One of the leaf switches, leaf-03, cannot establish OSPF adjacency with its spine switch, spine-01. The interfaces are up/up and the IP addresses are correctly configured. 'show ospf neighbor' on leaf-03 returns nothing. 'show ospf interface' shows the interface is in state DOWN. Both switches are configured with the same OSPF area (0.0.0.0) and the same hello interval (10 seconds). The MTU on both sides is 1500. Authentication is not configured. The spine switch has multiple OSPF neighbors from other leaves. The network is in production and other leaf switches are working fine. What is the most likely cause of the problem?

Accepted Answer

OSPF is disabled on the interface at the leaf switch.. Option B is correct because if the OSPF interface is down on leaf-03 but up/up at Layer 1, the most likely cause is that OSPF is disabled on that interface, or there is an OSPF passive interface configuration. Option A is wrong - router ID duplication would cause both to have issues, but other leaves are fine. Option C is wrong - if network type mismatch, the interface would still be up but adjacency would not form, but the interface state would be up (not down). Option D is wrong - MTU mismatch would be at L3, but interface state would be up with adjacency failing.

Answer

The OSPF network type is not consistent between the two switches.

Answer

The interface MTU is misconfigured on one side.

Answer

Duplicate router ID on leaf-03 and spine-01.

Question 12

You are troubleshooting a network connectivity issue at a medium-sized company that uses Juniper EX4300 switches. Users in VLAN 100 (10.10.100.0/24) report that they cannot access a server in VLAN 200 (10.10.200.0/24). The switch has IRB interfaces configured for both VLANs and is acting as the default gateway. The IRB interfaces are up and have correct IP addresses. The server and users have correct IP configurations. 'show arp' on the switch shows incomplete entries for some hosts in VLAN 100. 'show ethernet-switching table' shows MAC addresses for users are present on the correct access ports. However, pings from the switch to a user in VLAN 100 fail. What is the most likely issue?

Accepted Answer

The switch cannot resolve ARP requests because the user's host is not responding.. The 'incomplete' entries in the 'show arp' output indicate that the switch sent ARP requests for hosts in VLAN 100 but received no replies. Since the switch is the default gateway, it must resolve the Layer 2 MAC address of each host to forward traffic. If a user's host is not responding to ARP (e.g., due to a firewall, misconfiguration, or the host being offline), the switch cannot complete the ARP cache entry, causing pings from the switch to fail and potentially disrupting inter-VLAN routing.

Answer

Spanning Tree Protocol is blocking the user ports.

Answer

A static MAC address entry for the server is missing.

Answer

The IRB interface for VLAN 100 is not configured with a proxy ARP.

Question 13

You are troubleshooting an OSPF adjacency issue between two Juniper MX series routers, R1 and R2, both running Junos 18.2. They are directly connected via a Gigabit Ethernet link. R1's interface ge-0/0/0 is configured with IP 192.168.1.1/24, and R2's ge-0/0/0 is configured with IP 192.168.1.2/24. Both interfaces are in OSPF area 0.0.0.0. The physical connectivity is confirmed up, and both interfaces are administratively enabled. You have checked that the hello and dead intervals match (hello 10, dead 40), the area IDs are identical, and the subnet masks are /24. No firewall filters are blocking OSPF. The OSPF configuration on R1 includes 'authentication-type md5' and 'authentication-key juniper123' under the interface. On R2, the configuration also includes 'authentication-type md5' but the key is 'juniper321'. Both routers have loopback addresses as router IDs: 1.1.1.1 for R1 and 2.2.2.2 for R2. No explicit OSPF network type is set, so the default is broadcast. Despite these configurations, the OSPF adjacency state on both routers remains in INIT. What is the most likely cause of this issue?

Accepted Answer

The OSPF authentication keys do not match.. Option C is correct because OSPF authentication keys must match exactly for the adjacency to form. R1 uses key 'juniper123' while R2 uses 'juniper321', causing the MD5 digest computed on each Hello packet to differ. Since OSPF authentication is validated per-packet, mismatched keys prevent the routers from moving past the INIT state.

Answer

The interface MTU size is mismatched.

Answer

The OSPF network type is mismatched.

Answer

The router IDs are not properly configured.

Question 14

Which TWO statements about IPv4 addressing are correct?

Accepted Answer

The network 169.254.0.0/16 is used for link-local addressing.. Options A and B are correct. 127.0.0.1 is the loopback address, and 169.254.0.0/16 is used for link-local addressing (APIPA). Option C is false because 192.168.0.0/16 is a private address. Option D is false because 10.0.0.0/8 is private. Option E is false because a /24 subnet mask provides 254 usable host addresses, not 256.

Answer

A /24 subnet mask provides 256 usable host addresses.

Answer

The address 192.168.0.0/16 is a public address.

Answer

The address 10.0.0.0/8 is a public address.

Question 15

Refer to the exhibit. The administrator has verified that the remote router's interface uses PPP encapsulation. What is the most likely cause of the link not passing traffic?

Accepted Answer

The encapsulation type is mismatched.. Option C is correct. The exhibit shows the local interface is configured with HDLC encapsulation, while the remote uses PPP. This encapsulation mismatch prevents Layer 2 communication. Options A and B are incorrect because the physical link is up and enabled. Option D is incorrect because an IP address mismatch would allow Layer 2 connectivity but prevent Layer 3 routing.

Answer

The interface has a speed mismatch.

Answer

The interface is using the wrong IP address.

Answer

The interface is administratively down.