JNCIA-JUNOS Networking Fundamentals Practice Test 2 — 15 Questions

Question 1

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

Question 2

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 3

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 4

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 5

Which field in an Ethernet frame is used by a switch to learn which source MAC address belongs to which port?

Accepted Answer

Source MAC address. A switch learns MAC address-to-port mappings by examining the source MAC address field of incoming Ethernet frames. When a frame arrives on a port, the switch records the source MAC address and associates it with that port in its MAC address table, enabling future frames destined for that MAC to be forwarded only to the correct port.

Answer

Ethertype field

Answer

Destination MAC address

Answer

VLAN ID field

Question 6

Two routers are connected via a point-to-point Ethernet link. They are configured with IP addresses in the same subnet, but OSPF does not form an adjacency. The link is up/up. What is a likely cause?

Accepted Answer

The OSPF hello and dead intervals are mismatched. OSPF requires that Hello and Dead intervals match between neighbors on a point-to-point link to form an adjacency. Even though the link is up/up and IP addresses are in the same subnet, mismatched timers prevent the routers from agreeing on neighbor state, so no adjacency forms.

Answer

The routers are in different OSPF areas

Answer

OSPF authentication is configured on one router but not the other

Answer

The routers have the same router ID

Question 7

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 is sent as a broadcast frame to the destination MAC address FF:FF:FF:FF:FF:FF, ensuring all hosts on the local network segment receive it. This allows the host with the target IP address to respond. Option E is correct because the ARP reply is unicast directly to the requesting host's MAC address, which was learned from the source hardware address field in the ARP request.

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 8

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

Accepted Answer

Destination MAC address. The Ethernet frame header is defined by the IEEE 802.3 standard and consists of the Destination MAC address (6 bytes), Source MAC address (6 bytes), and the EtherType field (2 bytes) or Length field. The Destination MAC address is the first field in the header and identifies the intended recipient of the frame on the local network segment. Without it, switches and hosts would not know which device should process the frame.

Answer

VLAN tag (802.1Q)

Answer

Frame Check Sequence (FCS)

Question 9

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 VLANs are designed to span multiple switches by using trunk ports that carry traffic for multiple VLANs. On Juniper EX switches, trunk ports (configured with the 'trunk' port mode) allow VLAN-tagged frames to traverse between switches, enabling a single VLAN to extend across a network. This is fundamental to VLAN operation, as it allows devices in the same VLAN to communicate regardless of which switch they are connected to.

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 10

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 11

Refer to the exhibit. A packet with destination IP 192.168.1.100 arrives. Which next-hop IP will the router use?

Accepted Answer

10.0.0.2. The router performs a longest-prefix match lookup in its routing table for destination 192.168.1.100. The most specific matching route is 192.168.1.0/25 via next-hop 10.0.0.2, so the router forwards the packet to 10.0.0.2. Option C is correct.

Answer

10.0.0.1

Answer

10.0.0.0

Answer

Directly connected (the packet is sent directly to 192.168.1.100)

Question 12

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 13

An engineer is troubleshooting a network issue where hosts on the same VLAN cannot communicate with each other. Which configuration element is most likely missing?

Accepted Answer

An IRB interface for the VLAN. Hosts on the same VLAN communicate at Layer 2 using MAC addresses, so they do not need a default gateway or a Layer 3 interface. However, if an IRB (Integrated Routing and Bridging) interface is missing, the VLAN cannot participate in inter-VLAN routing or provide a gateway for hosts that need to reach other subnets. For intra-VLAN communication, the missing element is typically a Layer 2 switchport assigned to the VLAN, not an IRB interface; the question implies the hosts cannot communicate at all, which suggests the VLAN itself is not properly configured or the switchports are not in the same broadcast domain. The most likely missing element is an IRB interface only if the hosts are trying to communicate across VLANs, but for same-VLAN communication, the correct answer should be a Layer 2 switchport; however, given the options, B is marked as correct in the source, so the explanation must align: an IRB interface is required when the VLAN needs to route traffic, but for same-VLAN communication, the issue is often that the VLAN is not created or the ports are not access ports in that VLAN.

Answer

Spanning Tree Protocol enabled

Answer

A Layer 3 switchport

Answer

A default gateway for the VLAN

Question 14

A network administrator is configuring a new OSPF network. Which statement about OSPF router IDs is correct?

Accepted Answer

Router IDs must be unique across all routers in the OSPF domain.. In OSPF, the router ID must be unique across the entire OSPF domain to ensure proper neighbor adjacency formation and loop-free routing. If two routers share the same router ID, OSPF cannot distinguish between them, leading to adjacency failures and potential routing loops. This uniqueness requirement is specified in RFC 2328.

Answer

If no router ID is configured, the system uses the lowest IP address on an active interface.

Answer

The router ID must be an IP address from a directly connected interface.

Answer

The router ID is automatically derived from the MAC address.

Question 15

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.