Courseiva

CCNA Spanning Tree Questions

38 questions · Spanning Tree · All types, answers revealed

Practice these questionsDomain overviewAll questions
1
MCQmedium

You have an EX Series switch running RSTP. You want to configure a port connected to a printer to immediately transition to forwarding without going through the learning state. Which configuration statement should you use?

A.set protocols rstp interface ge-0/0/10 priority 0
B.set protocols rstp interface ge-0/0/10 cost 0
C.set protocols rstp interface ge-0/0/10 no-root-port
D.set protocols rstp interface ge-0/0/10 edge
AnswerD

The 'edge' configuration designates the port as an edge port, which means it connects to an end device and can immediately transition to forwarding upon link up, bypassing the learning state. This is ideal for a printer. In Junos, this is configured under the RSTP protocol hierarchy for the specific interface.

Why this answer

Configuring the port as an edge port with 'set protocols rstp interface ge-0/0/10 edge' allows it to bypass the learning state and immediately forward traffic when the link comes up. This is appropriate for devices like printers that do not participate in spanning tree. Other options do not achieve immediate transition and some are invalid commands.

Exam trap

The trap here is confusing port priority or cost with edge port configuration; only the edge setting enables immediate forwarding for end devices.

Practice this question →
2
Multi-Selectmedium

Which THREE parameters are included when calculating the MST region configuration digest? (Choose three.)

Select 3 answers
A.VLAN-to-instance mapping table
B.Configuration name
C.Root bridge MAC address
D.Bridge priority value
E.Revision number
AnswersA, B, E

The VLAN-to-instance mapping table forms part of the MST region configuration digest, since the digest must reflect exactly which VLANs each MSTI carries. Changing any VLAN-to-instance assignment alters the digest, causing neighbouring switches to treat the region as different and preventing them from sharing a common spanning-tree topology.

Why this answer

The MST region configuration digest is an MD5 hash computed over the region's configuration attributes, and it includes the VLAN-to-instance mapping table (A), the configuration name (B), and the revision number (E) — these three define the region's identity and ensure all switches agree on the same MST region. The VLAN-to-instance mapping (A) determines which VLANs belong to which MST instances, the configuration name (B) is the alphanumeric region identifier, and the revision number (E) is the 16-bit value that must match across all switches in the region. The root bridge MAC address (C) and bridge priority value (D) are not part of the digest; they are per-instance spanning-tree parameters used for root election and topology calculation, not region configuration attributes.

Exam trap

JNCIS-ENT often tests the misconception that bridge priority or MAC address affect the MST region digest — candidates confuse the digest parameters with the bridge ID fields used for root election.

Practice this question →
3
MCQeasy

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

A.It forwards traffic when the primary trunk fails without triggering a topology change.
B.It acts as a backup for the root port towards the root bridge.
C.It acts as a backup for a designated port on the same segment.
D.It is an operational state used exclusively during edge port transitions.
AnswerC

A Backup port receives BPDUs from its own designated bridge on a shared segment, so it stays discarding while that designated port forwards. It provides rapid failover only if the designated port fails, unlike an Alternate port, which backs up a root port.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the distinction between Alternate and Backup port roles — the trap is confusing 'backup for the root port' (Alternate) with 'backup for a designated port on the same segment' (Backup).

How to eliminate wrong answers

Option A is wrong because a port that forwards when a trunk fails without a topology change describes an Alternate port (backup to the root port), not a Backup port. Option B is wrong because backing up the root port is the role of the Alternate port — the Backup port backs up a designated port, not the root port. Option D is wrong because 'edge port transitions' relate to Edge ports (PortFast equivalents) and their immediate forwarding behaviour, not to the Backup port role.

Practice this question →
4
MCQhard

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?

A.The port receiving legacy BPDUs is immediately placed into a permanent error-disabled state.
B.The legacy STP BPDUs are dropped and logged as protocol violations.
C.The entire switch converts its spanning tree mode to legacy STP.
D.The RSTP switch falls back to legacy STP behavior on that specific interface.
AnswerD

RSTP detects legacy STP BPDUs on the link and operates in STP compatibility mode per port, transmitting and interpreting 802.1D BPDUs rather than RSTP's version. Other interfaces retain full RSTP, so degradation is scoped to that specific interface only.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the misconception that RSTP and STP cannot interoperate, leading candidates to choose answers that involve disabling ports or converting the entire switch, rather than the correct per-interface fallback.

How to eliminate wrong answers

Option A is wrong because RSTP does not error-disable a port upon receiving legacy BPDUs; it only does so for BPDU guard violations, not for standard STP interoperability. Option B is wrong because RSTP does not drop legacy BPDUs; it processes them to maintain loop-free topology. Option C is wrong because the entire switch does not convert to legacy STP; only the specific interface falls back, preserving RSTP benefits elsewhere.

Practice this question →
5
MCQmedium

You are configuring an EX Series switch and want to set the forward delay timer to 15 seconds. Which command is used?

A.set protocols rstp forward-delay 15
B.set bridge forward-delay 15
C.set spanning-tree forward-delay 15
D.set protocols rstp delay 15
AnswerA

The RSTP forward-delay timer is configured under the spanning-tree protocol hierarchy, so `set protocols rstp forward-delay 15` satisfies the stem's requirement of 15 seconds on the EX Series switch. RSTP retains a configurable forward-delay value even though its rapid transition mechanism usually bypasses the listening and learning waiting periods.

Why this answer

On Junos EX Series switches, RSTP is configured under the [edit protocols rstp] hierarchy, and the forward-delay timer is set with 'set protocols rstp forward-delay 15'. This command sets the RSTP forward delay to 15 seconds, controlling how long a port stays in the Listening and Learning states before transitioning to Forwarding.

Exam trap

JNCIS-ENT often tests Junos-specific CLI syntax — candidates familiar with Cisco or generic STP commands pick 'spanning-tree' or 'bridge' hierarchies that do not exist in Junos, where the correct hierarchy is 'protocols rstp'.

How to eliminate wrong answers

Option B is wrong because 'set bridge forward-delay 15' is not valid Junos syntax — Junos does not use a 'bridge' hierarchy for spanning-tree configuration on EX switches. Option C is wrong because 'set spanning-tree forward-delay 15' is not a Junos command; spanning-tree parameters are configured under 'protocols rstp' or 'protocols mstp', not a top-level 'spanning-tree' hierarchy. Option D is wrong because 'set protocols rstp delay 15' uses an incorrect keyword — the correct keyword is 'forward-delay', not 'delay'.

Practice this question →
6
MCQhard

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

A.The Internal Regional Boundary instance (IRBI)
B.The Designated Root Backup instance (DRB)
C.The Common and Internal Spanning Tree (CIST) instance
D.The Vlan-to-Instance Mapping table (VIM)
AnswerC

MSTP reserves instance 0 for the Common and Internal Spanning Tree, which carries all VLANs not explicitly mapped elsewhere and interoperates with other spanning-tree regions. This satisfies the stem's constraint that instance 0 has a fixed, reserved purpose rather than user-defined VLAN mapping.

Why this answer

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.

Exam trap

JNCIS-ENT often tests MSTP terminology; candidates might confuse Instance 0 with other reserved instances or think it's for a specific VLAN range, but it's always the CIST.

How to eliminate wrong answers

Option A is wrong because 'Internal Regional Boundary instance (IRBI)' is not a standard term in MSTP; the correct term is IST (Internal Spanning Tree) which is part of the CIST. Option B is wrong because 'Designated Root Backup instance (DRB)' is not a valid MSTP concept. Option D is wrong because 'Vlan-to-Instance Mapping table (VIM)' is not an instance; it's a mapping configuration, and Instance 0 is not reserved for that purpose.

Practice this question →
7
MCQmedium

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?

A.Bridge priority, root identifier, and Hello timer
B.Configuration name, revision number, and VLAN-to-instance mapping
C.Forward delay, max age, and region password
D.MAC address, system ID extension, and MST digest
AnswerB

MSTP region membership is defined by three identical attributes: the configuration name, the revision number, and the VLAN-to-instance mapping table. Any mismatch in these causes switches to belong to different regions, breaking the intended single spanning-tree topology across the EX Series estate.

Why this answer

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

Practice this question →
8
Multi-Selectmedium

Which THREE parameters must match across all switches for them to successfully form a single MST region? (Choose three.)

Select 3 answers
A.VLAN-to-instance mapping table
B.Configuration name
C.Revision number
D.Bridge priority value
E.Hello timer interval
AnswersA, B, C

The VLAN-to-instance mapping table must be identical on every switch, since MSTP computes separate topologies per instance. Mismatched mappings mean switches disagree about which VLANs share a spanning-tree instance, so they cannot form one common region.

Why this answer

For switches to form a single MST region, the MST configuration must be identical across all of them, and this is verified by three specific parameters. Option A, the VLAN-to-instance mapping table, must match because it defines which VLANs are mapped to which MST instances (MSTIs), and any difference causes the switches to be in different regions. Option B, the configuration name, is a 32-character alphanumeric string that identifies the MST region and must be identical on all switches.

Option C, the revision number, is a 16-bit value that must also match, as it is used together with the configuration name and mapping table to determine region membership. Options D and E are incorrect because bridge priority and hello timer are per-instance or per-port STP parameters that affect root election and BPDU timing, but they are not part of the MST region identifier and do not need to match for region formation.

Exam trap

JNCIS-ENT often tests the distinction between MST region identity parameters (name, revision, VLAN mapping) and per-instance tunables (bridge priority, timers) — candidates frequently include bridge priority because it feels like a global STP setting.

Practice this question →
9
MCQmedium

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?

A.Loop protection
B.BPDU guard
C.Root protection
D.Edge port configuration
AnswerC

Root protection forces a port into a root-inconsistent blocking state if it receives superior BPDUs, preventing a downstream switch with a lowered priority from becoming root. This preserves the core switch's root bridge role, satisfying the requirement to retain its position.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the differences between root protection, BPDU guard, and loop protection, and candidates may confuse their specific use cases and behaviors.

How to eliminate wrong answers

Option A is wrong because loop protection is used to prevent loops by disabling a port if no BPDUs are received, not to enforce root bridge status. Option B is wrong because BPDU guard disables a port upon receiving any BPDU, typically used on edge ports to prevent loops, not to protect the root bridge. Option D is wrong because edge port configuration is used to speed up convergence on ports connected to end devices, not to prevent root bridge takeover.

Practice this question →
10
Multi-Selecthard

Which TWO mechanisms are utilized by RSTP to achieve rapid convergence compared to legacy 802.1D STP? (Choose two.)

Select 2 answers
A.Proposal-agreement handshake mechanism
B.Mandatory 15-second listening timer enforcement
C.Centralized root bridge polling
D.Support for point-to-point link synchronization
E.Periodic topology change broadcasting by root bridges
AnswersA, D

The proposal-agreement handshake lets a designated port bypass the 802.1D listening and learning timers, synchronising the downstream bridge immediately. This satisfies the stem's rapid-convergence constraint by replacing timer-based waiting with explicit negotiation, so edge-facing topology changes propagate in seconds rather than the legacy 30–50 second convergence delay.

Why this answer

Option A (Proposal-agreement handshake mechanism) is correct because RSTP uses a proposal/agreement exchange on designated ports to rapidly move a port to Forwarding without waiting for the legacy STP timers, allowing quick synchronization between neighboring switches. Option D (Support for point-to-point link synchronization) is correct because RSTP explicitly distinguishes edge, point-to-point, and shared link types; on point-to-point links it can immediately transition ports via the sync process, which is fundamental to its rapid convergence. Option B is incorrect because RSTP does not enforce the legacy 15-second listening timer; it replaces timer-based transitions with handshake-based ones.

Option C is incorrect because RSTP does not use centralized root bridge polling; the root bridge is elected via BPDUs as in STP, and convergence is distributed. Option E is incorrect because RSTP does not rely on periodic topology change broadcasts from the root; topology changes are signaled via TC-flag BPDUs and propagated hop-by-hop.

Exam trap

JNCIS-ENT often tests the misconception that RSTP still uses the 15-second listening timer — candidates carry over 802.1D knowledge and miss that RSTP replaces timers with the proposal-agreement handshake.

Practice this question →
11
MCQmedium

When a topology change occurs in an RSTP network, how are MAC address forwarding table entries affected across the switches?

A.MAC addresses are relearned only after the root bridge sends a direct clearing command.
B.MAC address entries are retained until the standard 300-second aging timer expires.
C.MAC address tables are completely cleared for all ports, including edge ports.
D.MAC address table entries associated with non-edge ports are flushed immediately.
AnswerD

RSTP flushes MAC entries learned on non-edge ports immediately upon topology change, rather than ageing them out. This satisfies the stem's requirement for rapid reconvergence: stale forwarding entries are purged at once, preventing misdirected traffic, while edge ports retain their entries since they cannot create loops.

Why this answer

When a topology change occurs in RSTP, MAC address table entries associated with non-edge ports are flushed immediately. This allows the switches to relearn MAC addresses on the new topology, preventing forwarding to stale locations. Edge ports are not flushed because they connect to end devices and are not affected by topology changes.

Exam trap

JNCIS-ENT often tests the difference between STP and RSTP topology change handling; candidates may incorrectly think MAC entries age out over 300 seconds instead of being flushed immediately.

How to eliminate wrong answers

Option A is wrong because MAC addresses are not relearned only after a direct clearing command from the root bridge; RSTP uses a topology change notification mechanism that triggers immediate flushing. Option B is wrong because MAC entries are not retained for 300 seconds; that is the default aging time in classic STP, but RSTP flushes immediately on topology change. Option C is wrong because MAC address tables are not completely cleared for all ports; only non-edge ports are flushed, while edge ports retain their entries.

Practice this question →
12
MCQeasy

What is the primary purpose of the Hello timer in RSTP?

A.It sets the interval between topology change notifications.
B.It determines the maximum time a switch waits before declaring a neighbor dead.
C.It defines the duration of the listening and learning states during convergence.
D.It dictates how often configuration BPDUs are generated and sent by the root bridge.
AnswerD

RSTP Hello timer sets the interval at which the root bridge generates and transmits configuration BPDUs, defaulting to two seconds; downstream bridges relay these, so it governs topology information freshness rather than port state timers.

Why this answer

In RSTP (IEEE 802.1w), the Hello timer defines the interval at which the root bridge generates and transmits configuration BPDUs. Non-root bridges do not originate BPDUs; they relay the root's BPDUs downstream after updating them. The default Hello time is 2 seconds, and this timer is configured only on the root bridge (or the intended root) and propagated throughout the spanning tree via the BPDU fields.

Thus, the Hello timer directly controls the frequency of configuration BPDU generation and transmission by the root bridge.

Exam trap

JNCIS-ENT often tests the confusion between the Hello timer and other STP timers, such as Max Age, Forward Delay, and the topology change mechanism, leading candidates to incorrectly associate the Hello timer with failure detection or state durations.

How to eliminate wrong answers

Option A is wrong because topology change notifications in RSTP are not sent at a fixed interval; they are triggered immediately by a topology change and propagated via the TC flag in BPDUs, not governed by the Hello timer. Option B is wrong because the maximum time a switch waits before declaring a neighbor dead is determined by the 3× Hello timer rule (or the Message Age and Max Age in classic STP), not by the Hello timer alone; in RSTP, a neighbor is considered lost after three consecutive missed Hellos (default 6 seconds), but the Hello timer itself does not define that maximum wait. Option C is wrong because the duration of the listening and learning states is not defined by the Hello timer; in RSTP, these states are replaced by the learning and forwarding states with rapid transition, and any timing is governed by the Forward Delay timer in legacy STP, not the Hello timer.

Practice this question →
13
Multi-Selecteasy

Which TWO commands are valid Junos configuration hierarchy paths for setting up RSTP? (Choose two.)

Select 2 answers
A.set bridge rstp mode rapid-pvst
B.set protocols stp rstp-version true
C.set protocols rstp bridge-priority 4096
D.set protocols rstp interface ge-0/0/1.0 edge
E.set spanning-tree rstp enable
AnswersC, D

RSTP bridge priority is configured under the protocols rstp hierarchy, so 'set protocols rstp bridge-priority 4096' is a valid path. The value is a multiple of 4096, making it a legal priority that influences root bridge election.

Why this answer

Option C is correct because under the Junos RSTP hierarchy, the bridge priority is configured with 'set protocols rstp bridge-priority 4096', which sets the bridge priority value used in RSTP root election. Option D is correct because 'set protocols rstp interface ge-0/0/1.0 edge' is a valid Junos RSTP interface-level command that marks the specified interface as an edge port, allowing it to transition immediately to forwarding. Option A is not valid because 'bridge rstp mode rapid-pvst' is not a Junos configuration path and mixes Cisco-style syntax.

Option B is not valid because 'set protocols stp rstp-version true' is not a Junos command; RSTP is enabled under 'protocols rstp', not via an 'rstp-version' boolean under 'protocols stp'. Option E is not valid because 'set spanning-tree rstp enable' is not Junos syntax; Junos uses the 'protocols rstp' hierarchy rather than a 'spanning-tree' stanza.

Exam trap

JNCIS-ENT often tests Junos vs. Cisco syntax confusion — candidates pick Cisco-style commands like 'rapid-pvst' or 'spanning-tree rstp enable' that look familiar but are invalid in Junos.

Practice this question →
14
MCQeasy

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

A.61440
B.0
C.32768
D.4096
AnswerC

Juniper EX Series switches use 32768 as the default bridge priority for RSTP, satisfying the stem's request for the out-of-the-box value. This 16-bit field, combined with the switch's MAC address, forms the 8-byte bridge identifier that determines root bridge election; lower values win, so 32768 is the midpoint default.

Why this answer

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.

Exam trap

JNCIS-ENT often tests whether candidates remember the exact default bridge priority (32768) versus other valid values like 4096 or 61440 — the trap is confusing a commonly configured value with the actual default.

How to eliminate wrong answers

Option A is wrong because 61440 is a valid bridge priority value (15 × 4096) but is not the default — it is a high value often used to make a switch unlikely to become root. Option B is wrong because 0 is the lowest possible priority and is typically configured deliberately to force a switch to become root, not a default. Option D is wrong because 4096 is a low priority value (1 × 4096) used to influence root election, not the default.

Practice this question →
15
MCQhard

When configuring MSTP, how is the regional maximum hop count enforced across switches within the same MST region?

A.The regional root periodically floods drop-commands if the network diameter exceeds the limit.
B.BPDUs carry a hop count that is decremented by each bridge; when it reaches zero, the BPDU is dropped.
C.Switches drop any incoming BPDUs that exceed the statically configured Max Age timer.
D.Ports exceeding the hop count are automatically placed into the error-disabled state.
AnswerB

MSTP enforces the regional hop count through the BPDU's remaining-hops field, which each bridge decrements on receipt. Once the value hits zero the BPDU is discarded, preventing stale topology information from looping indefinitely across the region.

Why this answer

In MSTP, the regional maximum hop count is enforced by the hop count field in BPDUs. The regional root sends BPDUs with a hop count equal to the maximum hop count (default 20). Each bridge that receives the BPDU decrements the hop count by one before forwarding it.

When the hop count reaches zero, the BPDU is dropped, preventing loops and limiting the network diameter within the MST region.

Exam trap

JNCIS-ENT often tests the misconception that MSTP uses a static Max Age timer or error-disabled ports to enforce hop count, when in reality it uses a decrementing hop count in BPDUs.

How to eliminate wrong answers

Option A is wrong because MSTP does not use drop-commands; it uses BPDU hop count decrementing. Option C is wrong because Max Age timer is used for BPDU aging, not hop count enforcement. Option D is wrong because ports are not error-disabled due to hop count; the BPDU is simply dropped when hop count expires.

Practice this question →
16
MCQhard

An organization runs MSTP with multiple instances. A packet belonging to a VLAN that is not explicitly mapped to any MST instance is processed in which manner?

A.It is processed by the default RSTP instance running outside MSTP.
B.It is dropped by the switch because unmapped VLANs are considered invalid in MSTP.
C.It is automatically mapped to Instance 0 (the CIST).
D.It floods to all ports regardless of spanning tree port states.
AnswerC

MSTP maps every VLAN not explicitly assigned to an instance into the Common and Internal Spanning Tree, Instance 0. That instance carries the CIST topology, so unmapped VLAN traffic follows Instance 0 forwarding rather than being dropped or creating a separate instance.

Why this answer

In MSTP (Multiple Spanning Tree Protocol, IEEE 802.1s), VLANs that are not explicitly mapped to a specific MST instance are automatically assigned to the Common and Internal Spanning Tree (CIST), which is MST instance 0. The CIST provides connectivity for all VLANs that are not mapped to other instances and also interconnects MST regions. Therefore, the packet is processed by Instance 0, not dropped or flooded indiscriminately.

Exam trap

JNCIS-ENT often tests the misconception that unmapped VLANs are dropped or handled by a separate RSTP instance, when in fact they are automatically mapped to the CIST (Instance 0).

How to eliminate wrong answers

Option A is wrong because MSTP does not run a separate 'default RSTP instance' outside of MSTP; the CIST is an integral part of MSTP and operates as instance 0. Option B is wrong because unmapped VLANs are not invalid; they are automatically mapped to the CIST. Option D is wrong because flooding to all ports regardless of spanning tree state would create loops and violate STP operation; MSTP still enforces port states for the CIST.

Practice this question →
17
MCQhard

An administrator configures root protection on a designated port. If that port subsequently receives a superior BPDU from another switch, what exact state does the port transition into?

A.Alternate port state
B.Error-disabled state
C.Root-inconsistent (discarding) state
D.Listening state
AnswerC

Root protection on a designated port places the port into the root-inconsistent discarding state when a superior BPDU arrives, blocking that interface to prevent a rogue switch becoming root. The port recovers automatically once superior BPDUs cease.

Why this answer

When root protection is configured on a designated port and that port receives a superior BPDU, the port is placed into the root-inconsistent (discarding) state rather than being error-disabled. This prevents the port from becoming a root port and preserves the intended root bridge placement.

Exam trap

JNCIS-ENT often tests the distinction between Root Guard (root-inconsistent, auto-recovering) and BPDU Guard (error-disabled, manual recovery) — candidates conflate the two penalty states.

How to eliminate wrong answers

Option A is wrong because alternate port state is a normal RSTP role for a port that provides a backup path to the root, not the penalty state for root protection violations. Option B is wrong because error-disabled state is used by BPDU Guard (which shuts the port on any BPDU), not by Root Guard, which uses the root-inconsistent state. Option D is wrong because listening state is a legacy 802.1D transitional state, not the specific penalty applied by root protection.

Practice this question →
18
Multi-Selecteasy

Which TWO features help protect an RSTP topology from unauthorized switches or misconfigurations on access ports? (Choose two.)

Select 2 answers
A.Bridge priority adjustment
B.Hello timer scaling
C.Root protection
D.Forward delay timer optimization
E.BPDU guard
AnswersC, E

Root protection on a designated access port blocks superior BPDUs that would otherwise let an unauthorised switch become root, preserving the existing topology. It directly counters misconfiguration or rogue devices attempting to hijack the root role on access ports.

Why this answer

Root protection (Option C) is correct because when enabled on a port, it forces that port into a root-inconsistent blocking state if it receives a superior BPDU, preventing an unauthorized or misconfigured switch on an access port from becoming the root bridge and hijacking the spanning-tree topology. BPDU guard (Option E) is correct because it err-disables a port immediately upon receiving any BPDU, which stops rogue switches or misconfigured devices from participating in RSTP on access ports that should only connect to end hosts. Bridge priority adjustment (Option A) only influences which switch becomes root and does not defend against unauthorized switches on access ports.

Hello timer scaling (Option B) merely changes how often BPDUs are sent and does not provide protection. Forward delay timer optimization (Option D) tunes STP convergence timing and likewise offers no security against rogue or misconfigured access-port devices.

Exam trap

JNCIS-ENT often tests the distinction between STP tuning parameters (priority, timers) and STP security features (BPDU guard, root protection) — candidates pick tuning knobs thinking they provide protection.

Practice this question →
19
Multi-Selecthard

Which TWO port roles are defined in Rapid Spanning Tree Protocol (RSTP) but did not exist in legacy 802.1D STP? (Choose two.)

Select 2 answers
A.Master port
B.Root port
C.Backup port
D.Designated port
E.Alternate port
AnswersC, E

RSTP introduces the backup port role, which provides a redundant path to a segment already reached through a designated port on the same bridge. Legacy 802.1D STP defined only root, designated and blocked roles, so backup did not exist.

Why this answer

RSTP (IEEE 802.1w) introduced two new port roles that did not exist in legacy 802.1D STP: the Alternate port (option E) and the Backup port (option C). An Alternate port is a discarding port that receives superior BPDUs from another bridge and serves as an immediate replacement for the Root port if it fails, while a Backup port is a discarding port that receives superior BPDUs from its own bridge (a redundant connection to the same shared segment) and can immediately replace a Designated port. Both roles enable RSTP's rapid failover by allowing pre-computed, ready-to-use backup paths instead of waiting for STP timers.

The other options are not new to RSTP: Root port (B) and Designated port (D) are the two forwarding roles already defined in 802.1D STP, and Master port (A) is a role specific to Multiple Spanning Tree Protocol (MSTP, 802.1s) used on boundary ports toward the Common Spanning Tree region, not a role introduced by RSTP.

Exam trap

The trap is confusing RSTP's new roles (Alternate, Backup) with roles that existed in 802.1D (Root, Designated) or with MSTP-specific roles like Master.

Practice this question →
20
MCQmedium

You notice that an access switch running RSTP is continually dropping user traffic because a connected end-user device is sending inferior BPDUs that trigger topology changes. Which Juniper feature should you configure on the access ports to protect against this behavior?

A.set protocols rstp interface ge-0/0/1.0 loop-protection
B.set protocols rstp interface ge-0/0/1.0 bpdu-protection
C.set protocols rstp interface ge-0/0/1.0 root-protection
D.set protocols rstp interface ge-0/0/1.0 no-root-port
AnswerB

BPDU protection administratively disables an edge port that receives BPDUs, so inferior BPDUs from the attached device can no longer trigger RSTP topology changes. This directly stops the continual traffic drops described in the stem.

Why this answer

BPDU protection is the Juniper RSTP feature designed to shut down (or disable) an edge/access port if it receives a BPDU. Since end-user devices should never send BPDUs, receiving one indicates a rogue switch or misconfigured device. When bpdu-protection is enabled on the access interface, the port is immediately disabled (error-disabled) upon receiving a BPDU, preventing the inferior BPDU from triggering topology changes and disrupting user traffic.

Exam trap

JNCIS-ENT often tests the distinction between BPDU protection, root protection, and loop protection; candidates frequently confuse root-protection (which prevents a port from becoming root) with BPDU protection (which blocks BPDUs on edge ports).

How to eliminate wrong answers

Option A is wrong because loop-protection is used on root ports and alternate ports to prevent unidirectional link loops; it does not block BPDUs from end-user devices. Option C is wrong because root-protection is applied to designated ports to prevent them from becoming root ports; it does not stop BPDU processing on access ports. Option D is wrong because no-root-port is not a valid Juniper RSTP configuration statement; the correct concept is root-protection, not no-root-port.

Practice this question →
21
Multi-Selecteasy

Which TWO commands can be used to view spanning tree information on a Juniper EX Series switch? (Choose two.)

Select 2 answers
A.show bridge spanning-tree status
B.show spanning-tree interface
C.show spanning-tree bridge
D.show rstp summary
E.show protocols spanning-tree summary
AnswersB, C

`show spanning-tree interface` displays per-port spanning-tree state, role, cost and edge status on EX Series switches, satisfying the requirement to view spanning tree information. It queries the interface-level STP database directly, confirming which ports forward or block, unlike configuration or non-STP commands.

Why this answer

Option B, 'show spanning-tree interface', is correct because on Juniper EX Series switches running Junos, this operational command displays per-interface spanning-tree information such as role, state, port cost, and edge/point-to-point settings for the specified interface. Option C, 'show spanning-tree bridge', is correct because it displays bridge-level spanning-tree parameters, including the bridge ID, root bridge information, timers, and topology change counters. These two commands are the standard Junos CLI forms for inspecting STP/RSTP/MSTP on EX switches.

Option A is not a valid Junos command; the 'show bridge' hierarchy is not used for spanning tree on EX. Option D is invalid because 'show rstp summary' is not a Junos operational command. Option E is invalid because 'show protocols spanning-tree summary' is not a Junos command; 'show protocols' is used for routing protocol status, not spanning tree.

Exam trap

JNCIS-ENT often tests command syntax familiarity — candidates pick plausible-sounding but non-existent commands like 'show rstp summary' or 'show protocols spanning-tree' because they mirror Cisco or routing-protocol naming conventions rather than actual Junos hierarchy.

Practice this question →
22
Multi-Selectmedium

Which THREE statements describe the function of the Designated Port in RSTP? (Choose three.)

Select 3 answers
A.It provides the lowest cost path from the segment back toward the root bridge.
B.It remains permanently in the discarding state to prevent loops.
C.It forwards BPDUs onto the attached LAN segment.
D.There is exactly one designated port for every active LAN segment.
E.It acts as the primary root port for the local bridge.
AnswersA, C, D

The designated port for a segment advertises the lowest-cost path from that segment toward the root bridge, based on root path cost. It is the forwarding port elected per segment to carry traffic toward the root.

Why this answer

Option A is correct because the designated port is the port on a LAN segment that offers the lowest path cost back toward the root bridge, making it the forwarding port for that segment. Option C is correct because the designated port is responsible for sending (forwarding) BPDUs onto its attached LAN segment, which is how RSTP maintains topology information. Option D is correct because RSTP elects exactly one designated port per active LAN segment to avoid loops and ensure consistent forwarding.

Option B is incorrect because designated ports normally forward frames and BPDUs, not remain permanently discarding; discarding is a temporary or alternate/backup port state. Option E is incorrect because the primary root port is the port on a non-root bridge closest to the root bridge, which is a different role from the designated port.

Exam trap

The trap is conflating the Designated Port with the Root Port — candidates pick 'primary root port for the local bridge' (Option E), but that describes the Root Port, while the Designated Port is elected per segment and forwards BPDUs onto that segment.

Practice this question →
23
MCQhard

An EX Series switch running MSTP receives a BPDU from an upstream switch in a different MST region. How does the receiving switch identify the boundary between the two regions?

A.When the incoming BPDU contains an instance count greater than the local switch capacity.
B.When the CIST external path cost exceeds the maximum hop count.
C.By comparing the received MST configuration digest and region parameters with its own.
D.By detecting a mismatch in the system MAC address of the root bridge.
AnswerC

The receiving switch compares the received MST configuration digest and region parameters (name, revision, VLAN-to-instance mapping) against its own. A mismatch in any of these identifies the boundary, so the port becomes a boundary port linking the two distinct MST regions.

Why this answer

MSTP (IEEE 802.1s) identifies region boundaries by comparing the MST Configuration Identifier carried in BPDUs, which consists of the configuration name, revision level, and a 16-byte configuration digest computed via HMAC-MD5 over the VLAN-to-instance mapping. If any of these parameters differ from the local switch's own MST configuration, the receiving switch treats the port as a boundary port and interoperates with the neighbor using CIST (MST instance 0) rather than a shared MSTI. This is why option C is correct — the digest and region parameters are the definitive boundary markers.

Exam trap

JNCIS-ENT often tests the misconception that region boundaries are detected via root bridge MAC or instance count, when in fact the MST Configuration Identifier (name, revision, and MD5 digest of the VLAN mapping) is the sole determinant.

How to eliminate wrong answers

Option A is wrong because the MSTI instance count is not part of the MST Configuration Identifier and has no role in boundary detection — a switch can support up to 64 MSTIs regardless of neighbor configuration. Option B is wrong because CIST external path cost is used for root path selection and topology calculation, not for identifying region boundaries; hop count limits apply to RSTP/MSTP BPDU aging, not region detection. Option D is wrong because the root bridge system MAC address is used for root election tie-breaking, not for determining whether two switches belong to the same MST region.

Practice this question →
24
Multi-Selectmedium

Which TWO parameters are included when a Junos switch calculates the MST configuration digest? (Choose two.)

Select 2 answers
A.Bridge priority of Instance 0
B.Revision number
C.System MAC address
D.Configuration name
E.Hello timer interval
AnswersB, D

The digest is computed from the MSTP configuration name and the revision number, hashed together. The revision number lets administrators version the MST region configuration, so switches sharing a name but differing revisions land in separate regions.

Why this answer

The MST configuration digest is an MD5 hash computed from the MST configuration identifier, which consists of exactly three elements: the configuration name, the revision level (revision number), and the VLAN-to-MSTI mapping table. Option B, the revision number, is correct because it is one of the three components fed into the digest calculation. Option D, the configuration name, is also correct because the 32-character configuration name is likewise part of the MST configuration identifier hashed into the digest.

Options A, C, and E are incorrect: bridge priority of Instance 0, the system MAC address, and the hello timer interval are not part of the MST configuration identifier and therefore are not included in the digest computation.

Exam trap

JNCIS-ENT often tests the components of the MST configuration digest, and candidates may incorrectly include bridge priority or MAC address.

Practice this question →
25
MCQmedium

You need to modify the bridge priority of an EX Series switch running RSTP to ensure it becomes the root bridge. Which configuration command correctly sets the bridge priority to 4096?

A.set spanning-tree priority 4096
B.set protocols rstp priority 4096
C.set bridge-priority 4096
D.set protocols rstp bridge-priority 4096
AnswerD

RSTP bridge priority is configured under the rstp hierarchy using the bridge-priority statement, and 4096 is a valid multiple of 4096. This command sets the switch's priority low enough to win root election, satisfying the requirement to become root bridge.

Why this answer

On Juniper EX Series switches, RSTP is configured under the [edit protocols rstp] hierarchy. The bridge priority is set using the 'bridge-priority' command, so the correct full command is 'set protocols rstp bridge-priority 4096'. This value is used in the bridge ID and must be a multiple of 4096 (ranging from 0 to 61440).

Setting it to 4096 makes the switch highly likely to become the root bridge, assuming no other switch has a lower priority.

Exam trap

JNCIS-ENT often tests the exact Junos CLI hierarchy and keywords for RSTP, and candidates frequently confuse 'priority' with 'bridge-priority' or forget the 'protocols rstp' context.

How to eliminate wrong answers

Option A is wrong because 'set spanning-tree priority 4096' is not valid Junos syntax; spanning-tree is not a top-level configuration hierarchy on EX switches, and 'priority' is not the correct keyword under RSTP. Option B is wrong because 'priority' is not the correct keyword; the correct keyword is 'bridge-priority'. Option C is wrong because 'set bridge-priority 4096' is incomplete and missing the required 'protocols rstp' hierarchy, so it would not be accepted by the CLI.

Practice this question →
26
MCQmedium

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?

A.delete protocols rstp
B.set protocols rstp interface ge-0/0/10.0 disable
C.set interfaces ge-0/0/10.0 spanning-tree disable
D.set protocols rstp interface ge-0/0/10.0 mode off
AnswerB

The disable statement under protocols rstp applies to the named interface only, stopping RSTP participation on ge-0/0/10.0 while RSTP remains active on all other ports. This precisely satisfies the stem's requirement to disable spanning tree on one access interface.

Why this answer

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.

Exam trap

The trap here is mixing up global RSTP commands with per-interface commands — candidates who don't know Junos hierarchy may pick 'delete protocols rstp' or invent an 'interfaces ... spanning-tree' command that doesn't exist.

How to eliminate wrong answers

Option A is wrong because 'delete protocols rstp' removes RSTP from the entire switch, not just one interface, which is the opposite of the requirement. Option C is wrong because spanning-tree is not configured under the 'interfaces' hierarchy in Junos — it lives under 'protocols rstp', so this command is syntactically invalid. Option D is wrong because 'mode off' is not a valid RSTP interface mode; valid modes are point-to-point and shared, and disabling is done with the 'disable' keyword.

Practice this question →
27
MCQhard

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

A.It acts as the root switch for a specific MSTI within the local MST region.
B.It translates MST BPDUs into legacy STP BPDUs at the regional boundary.
C.It manages the CIST instance across all connected foreign MST regions.
D.It serves as the sole bridge responsible for generating the MST configuration digest.
AnswerA

Within an MST region, the Internal Root is the bridge elected as root for one specific MSTI, using internal path costs. It governs topology inside the region while the CIST root handles connectivity between regions.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the confusion between the Internal Root (per-MSTI root inside a region) and the CIST Regional Root (the bridge connecting the region to the CIST) — candidates conflate the two roles.

How to eliminate wrong answers

Option B is wrong because translating MST BPDUs to legacy STP BPDUs at the regional boundary is not the function of the Internal Root — boundary ports handle interoperation with other regions or STP domains. Option C is wrong because the CIST (Common and Internal Spanning Tree) is managed by the CIST Regional Root and the CIST Root, not by an MSTI Internal Root. Option D is wrong because the MST configuration digest is computed identically by every bridge in the region from the configuration name, revision number, and VLAN-to-instance mapping — no single bridge generates it.

Practice this question →
28
MCQhard

An administrator configures link aggregation (ae0) using LACP between two EX Series switches running RSTP. How does RSTP treat the aggregated interface ae0?

A.RSTP places all member links except the primary active link into a permanent discarding state.
B.RSTP disables itself on aggregated links to prevent conflicts with LACP.
C.RSTP treats the aggregate interface as a single logical port.
D.RSTP runs an independent spanning tree instance on each physical member link of the aggregate.
AnswerC

LACP bundles the physical member links into one IEEE 802.3ad aggregate, and RSTP runs over that aggregate as a single logical port. This satisfies the stem's constraint: RSTP never blocks individual member links, so no member is placed in a discarding state.

Why this answer

RSTP operates on the aggregate interface (ae0) as a single logical port, not on the individual physical member links. This means the entire bundle is either in a forwarding or discarding state based on the spanning tree topology, and LACP handles the distribution of traffic across the active member links. The aggregated interface presents a single MAC address and a single port cost to RSTP, so the spanning tree calculation sees it as one link.

Exam trap

JNCIS-ENT often tests the misconception that RSTP runs per physical link in an aggregate, leading candidates to choose options that describe independent spanning tree instances or discarding states for member links.

How to eliminate wrong answers

Option A is wrong because RSTP does not place individual member links into a discarding state; all active member links in the aggregate forward traffic, and LACP manages their state. Option B is wrong because RSTP and LACP are complementary protocols that operate at different layers; RSTP does not disable itself on aggregated links. Option D is wrong because RSTP does not run independent instances per physical link; it treats the aggregate as a single logical interface, avoiding multiple spanning tree instances on the same bundle.

Practice this question →
29
MCQhard

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?

A.MAC address, system ID extension, and MSTP version
B.Configuration name, revision number, and VLAN-to-instance mapping
C.Bridge priority, root identifier, and Hello timer
D.Max age, forward delay, and region map digest
AnswerB

MSTP regions require three matching attributes: the configuration name, the revision number, and the VLAN-to-instance mapping table. Switches sharing these identical values join the same region, so the stem's requirement for consistent region membership is satisfied. Mismatched mapping tables would place a switch in a separate region, breaking the intended topology.

Why this answer

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

Practice this question →
30
MCQeasy

What is the primary benefit of enabling edge port configuration on switch access ports connected to end devices?

A.It forces the port to operate exclusively in full-duplex mode.
B.It allows the port to transition immediately to the forwarding state, avoiding startup delays.
C.It increases the interface bandwidth by disabling spanning tree calculations entirely.
D.It enables automatic detection of unauthorized rogue bridges on access links.
AnswerB

Edge port, equivalent to RSTP edge or Cisco PortFast, lets an access port skip listening and learning and enter forwarding immediately when a link comes up. This removes the roughly 30-second STP startup delay before end devices can pass traffic.

Why this answer

Enabling edge port configuration on switch access ports connected to end devices allows the port to transition immediately to the forwarding state, bypassing the listening and learning states of STP/RSTP. This avoids startup delays (up to 30 seconds with classic STP) and improves convergence for end-user devices. Edge ports are also known as PortFast in Cisco terminology.

Exam trap

JNCIS-ENT often tests the misconception that edge ports disable STP entirely or affect duplex settings, when they only affect the port's initial STP state.

How to eliminate wrong answers

Option A is wrong because edge port configuration does not force full-duplex mode; duplex is typically auto-negotiated or manually set, independent of edge port status. Option C is wrong because edge ports do not disable spanning tree calculations entirely; they still participate in STP but are assumed to not create loops, so they can forward immediately. Option D is wrong because edge ports do not detect rogue bridges; they are designed for end devices, and if a BPDU is received, the port loses its edge status and participates in STP to prevent loops.

Practice this question →
31
MCQmedium

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?

A.Loop protection
B.Root protection
C.BPDU guard
D.TC protection
AnswerA

Loop protection detects a designated port that stops receiving BPDUs from its upstream neighbour. It then moves that port into a discarding state, preventing a unidirectional link failure from creating a bridging loop, which matches the described RSTP behaviour.

Why this answer

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.

Exam trap

The trap here is confusing loop protection with root protection or BPDU guard; candidates often mix up which feature reacts to loss of BPDUs versus receiving unexpected BPDUs.

How to eliminate wrong answers

Option B is wrong because root protection is applied to root ports to prevent them from becoming designated ports when superior BPDUs are received, guarding against a rogue switch becoming root; it does not react to loss of BPDUs on a designated port. Option C is wrong because BPDU guard is applied to edge ports (access ports) and shuts them down if any BPDU is received, which is the opposite condition (receiving BPDUs, not losing them). Option D is wrong because TC protection (Topology Change protection) is used to rate-limit or ignore topology change notifications, not to protect against loss of BPDUs on a designated port.

Practice this question →
32
MCQeasy

What action does the 'set protocols rstp max-age <seconds>' command perform on a Juniper switch?

A.It sets the maximum time a port can remain in the discarding state during bootup.
B.It limits the maximum age of MAC table entries learned via spanning tree.
C.It restricts the maximum number of hops an MSTP BPDU can traverse.
D.It defines how long BPDU information is stored before being considered expired.
AnswerD

Max-age sets the interval BPDU information is retained before being discarded as stale. Once this timer expires without a fresh BPDU from the root or designated bridge, the switch ages out that topology information and reconverges, preventing indefinite reliance on outdated spanning-tree data.

Why this answer

The 'max-age' timer in RSTP/MSTP (IEEE 802.1D-2004 / 802.1Q) defines how long a bridge retains received BPDU information before it declares that information stale and ages it out. When the max-age timer expires without receiving a fresh BPDU from the designated bridge, the port transitions and topology information is recalculated. On Juniper switches, 'set protocols rstp max-age <seconds>' tunes this BPDU-aging timer, with a default of 20 seconds and a valid range of 6–40 seconds.

Exam trap

JNCIS-ENT often tests the confusion between STP timers — candidates mix up max-age (BPDU expiry), forward delay (state transition), hello-time (BPDU interval), and max-hops (MST region diameter), so read each option carefully to identify which timer is actually being described.

How to eliminate wrong answers

Option A is wrong because the time a port stays in discarding during bootup is governed by the forwarding delay and the proposal/agreement handshake, not max-age. Option B is wrong because MAC table aging is controlled by the bridge's MAC aging timer (default 300 seconds on Juniper), which is independent of the STP max-age timer. Option C is wrong because the maximum hops an MSTP BPDU can traverse is controlled by the 'max-hops' parameter (default 20), not max-age.

Practice this question →
33
Multi-Selecthard

Which TWO statements are correct regarding the operation of MSTP instance 0 (CIST)? (Choose two.)

Select 2 answers
A.It handles any VLANs that are not explicitly mapped to another user-defined MST instance.
B.It operates independently without any interaction with external RSTP switches.
C.It is limited strictly to a maximum hop count of 1.
D.It acts as the Common and Internal Spanning Tree (CIST).
E.It prohibits the use of root protection on instance 0 ports.
AnswersA, D

MSTP instance 0, the CIST, carries all VLANs not explicitly mapped to another user-defined MST instance. This default mapping ensures every VLAN has a spanning-tree topology even when administrators define additional instances for load balancing.

Why this answer

Option A is correct because MSTP instance 0, the CIST, carries all VLANs that are not explicitly mapped to a user-defined MST instance (MSTI); these unmapped VLANs default to the CIST and are forwarded along the CIST topology. Option D is correct because instance 0 is by definition the Common and Internal Spanning Tree (CIST), which provides the single spanning-tree topology that interconnects all MST regions and also interoperates with external SST/RSTP regions. The unmarked options are wrong: B is false because the CIST does interact with external RSTP switches by treating each external region as a virtual bridge and running RSTP-based interoperation at region boundaries; C is false because MSTP uses the standard 802.1D hop-count limit of 20 (not 1) for the CIST; and E is false because root protection (root guard) can be configured on CIST/instance-0 ports to prevent them from becoming root ports.

Exam trap

JNCIS-ENT often tests the misconception that instance 0 is isolated from external switches or that it has special limitations, when it is actually the CIST that ensures interoperability.

Practice this question →
34
MCQhard

You have an EX Series switch running RSTP. You notice that a port connected to a server is flapping between forwarding and discarding states. The server NIC is configured for teaming but is not sending BPDUs. You have configured the port as an edge port using 'set protocols rstp interface ge-0/0/5 edge'. However, the port still flaps. What is the most likely cause?

A.The edge port configuration is not supported on EX Series switches running RSTP.
B.The physical link is flapping, causing RSTP to transition the port.
C.The server is sending BPDUs, causing the edge port to revert to a non-edge port.
D.The port is configured as an edge port but is receiving BPDUs from another switch on the same segment.
AnswerB

If the physical link is flapping (e.g., due to a bad cable or NIC), RSTP will detect link up/down events and transition the port accordingly. Even with edge port configured, link flapping causes the port to go down and up, leading to state changes. This is the most likely cause given the server is not sending BPDUs and edge is configured.

Why this answer

The most likely cause is a flapping physical link. Edge port configuration prevents RSTP from expecting BPDUs, but it does not prevent state changes due to link up/down events. If the link is unstable, the port will transition between discarding and forwarding.

Checking interface statistics for link flaps would confirm. Other options are less likely because edge is supported, and the server is not sending BPDUs.

Exam trap

The trap here is focusing on spanning-tree protocol behavior and overlooking Layer 1 physical link instability as the root cause of state changes.

Practice this question →
35
MCQhard

You are troubleshooting an RSTP network. You notice that a port on an EX Series switch is flapping between Discarding and Forwarding states. You suspect a BPDU inconsistency. Which command would you use to check for BPDU inconsistencies on the switch?

A.show spanning-tree interface detail
B.show spanning-tree statistics
C.show spanning-tree bridge
D.show spanning-tree interface
AnswerA

On Juniper EX Series switches, the 'show spanning-tree interface detail' command provides detailed information for each interface, including any BPDU inconsistencies detected, such as type or version mismatches. This is the correct command to identify why a port might be flapping due to inconsistent BPDUs.

Why this answer

The 'show spanning-tree interface detail' command gives comprehensive per-interface information, including BPDU inconsistencies that can cause port state flapping. Other commands provide statistics or general status but lack the specific inconsistency details needed for this troubleshooting scenario.

Exam trap

The trap here is assuming that 'show spanning-tree statistics' will reveal BPDU inconsistencies, when it only shows counts.

Practice this question →
36
MCQeasy

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

A.It forwards traffic between two different MST regions.
B.It connects the root bridge to all downstream non-root switches.
C.It provides the lowest-cost path from a non-root switch to the root bridge.
D.It serves as the backup forwarding path when the designated port fails.
AnswerC

RSTP elects one Root Port per non-root bridge: the port advertising the lowest cumulative root path cost toward the root bridge. This satisfies the stem's requirement, giving that switch its single best path to the root, with Designated Ports forwarding downstream.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the distinction between root port, designated port, and alternate port; candidates may confuse the root port with the port on the root bridge.

How to eliminate wrong answers

Option A is wrong because forwarding traffic between MST regions is the role of a boundary port, not the root port. Option B is wrong because the root port does not connect the root bridge to downstream switches; that is the role of designated ports on the root bridge. Option D is wrong because the backup forwarding path when a designated port fails is provided by an alternate port, not the root port.

Practice this question →
37
MCQhard

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

A.It encrypts the BPDU payload to prevent tampering by unauthorized access switches.
B.It allows neighboring switches to verify if they belong to the same MST region.
C.It acts as a dynamic bridge priority modifier for internal instance root selection.
D.It determines the Designated Master Bridge within the CIST instance.
AnswerB

The configuration digest is an MD5 hash computed from the region name, revision level and VLAN-to-instance mapping. Neighbours compare digests in BPDUs, so mismatched values prove the switches sit in different MST regions rather than merely different revisions.

Why this answer

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.

Exam trap

JNCIS-ENT often tests the components of MSTP region configuration and the role of the digest, and candidates may incorrectly assume it provides security or affects root election.

How to eliminate wrong answers

Option A is wrong because the digest does not encrypt BPDUs; it is a checksum for configuration verification, not a security mechanism. Option C is wrong because the digest does not modify bridge priority; it is used for region identification. Option D is wrong because the Designated Master Bridge is elected based on bridge priority and MAC address, not the digest.

Practice this question →
38
Multi-Selectmedium

Which THREE states are defined in Rapid Spanning Tree Protocol (RSTP)? (Choose three.)

Select 3 answers
A.Blocking
B.Discarding
C.Learning
D.Forwarding
E.Listening
AnswersB, C, D

RSTP defines only three port states: Discarding, Learning and Forwarding. Discarding merges the former Disabled, Blocking and Listening states, so it satisfies the stem's requirement for a defined RSTP state rather than a legacy STP state.

Why this answer

RSTP (IEEE 802.1w) defines only three port states: Discarding, Learning, and Forwarding. Option B (Discarding) is correct because RSTP merged the 802.1D Disabled, Blocking, and Listening states into a single Discarding state, in which the port neither learns MAC addresses nor forwards frames but still participates in the topology (e.g., receiving BPDUs). Option C (Learning) is correct because it is retained in RSTP: the port processes BPDUs and populates its MAC address table, but does not yet forward user data frames.

Option D (Forwarding) is correct because it is the fully operational state in which the port learns MAC addresses and forwards data frames. Option A (Blocking) is not correct because Blocking is a legacy 802.1D STP state that RSTP replaced with Discarding, and Option E (Listening) is not correct because Listening was also an 802.1D-only state eliminated in RSTP and folded into Discarding.

Exam trap

JNCIS-ENT often tests the state-count reduction between 802.1D and 802.1w, tricking candidates who memorized the five legacy STP states into selecting Blocking or Listening for RSTP.

Practice this question →

Ready to test yourself?

Try a timed practice session using only Spanning Tree questions.

Start 20-question session