JNCIS-ENT Spanning Tree Practice Test 3 — 15 Questions

Question 1

You manage a network of EX Series switches running RSTP. Users report intermittent connectivity issues. You suspect a unidirectional link failure on a point-to-point link between two switches. Which RSTP mechanism is designed to detect and mitigate this type of failure?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Unidirectional Link Detection (UDLD). UDLD actively monitors links and detects unidirectional failures by exchanging protocol packets. When a failure is detected, it can err-disable the port, preventing loops and connectivity issues. The other features are for different purposes: BPDU Guard for edge port protection, Root Guard for root bridge enforcement, and Loop Guard for preventing loops when BPDUs are lost, but not for detecting unidirectional links.

Answer

Loop Guard

Answer

BPDU Guard

Answer

Root Guard

Question 2

You are configuring RSTP on a Juniper EX Series switch. You want to ensure that the switch becomes the root bridge for a specific VLAN. Which two parameters can you adjust to influence the root bridge election? (Choose two.)

Accepted Answer

Bridge priority. The two parameters that influence root bridge election are bridge priority and MAC address. The switch with the lowest bridge priority wins; if priorities tie, the lowest MAC address wins. Configuring a lower bridge priority is the standard method to force a switch to become root. Hello time and forward delay affect BPDU timers but not the election outcome.

Answer

Forward delay

Answer

Hello time

Answer

Port cost

Question 3

You are configuring RSTP on a Juniper EX Series switch. You want to ensure that the switch does not participate in the spanning-tree topology for a specific VLAN, but still passes traffic for that VLAN. Which two statements are correct about disabling RSTP on a per-VLAN basis? (Choose two.)

Accepted Answer

In Junos, RSTP is not VLAN-aware; it runs per physical interface, so you cannot disable it per VLAN without using MSTP.. In Junos, RSTP is not VLAN-aware and runs per physical interface. To control spanning-tree per VLAN, MSTP must be used, where VLANs can be mapped to instances. Excluding a VLAN from all instances effectively disables spanning-tree for that VLAN. The other options describe invalid commands or concepts.

Answer

Disabling RSTP on a per-VLAN basis requires configuring the VLAN as an edge VLAN.

Answer

You can exclude a VLAN from RSTP by using the 'no spanning-tree' command under the VLAN configuration.

Answer

You can disable RSTP for a specific VLAN using the 'disable' command under the spanning-tree protocol configuration.

Question 4

You are troubleshooting an RSTP network on EX Series switches. You need to identify which port on a non-root switch is the Root Port. Which command displays the Root Port for a given VLAN?

Accepted Answer

show spanning-tree interface detail. The command 'show spanning-tree interface detail' displays the role of each interface, including the Root Port. On a non-root switch, the Root Port is the port with the lowest path cost to the root bridge. This command provides the necessary detail to identify it. Other commands either show bridge-level information or statistics, not port roles.

Answer

show spanning-tree statistics

Answer

show spanning-tree interface

Answer

show spanning-tree bridge

Question 5

You are deploying RSTP in a network with EX Series switches. You want to ensure that a specific switch is always the root bridge for a particular VLAN. Which configuration would you apply?

Accepted Answer

Enable MSTP and map the VLAN to an instance where the switch has the lowest priority.. To ensure a switch is root for a specific VLAN, MSTP must be used because RSTP does not support per-VLAN root selection. Mapping the VLAN to an MST instance and setting the lowest priority on the desired switch achieves this. Other methods either affect all VLANs or are not configurable.

Answer

Set the bridge priority to 0 on the switch for that VLAN.

Answer

Configure the switch with the lowest MAC address in the network.

Answer

Set the port cost to 0 on all ports of the switch.

Question 6

You are configuring RSTP on an EX Series switch. You want to enable RSTP on a trunk interface that carries multiple VLANs. Which command is correct?

Accepted Answer

set protocols rstp interface ge-0/0/1.0. RSTP is enabled per interface in Junos, and for a trunk, you specify the logical unit (e.g., ge-0/0/1.0). RSTP is not VLAN-aware, so it applies to all VLANs on that interface. The other commands are invalid syntax or incorrect concepts.

Answer

set protocols rstp vlan all interface ge-0/0/1

Answer

set protocols rstp interface ge-0/0/1.0.0

Answer

set protocols rstp interface ge-0/0/1.0 vlan-id 10

Question 7

What is the default bridge priority value assigned to Juniper EX Series switches when RSTP is enabled?

Accepted Answer

32768. Juniper EX Series switches use the IEEE 802.1D/802.1w default bridge priority of 32768 when RSTP is enabled. Bridge priority is a 16-bit value expressed in increments of 4096, and 32768 is the midpoint of the range, making it the standard default across vendors. The bridge with the lowest priority (and then lowest MAC) becomes root.

Answer

61440

Answer

0

Answer

4096

Question 8

An administrator needs to configure MSTP on a group of EX Series switches. Which three parameters must be identical across all switches in the same MST region?

Accepted Answer

Configuration name, revision number, and VLAN-to-instance mapping. Switches belong to the same MST region only if they share the exact same configuration name, revision number, and VLAN-to-instance mapping.

Answer

MAC address, system ID extension, and MSTP version

Answer

Bridge priority, root identifier, and Hello timer

Answer

Max age, forward delay, and region map digest

Question 9

You need to ensure that a core switch retains its position as the root bridge even if a new switch with a manually lowered bridge priority is added to the network. Which feature should you configure on the core switch's ports facing the downstream switches?

Accepted Answer

Root protection. Root protection is configured on ports that should not receive superior BPDUs. When enabled on the core switch's downstream ports, it ensures that if a superior BPDU (indicating a new root bridge) is received, the port enters a root-inconsistent state, effectively preventing the core switch from losing its root bridge status. This feature is designed to enforce the root bridge location.

Answer

Loop protection

Answer

BPDU guard

Answer

Edge port configuration

Question 10

While examining an RSTP topology, you notice that a designated port has suddenly stopped receiving BPDUs from its upstream neighbor, causing the port to transition into a discarding state to prevent potential loops. Which feature is actively providing this protection?

Accepted Answer

Loop protection. Loop protection is the RSTP feature designed to prevent loops on designated ports that stop receiving BPDUs from their upstream neighbor. When a designated port no longer receives BPDUs, it would normally age out and potentially cause a loop; loop protection detects this condition and places the port into a discarding state (or blocks it) to prevent the loop. This matches the scenario exactly.

Answer

Root protection

Answer

BPDU guard

Answer

TC protection

Question 11

What is the role of the Root Port in an RSTP topology?

Accepted Answer

It provides the lowest-cost path from a non-root switch to the root bridge.. The Root Port in RSTP is the port on a non-root switch that provides the lowest-cost path to the root bridge. It is the port through which the switch sends traffic toward the root and receives traffic from the root. Each non-root switch has exactly one root port, which is elected based on path cost, sender bridge ID, sender port ID, and local port ID.

Answer

It forwards traffic between two different MST regions.

Answer

It connects the root bridge to all downstream non-root switches.

Answer

It serves as the backup forwarding path when the designated port fails.

Question 12

An enterprise network runs MSTP. Instance 0 is always reserved for which specific purpose?

Accepted Answer

The Common and Internal Spanning Tree (CIST) instance. In MSTP, Instance 0 is always reserved for the Common and Internal Spanning Tree (CIST). The CIST is the spanning tree that connects all MST regions and also includes the IST (Internal Spanning Tree) within a region. Therefore, option C is correct.

Answer

The Internal Regional Boundary instance (IRBI)

Answer

The Designated Root Backup instance (DRB)

Answer

The Vlan-to-Instance Mapping table (VIM)

Question 13

What is the primary function of the RSTP port role known as the 'Backup' port?

Accepted Answer

It acts as a backup for a designated port on the same segment.. An RSTP Backup port is a discarding port that provides a backup path to a designated port on the same shared segment. It exists only on shared (multi-access) segments where a switch has multiple ports connected to the same LAN, and it can quickly transition to forwarding if the designated port on that segment fails.

Answer

It forwards traffic when the primary trunk fails without triggering a topology change.

Answer

It acts as a backup for the root port towards the root bridge.

Answer

It is an operational state used exclusively during edge port transitions.

Question 14

When configuring MSTP, what purpose does the regional configuration digest serve?

Accepted Answer

It allows neighboring switches to verify if they belong to the same MST region.. The MSTP regional configuration digest is a hash value computed from the MST region's configuration (name, revision level, and VLAN-to-instance mapping). It is included in BPDUs so that neighboring switches can verify whether they are in the same MST region. If the digest does not match, the switches consider themselves in different regions, and the boundary ports operate as if connecting to different MST regions.

Answer

It encrypts the BPDU payload to prevent tampering by unauthorized access switches.

Answer

It acts as a dynamic bridge priority modifier for internal instance root selection.

Answer

It determines the Designated Master Bridge within the CIST instance.

Question 15

An operator configures RSTP across multiple EX Series switches. A rogue switch running legacy STP is connected to an RSTP switch. How does the RSTP switch handle interaction with the legacy STP switch?

Accepted Answer

The RSTP switch falls back to legacy STP behavior on that specific interface.. RSTP (IEEE 802.1w) is backward compatible with legacy STP (802.1D). When an RSTP switch receives legacy STP BPDUs on a port, it detects the neighbor as an STP bridge and falls back to legacy STP behavior on that specific interface only. The rest of the RSTP domain continues to operate in RSTP mode, ensuring interoperability without disrupting the entire network.

Answer

The port receiving legacy BPDUs is immediately placed into a permanent error-disabled state.

Answer

The legacy STP BPDUs are dropped and logged as protocol violations.

Answer

The entire switch converts its spanning tree mode to legacy STP.