Important Juniper JN0-281 Exam Questions

Answer: C, D

Explanation:

On Junos devices used in data centers, an aggregated Ethernet bundle can run either static bundling or dynamic bundling using LACP. When LACP is used, Junos supports two negotiation modes: active and passive. These modes control whether the device initiates LACP negotiation by transmitting LACP Data Units or whether it waits to respond to LACP Data Units sent by its peer. In active mode, the system periodically sends LACP control frames to begin and maintain the negotiation. In passive mode, the system does not initiate negotiation but will respond if it receives LACP control frames from the neighbor.

From an operational perspective, active is typically recommended on at least one side of the link so the bundle reliably forms even if the peer is configured to be passive. If both ends are passive, each side waits for the other to start, and the aggregated link might not come up as expected. This is a common cause of down or partially formed bundles in leaf spine uplinks and server dual-homing scenarios.

Options aggressive and mixed are not Junos LACP modes for aggregated Ethernet. In Junos, you configure LACP under the aggregated-ether-options hierarchy and select active or passive behavior, along with optional timing such as periodic fast for quicker detection.

Answer: D

Explanation:

An IP fabric underlay is the routed foundation of a modern leaf-spine data center. Its purpose is to provide scalable, deterministic Layer 3 reachability between all fabric nodes, typically using point-to-point routed links between leaves and spines. In this design, EBGP is commonly used as an underlay routing protocol because it scales well, supports clear policy boundaries, and enables fast convergence and operational simplicity. Each leaf forms EBGP sessions to each spine, advertising loopback addresses and link subnets so that overlay endpoints and control plane services can reach one another reliably.

RSTP is a Layer 2 spanning tree mechanism and is not the standard protocol for a routed underlay. EVPN is an overlay control plane used to distribute tenant reachability and multihoming information; it is not the underlay routing protocol itself. VXLAN is a data plane encapsulation used by the overlay to transport Layer 2 segments across a Layer 3 fabric; it also is not the underlay routing protocol.

In Juniper data center architectures, the underlay is intentionally kept simple and purely routed, while overlays such as EVPN VXLAN deliver multi-tenant Layer 2 and Layer 3 services on top of that underlay. EBGP fits the underlay requirement among the provided options.

Answer: A

Explanation:

Qualified next hop is the Junos mechanism that lets you configure more than one next hop for the same static route and control which one is preferred. This is most commonly used to build primary and backup forwarding behavior with deterministic selection. You configure a primary next hop and then add one or more qualified next hops with a different preference value. The route installs the best, lowest preference next hop as active, while keeping the alternate next hop available as standby. If the primary next hop becomes unusable, Junos can switch to the qualified next hop so traffic continues to flow.

This approach is widely used in data center edge and services routing, for example toward management networks, service appliances, or external gateways where you want a static default or summary route with predictable failover. It is not the same as resolving a non-directly-connected next hop. That scenario is handled by recursive resolution behavior, not by qualification. Qualified next hop also does not depend on any specific routing protocol, and it is not limited to link-state environments.

Finally, qualified next hop does not rely on sending pings to test reachability. Health tracking can be achieved through other mechanisms such as interface state, next hop resolution changes, or integration with failure detection features, but the qualified next hop feature itself is about preference-ranked redundancy for static routes.

Answer: B, C

Explanation:

The commit error indicates that the interface is being treated as an access port while the configuration attempts to associate it with more than one VLAN. In Junos Ethernet switching, an access mode interface represents a single untagged VLAN membership. Because access ports accept and transmit frames without 802.1Q tags, the switch must map all ingress untagged traffic to exactly one VLAN. For that reason, Junos enforces the rule that an access interface can be part of only one VLAN, and it rejects configurations that try to add multiple VLAN members under access mode.

There are two valid ways to resolve this, depending on the intended design. First, if the port truly connects to a single endpoint that should live in only one broadcast domain, configure the interface as a member of only one VLAN. This aligns with access port semantics and eliminates the conflict that causes the commit to fail.

Second, if the endpoint or downstream device needs to carry multiple VLANs over the same physical link, change the interface to trunk mode. A trunk port is designed to transport multiple VLANs using 802.1Q tagging, so multiple VLAN members are valid and expected. In data center environments, trunking is common for server virtualization hosts, appliance uplinks, and switch-to-switch links.

Connecting the interface to the network does not affect configuration validation, and logical unit numbering is unrelated to VLAN membership rules for access ports.

Answer: B, D

Explanation:

On Junos Ethernet switching, access and trunk ports serve different purposes and therefore treat VLAN tags differently by default. An access port is intended for a single VLAN and is designed to connect to endpoints that do not tag their frames. Because of that, access ports forward traffic as untagged on the wire and internally associate those untagged frames to the configured access VLAN. This makes access ports the standard choice for single-VLAN server NICs, management devices, and any endpoint expecting a plain Ethernet connection.

A trunk port is intended to carry traffic for multiple VLANs over a single link, which is typical for switch-to-switch uplinks, leaf-to-spine connectivity where VLAN services are extended, and hosts or appliances that use VLAN tagging. By default, trunk ports forward tagged traffic and require VLAN tags to identify the VLAN membership of each frame. Untagged behavior on a trunk is not assumed by default and is typically governed by configuring a native VLAN or equivalent untagged VLAN handling, depending on platform and design. Without such configuration, untagged frames are not treated as a normal expected case for a trunk link in data center fabrics.

Therefore, the correct default statements are that access ports forward untagged traffic and trunk ports forward tagged traffic, matching options B and D.