JNCIS-ENT Spanning Tree — 20 Questions

Question 1

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 2

Which protocol standard defines Rapid Spanning Tree Protocol (RSTP)?

Accepted Answer

IEEE 802.1W. RSTP is standardized under IEEE 802.1W, which was later incorporated into IEEE 802.1Q.

Answer

IEEE 802.3ad

Answer

IEEE 802.1X

Question 3

Which TWO conditions will cause an RSTP port configured with loop protection to transition into a discarding state? (Choose two.)

Accepted Answer

Absence of incoming BPDUs on a point-to-point root port. Loop protection triggers when BPDUs stop arriving on a point-to-point link or when an alternate/root port loses expected BPDU keepalives.

Answer

Receipt of any BPDU on an access edge port

Answer

Exhaustion of the regional hop count limit

Question 4

You are configuring Multiple Spanning Tree Protocol (MSTP) across multiple EX Series switches in your enterprise network. Which three parameters must be identical across all switches to ensure they belong to the same MST region?

Accepted Answer

Configuration name, revision number, and VLAN-to-instance mapping. MST region configuration requires identical configuration name, revision number, and VLAN-to-instance mapping.

Answer

Bridge priority, root identifier, and Hello timer

Answer

Forward delay, max age, and region password

Answer

MAC address, system ID extension, and MST digest

Question 5

Which TWO actions occur when you configure an interface as an edge port using 'set protocols rstp interface ge-0/0/10.0 edge' on a Juniper switch? (Choose two.)

Accepted Answer

The port immediately transitions to the forwarding state, bypassing listening and learning phases.. Edge ports immediately transition to the forwarding state and do not generate topology change notifications (TCNs) when their link status toggles.

Answer

The port automatically assumes a Root Port role for the local switch.

Answer

The port will drop any incoming configuration BPDUs.

Question 6

When configuring and verifying MSTP on Junos devices, which THREE statements are correct regarding regional boundaries and communication? (Choose three.)

Accepted Answer

All switches within the same MST region share a common MST configuration identifier digest.. MSTP uses an Internal Spanning Tree (IST) inside a region and communicates with other regions and legacy STP via the Common Spanning Tree (CST) and region boundary ports.

Answer

MSTP regions must have identical bridge priorities for instance 0 to function.

Question 7

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

Accepted Answer

32768. The default bridge priority for JUNOS devices running STP, RSTP, or MSTP is 32768.

Answer

61440

Answer

0

Answer

4096

Question 8

Which command allows an operator to view the designated root bridge identifier and port roles for all active VLANs under RSTP?

Accepted Answer

show spanning-tree bridge. The command 'show spanning-tree interface' or 'show spanning-tree bridge' displays root bridge information. Specifically, 'show spanning-tree bridge' displays the root bridge identifier.

Answer

show spanning-tree statistics

Answer

show spanning-treemstp region

Answer

show spanning-tree interface detail

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 prevents a designated port from becoming a root port, ensuring that if a superior BPDU arrives on that port, the port transitions to a root-inconsistent (discarding) state rather than yielding the root role.

Answer

Loop protection

Answer

BPDU guard

Answer

Edge port configuration

Question 10

An EX Series switch running RSTP receives BPDUs with superior path cost information from an unauthorized access switch. Which feature should you enable on access-facing ports to prevent these unauthorized BPDUs from altering the spanning tree topology?

Accepted Answer

set protocols rstp interface ge-0/0/5.0 bpdu-guard. BPDU protection (bpdu-guard) disables a designated edge port if it receives a BPDU, protecting the network from unauthorized switches attempting to inject themselves into the topology.

Answer

set protocols rstp interface ge-0/0/5.0 no-root-port

Answer

set protocols rstp interface ge-0/0/5.0 root-protection

Answer

set protocols rstp interface ge-0/0/5.0 loop-protection

Question 11

You are troubleshooting a suboptimal spanning tree topology where an EX4300 switch is chosen as the STP Root Bridge despite having a high bridge priority. What command helps verify the current bridge priority and local bridge identifier parameters?

Accepted Answer

show spanning-tree bridge. The 'show spanning-tree bridge' command displays the local bridge identifier, bridge priority, and root bridge information, allowing administrators to inspect active STP parameters.

Answer

show spanning-tree statistics

Answer

show spanning-tree interface

Answer

show spanning-treemstp configuration

Question 12

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 detects when BPDUs are missing on a point-to-point link and places the port into a discarding state to prevent loops caused by unidirectional link failures.

Answer

Root protection

Answer

BPDU guard

Answer

TC protection

Question 13

An administrator has deployed a new EX Series switch running RSTP and notices that access ports connected to end-user workstations take up to 30 seconds to transition to the forwarding state. Which configuration command should be applied to immediately resolve this delay for access ports?

Accepted Answer

set protocols rstp interface ge-0/0/1.0 edge. In RSTP, edge ports transition directly to the forwarding state, bypassing the discarding and learning states. Configuring 'no-root-port' or designating the port as an edge port using 'set protocols rstp interface <interface> edge' allows workstations to connect without experiencing STP convergence delay.

Answer

set protocols rstp force-version stp

Answer

set protocols rstp interface ge-0/0/1.0 mode point-to-point

Answer

set protocols rstp bridge-priority 0

Question 14

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 is the port on a non-root switch that provides the lowest cost path to the root bridge.

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 15

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

Accepted Answer

The Common and Internal Spanning Tree (CIST) instance. Instance 0 in MSTP is the Common and Internal Spanning Tree (CIST) instance, which maintains connectivity across the entire bridged network and communicates with legacy STP/RSTP regions.

Answer

The Internal Regional Boundary instance (IRBI)

Answer

The Designated Root Backup instance (DRB)

Answer

The Vlan-to-Instance Mapping table (VIM)

Question 16

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.. A backup port provides a redundant path to a segment and is a backup for a designated port, receiving superior BPDUs from its own bridge on the same segment.

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 17

You are configuring MSTP and want to assign VLANs 10 through 50 to MST instance 2. Which configuration hierarchy is correct in Junos OS?

Accepted Answer

set protocols mstp configuration-name RegionA revision 1 instance msti2 vlan 10-50. In Junos OS, MSTP instances and their associated VLAN mappings are defined under the 'protocols mstp' hierarchy.

Answer

set bridge-domains domain1 mstp-instance 2 vlan 10-50

Answer

set protocols rstp msti 2 vlan-list 10-50

Answer

set vlans vlan-range 10-50 mstp instance 2

Question 18

You suspect that a network loop has formed due to a misconfigured RSTP topology on your EX Series switches. Which command shows real-time topology change notifications and counts on the switch?

Accepted Answer

show spanning-tree statistics. The command 'show spanning-tree statistics' displays counters related to topology changes, BPDU transmission, and reception per interface.

Answer

show bridge mac-table

Answer

show spanning-tree interface detail

Answer

show rstp neighbor

Question 19

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 configuration digest is an MD5 hash calculated from the region name, revision, and VLAN-to-instance mapping, included in BPDUs to verify that neighboring switches share the exact same region configuration.

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 20

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 switches have a migration mechanism that automatically detects legacy STP BPDUs on a per-port basis and falls back to running legacy 802.1D STP behavior on that specific port.

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.