JNCIS-ENT Practice Test 23 — 15 Questions

Question 1

You are troubleshooting a Layer 2 connectivity problem where a trunk port is dropping tagged traffic for VLAN 50. Which TWO configuration checks should be performed? (Choose two.)

Accepted Answer

Verify that VLAN 50 is included in the trunk port's member list (or not excluded via 'except').. Option B is correct because on a trunk port, tagged traffic for a VLAN is only forwarded if that VLAN is explicitly listed as a member (e.g., under 'vlan members' or 'vlan members all' with 'except' not excluding it); if VLAN 50 is missing from the member list or excluded via 'except', its tagged frames are dropped. Option E is correct because the VLAN must first exist under the [edit vlans] hierarchy with a valid vlan-id (and typically a matching 802.1Q tag) before it can be assigned to a trunk interface; a missing or misconfigured vlan-id means the switch cannot map the tagged frames to VLAN 50. Option A is incorrect because a Layer 2 trunk interface does not require an IPv4 address in family inet to forward tagged traffic, and adding one is irrelevant to VLAN 50 connectivity. Option C is incorrect because spanning-tree bridge priority affects root bridge election, not whether tagged VLAN 50 traffic is accepted on a trunk. Option D is incorrect because setting the port to access mode would strip tags and place the port in a single VLAN, which is the opposite of what a trunk carrying VLAN 50 requires.

Answer

Verify that the physical interface has an IPv4 address assigned in family inet.

Answer

Verify that spanning-tree bridge priority is set to 0.

Answer

Verify that the port mode is set to access.

Question 2

You want to disable spanning tree protocol entirely on a specific access interface (ge-0/0/10.0) of an EX Series switch while keeping RSTP active globally. Which configuration command achieves this?

Accepted Answer

set protocols rstp interface ge-0/0/10.0 disable. Junos supports per-interface RSTP disable via 'set protocols rstp interface <name> disable', which stops BPDU processing and STP participation on that specific interface while leaving RSTP running globally on the rest of the switch. This is the correct way to exempt an access port from spanning tree without affecting other ports.

Answer

delete protocols rstp

Answer

set interfaces ge-0/0/10.0 spanning-tree disable

Answer

set protocols rstp interface ge-0/0/10.0 mode off

Question 3

You are configuring OSPF route summarization on an ABR in a Junos environment. You want to summarize routes from Area 1 into Area 0. Which Junos configuration hierarchy is used to define this summary?

Accepted Answer

set protocols ospf area 0.0.0.1 range 192.168.0.0/16. To summarize routes from Area 1 into Area 0 on an ABR in Junos, you configure the `range` statement under the OSPF area that contains the routes to be summarized. The correct hierarchy is `set protocols ospf area 0.0.0.1 range 192.168.0.0/16`, where 0.0.0.1 is Area 1. This instructs the ABR to advertise the summary route into other areas.

Answer

set routing-options static route 192.168.0.0/16 discard

Answer

set protocols ospf summary-prefix 192.168.0.0/16

Answer

set protocols ospf export summarize-policy

Question 4

An administrator configures a Layer 2 firewall filter to drop all ARP packets with a specific invalid sender hardware address. The filter is applied to a VLAN interface. What special consideration must be taken when applying Layer 2 filters to VLANs in Junos?

Accepted Answer

Layer 2 filters on VLANs evaluate traffic across all member ports belonging to that VLAN instance.. In Junos, when a Layer 2 firewall filter is applied to a VLAN, it evaluates traffic across all member ports of that VLAN instance, not just a single physical interface. This is because the VLAN is a logical Layer 2 broadcast domain, and the filter operates at the VLAN level, inspecting frames as they traverse the VLAN's forwarding path. This behavior is critical when filtering protocols like ARP, which are broadcast within the VLAN.

Answer

Layer 2 filters applied to VLANs require family inet to inspect ARP payloads.

Answer

Layer 2 filters cannot be applied to VLANs; they must be applied to physical interfaces only.

Answer

VLAN-based Layer 2 filters only process egress traffic leaving the VLAN trunk ports.

Question 5

You are troubleshooting a Link Aggregation Group (LAG) between an EX4300 switch and a third-party switch. Member links are flapping intermittently. Examination of system logs reveals LACP timeout errors. What is the most likely cause?

Accepted Answer

A mismatch in LACP timeout settings (e.g., one side configured for fast timeout while the other sends slow intervals) causing dropped sessions.. LACP timeout errors and flapping member links are typically caused by a mismatch in LACP timeout configuration between the two sides. If one side uses fast (short) timeout and the other sends slow (long) intervals, the fast side may declare the LACPDU timeout and drop the session, causing intermittent flapping. Ensuring both sides use matching timeout modes resolves the issue.

Answer

The Virtual Chassis primary routing engine priority is set too low.

Answer

VRRP preemption is enabled on the aggregated interface.

Answer

GRES is disabled on the EX4300 switch.

Question 6

An enterprise network uses MSTP. What is the function of the Internal Root in an MST region?

Accepted Answer

It acts as the root switch for a specific MSTI within the local MST region.. In MSTP (IEEE 802.1Q), each MST instance (MSTI) within a region elects its own root bridge, called the Internal Root (or regional root for that MSTI). The Internal Root is the bridge with the lowest bridge ID for that specific MSTI inside the local region, and it is distinct from the CIST Regional Root, which connects the region to the outside. The Internal Root governs topology within the MSTI's scope inside the region.

Answer

It translates MST BPDUs into legacy STP BPDUs at the regional boundary.

Answer

It manages the CIST instance across all connected foreign MST regions.

Answer

It serves as the sole bridge responsible for generating the MST configuration digest.

Question 7

When configuring load balancing in Junos OS, which THREE header fields can be included in the hashing algorithm to compute the ECMP path? (Choose three)

Accepted Answer

Layer 4 TCP/UDP port numbers. In Junos OS, the ECMP hashing algorithm used for load balancing can incorporate Layer 3 and Layer 4 header fields to distribute traffic across multiple equal-cost next hops. Option B (Layer 4 TCP/UDP port numbers) is correct because including transport-layer port numbers enables per-flow load balancing, so different sessions between the same hosts can take different paths. Option C (Source IP address) is correct because the source IP is a standard Layer 3 field used in the hash, allowing traffic from different sources to be spread across ECMP paths. Option E (Destination IP address) is correct because the destination IP is also a standard Layer 3 field in the hash, ensuring flows to different destinations are distributed. Option A (MAC address destination) is not included because ECMP hashing in Junos operates on Layer 3/Layer 4 fields, not on the destination MAC address. Option D (VLAN ID tagging bits) is not included because VLAN ID is a Layer 2 field and is not part of the ECMP hash computation in Junos OS.

Answer

MAC address destination

Answer

VLAN ID tagging bits

Question 8

An engineer needs to verify which BGP routes are currently installed in the forwarding table (FIB). Which command should be used?

Accepted Answer

show route forwarding-table protocol bgp. The forwarding table is viewed using 'show route forwarding-table'.

Answer

show bgp forwarding

Answer

show route table inet.0

Answer

show route summary

Question 9

Which command is used to display all configured static routes along with their active/inactive status in Junos?

Accepted Answer

show route protocol static. The command `show route protocol static` queries the routing table (by default inet.0) and filters for routes whose protocol is static. This output includes both active routes (marked with `*`) and inactive routes (marked with `-`), along with next-hop, preference, and metric information. It directly answers the question by showing all configured static routes and their active/inactive status.

Answer

show static routes summary

Answer

show configuration routing-options static

Answer

show route table inet.0 static

Question 10

A network engineer wants to inject external routes into an OSPF domain using Junos OS. The routes originate from an external BGP process. Which OSPF LSA type is generated by the ASBR to describe these external destinations?

Accepted Answer

Type 5 AS-external LSA. When an ASBR redistributes external routes (such as those learned via BGP) into an OSPF domain, it originates a Type 5 AS-external LSA to describe each external destination. Type 5 LSAs are flooded throughout the OSPF area (except stub/NSSA areas) and carry the external metric and forwarding address. This is the standard mechanism for injecting non-OSPF routes into OSPF.

Answer

Type 7 NSSA LSA

Answer

Type 3 Summary LSA

Answer

Type 4 Summary LSA

Question 11

An administrator wants to configure load balancing across multiple static routes. Which THREE configuration elements are required to successfully implement ECMP load balancing for static routes in Junos? (Choose three)

Accepted Answer

Export of the load-balancing policy under 'routing-options forwarding-table export'.. To implement ECMP for static routes in Junos, you must first have multiple static routes to the same destination with equal preference and metric, because ECMP only installs paths of identical preference/metric into the forwarding table (option E). You then need a routing policy that contains the 'then load-balance' action, which tells Junos to install multiple next-hops for that destination rather than selecting a single best path (option C). Finally, that policy must be applied to the forwarding table via 'routing-options forwarding-table export', which is the mechanism that activates per-prefix load balancing for the routes (option B). BFD is not required for ECMP; it is an optional failure-detection mechanism (option A). A virtual-router routing instance is unrelated to ECMP and is used for routing-instance separation, not load balancing (option D).

Answer

BFD enabled on all static route next-hops.

Answer

A virtual-router routing instance wrapper.

Question 12

A BGP speaker receives an UPDATE message containing a malformed attribute. According to BGP specifications, which action does the receiving router take?

Accepted Answer

Send a NOTIFICATION message and terminate the BGP session.. A malformed attribute or critical error triggers a NOTIFICATION message with an appropriate error code, followed by session teardown.

Answer

Strip the attribute and propagate the route to iBGP peers.

Answer

Send an UPDATE message requesting retransmission.

Answer

Silently ignore the attribute and install the route.

Question 13

You are configuring a new eBGP session between two Juniper routers in different autonomous systems. The physical link is up and you can ping the remote interface, but the BGP session remains in the Active state. Reviewing the configuration, you notice that the neighbor statement on both routers specifies the loopback address of the peer, but no additional configuration is present. What is the most likely cause of the problem?

Accepted Answer

The BGP session is not configured with multihop because the peers are using loopback addresses.. The session remains in Active because the routers cannot establish a TCP connection to each other's loopback addresses. By default, eBGP uses a TTL of 1, so packets to a non-directly connected address are dropped. Configuring multihop with a TTL greater than 1 allows the BGP session to be established over multiple hops, resolving the Active state.

Answer

The BGP session is not configured with authentication, which is required for eBGP.

Answer

The BGP router ID is not configured and defaults to the loopback address.

Answer

The autonomous system numbers are mismatched on the two routers.

Question 14

You are configuring OSPF on a Juniper router. The router must not generate any OSPF packets or form adjacencies on interface ge-0/0/1, but the interface must remain in the OSPF configuration for future use. Which configuration statement accomplishes this?

Accepted Answer

set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 passive. A passive OSPF interface remains in the OSPF configuration but does not send or receive OSPF control packets, so it cannot form adjacencies. The connected network is still advertised as a stub network into the area. This is the standard Junos approach when an interface must be included in OSPF for reachability but must not participate in adjacency formation.

Answer

set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 priority 0

Answer

set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 no-hello

Answer

set protocols ospf area 0.0.0.0 interface ge-0/0/1.0 disable

Question 15

You are configuring a static route on a Junos device to a remote network reachable via a point-to-point interface. After committing, you notice the route appears in the routing table but is marked as hidden. Which of the following is the most likely cause?

Accepted Answer

The next-hop address is not reachable through a direct interface.. A static route is hidden when its next-hop cannot be resolved to a direct interface. Junos requires that the next-hop be reachable via a connected route for the static route to be active. If the interface is down or the next-hop is not on a directly connected subnet, the route remains hidden until reachability is restored.

Answer

The static route lacks a valid preference value.

Answer

The static route is configured with the 'no-readvertise' option.

Answer

The route is being rejected by an import policy.