JN0-106 Networking Fundamentals Practice Test 3 — 15 Questions

Question 1

Which TWO statements about ARP are correct? (Select two.)

Accepted Answer

ARP request is sent to the broadcast MAC address. Option A is correct because an ARP request must reach every device on the local segment, so it is encapsulated in a frame with the broadcast destination MAC address FF:FF:FF:FF:FF:FF. Option E is correct because the ARP reply is unicast: the responding host already knows the requester's MAC address from the sender hardware address field in the request, so it sends the reply directly to that MAC address. Option B is wrong because ARP requests use the broadcast MAC address, not a multicast address (IPv6 uses multicast with Neighbor Discovery instead of ARP). Option C is wrong because the reply is unicast to the requester, not broadcast, which would waste bandwidth and force all hosts to process it. Option D is wrong because ARP resolves IPv4 addresses to MAC addresses; IPv6 uses Neighbor Discovery (NDP) with ICMPv6 rather than ARP.

Answer

ARP request is sent to a multicast MAC address

Answer

ARP reply is sent to the broadcast MAC address

Answer

ARP is used for both IPv4 and IPv6

Question 2

Which THREE fields are part of an Ethernet frame header? (Select three.)

Accepted Answer

Destination MAC address. The Ethernet frame header always begins with the Destination MAC address (Option A), a 6-byte field identifying the intended recipient's hardware address, which is essential for Layer 2 forwarding. The Source MAC address (Option E) is the next 6-byte field, identifying the sending NIC so replies and switch MAC-address-table entries can be built. The EtherType field (Option C) follows (or follows the optional 802.1Q tag) and is a 2-byte value such as 0x0800 for IPv4 or 0x86DD for IPv6, telling the receiver which upper-layer protocol is encapsulated. Option B (VLAN tag 802.1Q) is not part of the standard Ethernet header; it is an optional 4-byte tag inserted only on trunk links, so it is not one of the three basic header fields. Option D (FCS) is a 4-byte CRC trailer appended to the end of the frame for error detection, not a header field.

Answer

VLAN tag (802.1Q)

Answer

Frame Check Sequence (FCS)

Question 3

Which THREE statements are true about VLANs on Juniper EX switches? (Select three.)

Accepted Answer

A single VLAN can extend across multiple switches via trunk ports. Option B is correct because a VLAN is a Layer 2 broadcast domain that can span multiple Juniper EX switches when the inter-switch links are configured as trunk ports carrying 802.1Q tags, allowing the same VLAN ID to be present on each switch. Option C is correct because within a single switch the VLAN ID (1–4094) must be unique; you cannot create two distinct VLANs with the same ID on one EX device, as the ID is the local identifier for that broadcast domain. Option D is correct because VLANs segment a switch into separate broadcast domains, so broadcast, unknown unicast, and multicast traffic from one VLAN is not flooded into another VLAN, providing broadcast isolation. Option A is not correct because hosts in different VLANs are in separate Layer 3 subnets and require a router or Layer 3 interface (e.g., an IRB/RVI) to communicate; no communication occurs by default. Option E is not correct because on Juniper EX switches, assigning an interface to a VLAN requires the VLAN to already exist (or be created with 'set vlans vlan-id '); it is not automatically created by the interface assignment.

Answer

Hosts in different VLANs can communicate without any additional configuration

Answer

VLANs are automatically created when you assign an interface to an untagged VLAN

Question 4

Refer to the exhibit. A frame with destination MAC 00:0c:29:2a:3b:4d arrives on interface ge-0/0/0. What action will the switch take?

Accepted Answer

Forward the frame out of ge-0/0/2 only. The switch learns MAC addresses and their associated VLANs from incoming frames. Since the destination MAC 00:0c:29:2a:3b:4d is already in the MAC address table and mapped to interface ge-0/0/2 within the same VLAN, the switch forwards the frame only out of ge-0/0/2. This is the fundamental behavior of transparent bridging: unicast frames are forwarded only to the port where the destination MAC was last seen.

Answer

Flood the frame to all ports except ge-0/0/0

Answer

Drop the frame because the VLAN does not match

Answer

Forward the frame out of both ge-0/0/1 and ge-0/0/2

Question 5

Refer to the exhibit. The OSPF neighbor adjacency repeatedly goes up and down on R1. What is a likely cause?

Accepted Answer

MTU mismatch between the two routers. An MTU mismatch between OSPF neighbors can cause the adjacency to flap because OSPF includes the interface MTU in the Database Description (DBD) packets. If the MTU values do not match, the receiving router will reject the DBD packet, preventing the exchange of LSAs and causing the neighbor state to reset. This is a common cause of repeated up/down OSPF adjacencies even when the physical link is stable.

Answer

The physical link is flapping

Answer

OSPF authentication is misconfigured

Answer

The routers are in different OSPF areas

Question 6

A network engineer is designing a redundant network using RSTP. For faster convergence, what is the recommended method to avoid waiting for the forward delay timer?

Accepted Answer

Configure interfaces connected to end hosts as edge ports.. RSTP (Rapid Spanning Tree Protocol, IEEE 802.1w) achieves faster convergence by eliminating the listening and learning states for edge ports. Configuring interfaces connected to end hosts as edge ports allows them to transition directly to the forwarding state without waiting for the forward delay timer (default 15 seconds), because no BPDUs are expected on those ports and no loop can form.

Answer

Manually specify the root bridge and root port.

Answer

Use link aggregation to bundle multiple links.

Answer

Increase the hello timer to speed up BPDU exchange.

Question 7

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 8

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 9

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 10

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 11

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

Accepted Answer

Trunk links can carry traffic for multiple VLANs.. Option C is correct because a trunk link (typically 802.1Q) tags frames so it can carry traffic for multiple VLANs between switches or to a router/switch, unlike an access port. Option D is correct because each VLAN defines its own broadcast domain, so broadcasts are confined within the VLAN and do not propagate to other VLANs, which is the core purpose of VLAN segmentation. Option A is incorrect because MAC addresses are not shared across VLANs; each VLAN maintains its own MAC address table and forwarding domain. Option B is incorrect because a VLAN does not strictly require a unique IP subnet — VLANs can be Layer 2 only, and multiple VLANs can theoretically share a subnet in unusual designs, though best practice is one subnet per VLAN. Option E is incorrect because a single switch port can belong to multiple VLANs when configured as a trunk (or in some vendor-specific modes), not only one VLAN.

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 12

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.. Although both hosts are in the same VLAN and can communicate at Layer 2 (MAC addresses), IP communication requires them to be on the same IP subnet. If the hosts have IP addresses on different subnets, they will attempt to use a default gateway for routing. Without a router or IRB interface configured, the traffic cannot be delivered, leading to unreachability.

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 13

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.. 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 14

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 15

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.. 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.