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Rapid PVST+ Spanning Tree Protocol Operations and Roles

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Question 1Question →

Which two statements accurately describe Rapid PVST+ port roles and operational port states? (Select two.)

Select all that apply

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Answer: An Alternate port maintains a backup path to the root bridge and remains in the Discarding state.; Rapid PVST+ reduces the legacy 802.1D Disabled, Blocking, and Listening states into a single Discarding state.

Answer

An Alternate port maintains a backup path to the root bridge and remains in the Discarding state, and Rapid PVST+ reduces the legacy 802.1D Disabled, Blocking, and Listening states into a single Discarding state.
Rapid PVST+ (802.1w) optimizes Spanning Tree by consolidating the 802.1D Disabled, Blocking, and Listening states into a single Discarding state. Under this model, an Alternate port provides a backup path to the root bridge and stays in the Discarding state during normal operation.

Step-by-Step Solution

1
Identify the port states supported by Rapid PVST+ (802.1w).
Rapid PVST+ consolidates legacy 802.1D port states (Disabled, Blocking, Listening) into a single Discarding state.
This reduces operational complexity and improves convergence time.
2
Analyze the function and state of an Alternate port role in Rapid PVST+.
An Alternate port offers an alternate path to the root bridge and remains discarded (not forwarding traffic) unless the primary root port loses connectivity.
Alternate ports provide rapid failover without needing to undergo standard convergence delays.

Key Concept

Rapid PVST+ Port Roles and Port States
Question 2Question →

A network administrator is evaluating Rapid PVST+ spanning-tree behavior across a multi-switch enterprise topology containing VLAN 10 and VLAN 20. Switch-A is configured with `spanning-tree vlan 10 priority 4096`, while Switch-B is configured with `spanning-tree vlan 20 priority 8192`. Switch-C connects to both Switch-A and Switch-B via 802.1Q trunk links with default Rapid PVST+ settings. Which two statements correctly describe the resulting Rapid PVST+ bridge operations, port roles, and configuration rules? (Select two choices.)

Select all that apply

Show answer & explanation

Answer: For VLAN 10, Switch-A advertises a total Bridge ID priority value of 4106 in its outbound BPDUs due to the addition of the 12-bit System ID Extension.; On Switch-C, an interface assigned the Alternate port role remains in the Discarding state while receiving superior BPDUs from a designated switch.

Answer

The correct statements are that Switch-A advertises a total priority value of 4106 for VLAN 10 due to the System ID Extension, and an Alternate port on Switch-C remains in the Discarding state while receiving superior BPDUs.
In Rapid PVST+, the Bridge ID priority calculation incorporates the 12-bit System ID Extension, resulting in an advertised priority of 4096+10=41064096 + 10 = 4106 for VLAN 10 on Switch-A. Additionally, an Alternate port provides a backup root path and resides in the Discarding state while continuing to process superior BPDUs from the designated bridge.

Step-by-Step Solution

1
Calculate the total Bridge ID priority advertised by Switch-A for VLAN 10.
Total Priority = Base Priority (4096) + System ID Extension (VLAN 10) = 4106.
Rapid PVST+ dynamically includes the VLAN ID inside the 12-bit System ID Extension field of the 16-bit Bridge Priority structure.
2
Verify Cisco IOS syntax rules for configuring STP bridge priority.
Configuring a value such as 4100 is rejected by Cisco IOS.
STP base priority values must be exact multiples of 4096 (0,4096,8192,…,614400, 4096, 8192, \dots, 61440).
3
Determine the state and behavior of an Alternate port in Rapid PVST+.
The Alternate port is placed in the Discarding state.
An Alternate port offers an alternate path to the root bridge but remains blocked (Discarding state) as long as it receives superior BPDUs from another switch.
4
Evaluate the operational mechanics of PortFast on switch links.
PortFast transitions ports directly to Forwarding, not Learning, and does not disable BPDU traffic.
PortFast is intended exclusively for single-host end devices to skip STP listening/learning phases. Applying it to trunk links connecting switches threatens network stability.

Key Concept

Rapid PVST+ Bridge ID Structure, Port Roles, and Port States
Question 3Question →

Which two statements accurately describe the operational characteristics of port roles and port states in Cisco Rapid PVST+ (802.1w)?

Select all that apply

Show answer & explanation

Answer: An Alternate port provides an alternate path toward the Root Bridge and remains in the Discarding state under normal operation.; Rapid PVST+ consolidates the legacy 802.1D Listening, Blocking, and Disabled states into a single Discarding state.

Answer

The statement that an Alternate port serves as a backup path in the Discarding state and the statement that Rapid PVST+ merges Listening, Blocking, and Disabled states into a single Discarding state are both correct.
Rapid PVST+ streamlines STP operation by using only three states: Discarding, Learning, and Forwarding. The Discarding state merges the old 802.1D Disabled, Blocking, and Listening states. An Alternate port is a designated backup path toward the Root Bridge that remains in the Discarding state while receiving BPDUs from other switches.

Step-by-Step Solution

1
Analyze Rapid PVST+ port states
Rapid PVST+ (802.1w) reduces port states to Discarding, Learning, and Forwarding.
Legacy 802.1D states of Disabled, Blocking, and Listening are combined into the Discarding state in 802.1w.
2
Analyze Rapid PVST+ port roles
An Alternate port is a backup path to the Root Bridge that stays in Discarding state until the primary Root Port fails.
Alternate ports receive superior BPDUs from neighboring switches but are blocked from forwarding frame traffic to maintain a loop-free topology.
3
Evaluate distractor regarding PortFast
PortFast is designed only for end-host (edge) access ports, not switch-to-switch trunks.
Configuring PortFast on trunks can lead to transient or persistent Layer 2 switching loops.
4
Evaluate distractor regarding bridge priority configuration
Bridge priority values must be assigned in multiples of 4096.
The 16-bit Bridge ID consists of a 4-bit priority field (increments of 4096) and a 12-bit System ID Extension carrying the VLAN ID.

Key Concept

Rapid PVST+ Port Roles, States, and Priority Configuration
Question 4Question →

A network engineer connects a new point-to-point link between Switch-1 and Switch-2 in an enterprise network running Rapid PVST+. Place the steps of the RSTP Proposal/Agreement fast convergence mechanism in the correct chronological order from link initialization to the Designated Port entering the Forwarding state.

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Answer

The correct chronological sequence for the Rapid PVST+ Proposal/Agreement handshake is: 1) Switch-1 sends a BPDU with the Proposal bit set on its Designated Port (in Discarding state) toward Switch-2. 2) Switch-2 receives the superior BPDU, selects the receiving port as its new Root Port, and enters the synchronization phase. 3) Switch-2 blocks all of its non-edge Designated Ports by placing them into the Discarding state. 4) Switch-2 transmits an Agreement BPDU back to Switch-1 out of its newly selected Root Port. 5) Switch-1 receives the Agreement BPDU and immediately transitions its Designated Port to the Forwarding state.
In Rapid PVST+ (IEEE 802.1w), fast convergence on point-to-point links relies on an explicit proposal/agreement handshake. When a link initializes, the upstream switch sends a BPDU with the Proposal bit set while its Designated Port is in the Discarding state. The downstream switch evaluates the BPDU, adopts the interface as its new Root Port, and enters the sync phase by blocking (placing in Discarding) all non-edge Designated Ports to eliminate loop potential. Once synchronized, the downstream switch sends an Agreement BPDU back out its Root Port. Upon receiving the Agreement BPDU, the upstream switch immediately transitions its Designated Port into the Forwarding state without waiting for traditional 802.1D timers.

Step-by-Step Solution

1
Initiate the handshake proposal
Switch-1 sends a BPDU with the Proposal bit set while its port is in the Discarding state.
Rapid PVST+ relies on active proposal/agreement handshake frames across point-to-point links rather than passive timer delays.
2
Process the proposal on the receiving switch
Switch-2 identifies the interface as its new Root Port upon evaluating the superior BPDU.
Accepting a superior BPDU triggers Switch-2 to sync its topology before confirming agreement.
3
Synchronize downstream ports to prevent loops
Switch-2 forces all non-edge Designated Ports into the Discarding state (sync process).
Isolating local non-edge ports guarantees that no active switching loop can form while transitioning the new Root Port.
4
Send agreement confirmation
Switch-2 transmits an Agreement BPDU back to Switch-1.
The Agreement BPDU explicitly informs Switch-1 that Switch-2 has safely synchronized its downstream ports.
5
Complete rapid transition to Forwarding
Switch-1 transitions its Designated Port directly into the Forwarding state.
Because Switch-2 confirmed loop safety via the Agreement BPDU, Switch-1 can bypass legacy 802.1D listening and learning timers.

Key Concept

Rapid PVST+ Proposal/Agreement Handshake Mechanism
Question 5Question →

A network administrator configures a Cisco Catalyst switch running Rapid PVST+ with the command `spanning-tree vlan 50 priority 8192`. What total bridge priority value will this switch include in its outgoing Bridge Protocol Data Units (BPDUs) for VLAN 50?

Show answer & explanation

Answer: 82428242

Answer

The total bridge priority included in BPDUs for VLAN 50 is 82428242.
In Cisco Rapid PVST+, the 16-bit Bridge Priority field is divided into a 4-bit configurable priority value (in increments of 4096) and a 12-bit System ID Extension representing the VLAN ID. When a base priority of 81928192 is configured for VLAN 50, the switch combines the base priority with the VLAN ID (8192+50=82428192 + 50 = 8242) to form the total bridge priority advertised in BPDUs.

Step-by-Step Solution

1
Identify the base bridge priority configured on the switch.
The configured base bridge priority is 81928192.
The CLI command explicitly sets the base priority to 81928192.
2
Determine the System ID Extension for the specified VLAN.
The System ID Extension is equal to the VLAN ID, which is 5050.
Rapid PVST+ uses the 12-bit System ID Extension field to encode the VLAN number into the Bridge ID.
3
Calculate the total bridge priority advertised in BPDUs.
8192+50=82428192 + 50 = 8242.
Total Bridge Priority = Base Priority + System ID Extension.

Key Concept

Rapid PVST+ Bridge ID Structure and System ID Extension
Question 6Question →

A network administrator is configuring Rapid PVST+ on a Cisco Catalyst switch. The administrator intends for the switch to advertise a total Bridge Priority of 2053020530 for VLAN 5050 in its outgoing BPDUs to influence the Root Bridge election. Which priority value must be specified in the `spanning-tree vlan 50 priority <value>` command?

Show answer & explanation

Answer: 20480

Answer

The base priority value that must be configured is 20480.
In Cisco Rapid PVST+, the Bridge Priority is composed of a 4-bit Base Priority field and a 12-bit System ID Extension field containing the VLAN ID. The Base Priority must be configured as a multiple of 4096 (0,4096,8192,dots,614400, 4096, 8192, dots, 61440). When the command `spanning-tree vlan 50 priority 20480` is executed, the switch automatically adds the VLAN ID (5050) to the base priority (2048020480), producing a total advertised Bridge Priority of 2053020530.

Step-by-Step Solution

1
Understand the Spanning Tree Bridge Identifier (BID) priority composition in Rapid PVST+.
Total Bridge Priority = Base Priority + System ID Extension (VLAN ID).
Rapid PVST+ uses 802.1t System ID Extension, allocating 4 bits for the base priority (in increments of 4096) and 12 bits for the VLAN ID.
2
Calculate the required base priority for VLAN 50 given a total priority of 20530.
Base Priority = 20530 - 50 = 20480.
Subtracting the VLAN ID (50) from the target total Bridge Priority yields the exact base priority.
3
Verify that the base priority is a valid increment of 4096.
20480 / 4096 = 5 (an exact integer multiple).
The Cisco IOS CLI accepts only values in increments of 4096 (e.g., 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768).

Key Concept

Rapid PVST+ System ID Extension and Bridge Priority Increment Calculation
Question 7Question →

A enterprise network topology consists of three Cisco Catalyst switches (SwitchA, SwitchB, and SwitchC) interconnected in a redundant ring running Rapid PVST+ for VLAN 10. SwitchA is configured as the Root Bridge with a base priority of 4096. SwitchB has a base priority of 28672, and SwitchC has a base priority of 32768. All inter-switch links operate at 1 Gbps GigabitEthernet speeds (STP path cost = 4). SwitchC connects directly to SwitchA via interface GigabitEthernet0/1 and to SwitchB via interface GigabitEthernet0/2. Which Rapid PVST+ port role and state will interface GigabitEthernet0/2 assume on SwitchC for VLAN 10 after STP convergence completes?

Show answer & explanation

Answer: Alternate port in the Discarding state

Answer

Interface GigabitEthernet0/2 on SwitchC will become an Alternate port in the Discarding state.
SwitchC selects GigabitEthernet0/1 as its Root Port because its direct path to SwitchA has a cumulative path cost of 4, compared to a cost of 8 through SwitchB. For the link between SwitchB and SwitchC, both switches have an equal cost to the Root Bridge (4), so the switch with the lower Bridge ID (SwitchB at 28682 vs SwitchC at 32778) claims the Designated Port role. As a result, SwitchC's GigabitEthernet0/2 interface becomes an Alternate port and enters the Discarding state.

Step-by-Step Solution

1
Calculate effective Bridge IDs including the System ID Extension (VLAN 10).
SwitchA BID = 4096 + 10 = 4106 (Root Bridge). SwitchB BID = 28672 + 10 = 28682. SwitchC BID = 32768 + 10 = 32778.
Rapid PVST+ appends the 12-bit VLAN ID to the base priority to form the bridge priority.
2
Evaluate root path costs for SwitchC to select its Root Port.
Path via Gi0/1 (direct to SwitchA) cost = 4. Path via Gi0/2 (to SwitchB to SwitchA) cost = 4 + 4 = 8. Gi0/1 becomes Root Port (Forwarding).
Each non-root switch selects the single port with the lowest cumulative cost toward the Root Bridge.
3
Determine Designated Port role on the segment between SwitchB and SwitchC.
Both SwitchB and SwitchC have a root path cost of 4 to SwitchA. SwitchB has a lower BID (28682) than SwitchC (32778), so SwitchB's interface becomes the Designated Port (Forwarding).
When root path costs on a segment are tied, the switch with the lowest Bridge ID wins the Designated Port role.
4
Assign the remaining port role for SwitchC's GigabitEthernet0/2 interface.
GigabitEthernet0/2 receives superior BPDUs from SwitchB and is assigned the Alternate port role in the Discarding state.
In Rapid PVST+ (802.1w), a port receiving BPDUs from another switch on a non-designated link serves as an Alternate port to provide a backup path to the root.

Key Concept

Rapid PVST+ Port Role and State Determination
Question 8Question →

A network engineer configures a Cisco switch running Rapid PVST+ to serve as the root bridge for VLAN 12. The engineer executes the global configuration command `spanning-tree vlan 12 priority 12288`. What total Bridge Priority value will be carried in the BPDUs transmitted by this switch for VLAN 12?

Show answer & explanation

Answer: 12288

Answer

12300
In Rapid PVST+, Cisco switches use the 802.1t System ID Extension, which split the 16-bit Bridge Priority field into a 4-bit Base Priority field (increments of 4096) and a 12-bit System ID Extension field carrying the VLAN ID. When `spanning-tree vlan 12 priority 12288` is configured, the total priority field transmitted in BPDUs is the sum of the base priority (1228812288) and the VLAN ID (1212), resulting in 1230012300.

Step-by-Step Solution

1
Identify the configured base bridge priority value.
The base priority is configured as 12288 (a valid multiple of 4096).
STP base priority values must be specified in increments of 4096.
2
Identify the System ID Extension (VLAN ID).
The System ID Extension is 12 for VLAN 12.
Rapid PVST+ uses the 12-bit System ID Extension field in the Bridge Identifier to encode the VLAN number.
3
Calculate the total Bridge Priority transmitted in BPDUs.
Total Priority = 12288 + 12 = 12300.
The total priority contained within BPDUs is the sum of the configured base priority and the System ID Extension.

Key Concept

Rapid PVST+ System ID Extension and Total Bridge Identifier Priority Calculation
Question 9Question →

Which two statements accurately describe port roles, states, and configuration rules in Rapid PVST+? (Select two choices.)

Select all that apply

Show answer & explanation

Answer: An Alternate port provides a backup path to the Root Bridge and remains in the discarding state under normal operation.; Rapid PVST+ removes the 802.1D Listening port state and consolidates non-forwarding states into the Discarding state.

Answer

The correct statements are that an Alternate port provides a backup path to the Root Bridge while remaining in a discarding state, and that Rapid PVST+ eliminates the 802.1D Listening state by using the Discarding state.
Rapid PVST+ introduces the Alternate port role, which provides an instant backup path to the Root Bridge while staying in a discarding state. Additionally, Rapid PVST+ streamlines the IEEE 802.1D port states (Disabled, Blocking, Listening, Learning, Forwarding) down to three operational states: Discarding, Learning, and Forwarding.

Step-by-Step Solution

1
Evaluate the role of an Alternate port in Rapid PVST+.
Confirm that an Alternate port receives BPDUs from another switch and acts as an alternate path to the Root Bridge in the discarding state.
RSTP/Rapid PVST+ defines Alternate ports for fast convergence when a Root port link fails.
2
Evaluate Rapid PVST+ operational port states.
Identify that 802.1w simplifies port states to Discarding, Learning, and Forwarding, combining Disabled, Blocking, and Listening into Discarding.
This structural simplification enables faster overall protocol operation.
3
Verify bridge priority increments and PortFast guidelines.
Reject custom non-multiplier priority values and reject applying PortFast on switch interconnects.
Priority values must adhere to steps of 4096 due to the System ID Extension, and PortFast is reserved exclusively for edge access interfaces.

Key Concept

Rapid PVST+ Port Roles, States, and Priority Calculations
Question 10Question →

Place the tie-breaking criteria evaluated by a non-root switch running Rapid PVST+ in the correct sequence (from first evaluated to last evaluated) when selecting its Root Port.

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Answer

The correct sequence for Rapid PVST+ Root Port selection tie-breaking is: 1) Lowest cumulative Root Path Cost, 2) Lowest Designated (Sender) Bridge ID, 3) Lowest Designated (Sender) Port ID, and 4) Lowest Local Port ID on the receiving switch.
Rapid PVST+ evaluates Root Port selection strictly in sequence: cumulative path cost first, followed by upstream sender Bridge ID, upstream sender Port ID, and finally the local receiving Port ID as a last resort.

Step-by-Step Solution

1
Identify the primary criterion for Root Port selection in Spanning Tree operations.
Path cost evaluation is performed first.
The switch determines the shortest administrative distance to the Root Bridge by adding the local port cost to the root path cost reported in the incoming BPDU.
2
Evaluate the first tie-breaker if path costs are equal.
Designated (Sender) Bridge ID is evaluated.
When equal-cost paths originate from different neighboring switches, the switch prefers the neighbor with the lowest Bridge ID (Priority + System ID Extension + MAC Address).
3
Evaluate the second tie-breaker if receiving BPDUs from the same neighbor across parallel links.
Designated (Sender) Port ID is evaluated.
When parallel links connect to the same upstream switch, the neighbor Bridge ID is identical, so the sender's Port ID (Port Priority + Port Number) breaks the tie.
4
Evaluate the final tie-breaker if BPDUs arrive on different local interfaces from the exact same remote port.
Local Port ID is evaluated.
If all sender parameters are identical, the local receiving switch uses its own lowest Port ID (Port Priority + Port Number) as the definitive tie-breaker.

Key Concept

Rapid PVST+ Root Port Selection Algorithm
Question 11Question →

Place the steps of the Rapid PVST+ Proposal and Agreement handshake process in the correct chronological order, starting from when a designated switch initiates fast convergence on a point-to-point link to when its port enters the Forwarding state.

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Answer

The correct sequence starts with the designated switch proposing the link state, followed by the receiving switch synchronizing non-edge ports, returning an Agreement BPDU, and finally the designated switch transitioning directly to the Forwarding state.
The Rapid PVST+ (802.1w) sync process operates in a strict sequence on point-to-point links: first, a designated port proposes rapid transition; second, the neighbor switch synchronizes by blocking non-edge designated ports; third, the neighbor sends an Agreement BPDU back; fourth, the proposing port immediately moves to the Forwarding state.

Step-by-Step Solution

1
Identify the initiation of the handshake.
The designated switch transmits a Proposal BPDU to request rapid transition on the point-to-point link.
Rapid PVST+ relies on explicit point-to-point messaging rather than timer-based delays.
2
Determine the synchronization action taken by the receiver.
The receiver puts non-edge ports into the Discarding state.
This guarantees no active loops can form while the topology adapts.
3
Identify the response message.
The receiver sends an Agreement BPDU back to the proposal sender.
The agreement signals that the receiving switch has safely isolated its local ports.
4
Determine the final state transition.
The initiating designated port transitions immediately into the Forwarding state.
Upon receiving agreement, the port moves directly to forwarding without waiting for 802.1D forward delay timers.

Key Concept

Rapid PVST+ Proposal and Agreement Handshake Mechanism
Question 12Question →

A network administrator needs to manually configure a Catalyst switch as the primary Root Bridge for VLAN 30 using Rapid PVST+. The switch currently has default STP settings. Which configuration change correctly sets the bridge priority, and what is the resulting Bridge Priority value advertised in the Bridge Protocol Data Units (BPDUs) for VLAN 30?

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Answer: Execute 'spanning-tree vlan 30 priority 4096'; the advertised Bridge Priority in BPDUs will be 4126.

Answer

Execute 'spanning-tree vlan 30 priority 4096'; the advertised Bridge Priority in BPDUs will be 4126.
In Rapid PVST+, the Bridge ID priority consists of a 4-bit base priority and a 12-bit System ID Extension representing the VLAN ID. The command 'spanning-tree vlan 30 priority 4096' sets the base priority to a valid multiple of 4096. When combined with the System ID Extension of 30, the switch advertises a total Bridge Priority of 4126 (4096+304096 + 30) in its BPDUs.

Step-by-Step Solution

1
Determine valid STP bridge priority configuration increments.
STP bridge priority must be configured in increments of 4096 (e.g., 0, 4096, 8192, 12288, etc.).
The 16-bit Priority field in the Bridge ID (BID) is divided into a 4-bit Priority field (multiples of 4096) and a 12-bit System ID Extension field.
2
Calculate the total advertised Bridge Priority for VLAN 30.
Base Priority (4096) + System ID Extension (VLAN 30) = 4126.
Rapid PVST+ appends the 12-bit VLAN ID to the 4-bit base priority value to form the complete 16-bit priority component of the Bridge ID.

Key Concept

Rapid PVST+ System ID Extension and Bridge Priority Calculation
Question 13Question →

A Cisco Catalyst switch running Rapid PVST+ receives BPDUs on multiple interfaces from different neighboring switches connected to the same Root Switch. Place the decision criteria in the exact sequence the switch uses to select its Root Port, starting from the first criterion evaluated to the final tie-breaker.

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Answer

The correct evaluation sequence for Root Port selection in Rapid PVST+ is: 1. Lowest cumulative Root Path Cost received in the BPDUs, 2. Lowest Sender (Designated) Bridge ID, 3. Lowest Sender (Designated) Port ID, 4. Lowest local (Receiver) Port ID.
Rapid PVST+ follows a strict 4-step decision sequence to select a single Root Port on non-root switches: 1) Lowest cumulative Root Path Cost, 2) Lowest Sender Bridge ID, 3) Lowest Sender Port ID, and 4) Lowest local (Receiver) Port ID.

Step-by-Step Solution

1
Evaluate incoming Root Path Cost
The switch identifies the port(s) offering the shortest cumulative cost to the Root Bridge.
Path cost reflects link speed and topology length, making it the primary factor for optimal routing toward the root.
2
Break ties using upstream switch identity
If path costs are equal, the port receiving BPDUs from the upstream switch with the lowest Bridge ID (Priority + MAC) is preferred.
Bridge ID serves as the first tie-breaker when multiple paths offer equal cost through different neighboring switches.
3
Break ties using upstream port identity
If candidate ports connect to the exact same upstream switch, the port connected to the lower upstream Port ID (Priority + Port Number) is selected.
Designated Port ID identifies which specific interface on the neighboring switch is sending the BPDU.
4
Break ties using local port identity
If all preceding criteria are identical (e.g., connected via a hub), the switch selects its own local interface with the lowest Port ID.
Local Port ID guarantees a deterministic decision when upstream information offers no distinction.

Key Concept

Rapid PVST+ Root Port Selection Tie-Breaker Logic
Estimated Time:1m 30s
Question 14Question →

A network engineer is configuring Rapid PVST+ on a Cisco Catalyst switch to force it to become the Root Bridge for VLAN 40. The engineer attempts to configure a custom bridge priority value using the Cisco IOS CLI command line syntax. Which command entry contains a valid bridge priority value that Cisco IOS will accept, and what total bridge priority value will the switch advertise in its outbound Spanning Tree Protocol BPDUs for VLAN 40?

Show answer & explanation

Answer: The command 'spanning-tree vlan 40 priority 4096' will be accepted, and the advertised total bridge priority will be 4136.

Answer

The command 'spanning-tree vlan 40 priority 4096' will be accepted, and the advertised total bridge priority will be 4136.
In Cisco Rapid PVST+, the 16-bit priority portion of the 8-byte Bridge ID (BID) is divided into a 4-bit Bridge Priority and a 12-bit System ID Extension. Because only 4 bits are allocated to user configuration (24=162^4 = 16 levels), priority values in CLI commands must increase in steps of 4096 (4096×1=40964096 \times 1 = 4096). When the command 'spanning-tree vlan 40 priority 4096' is executed, Cisco IOS validates 40964096 as a valid base priority. During BPDU generation for VLAN 40, the switch adds the VLAN ID (4040) as the System ID Extension, resulting in a total priority value of 4096+40=41364096 + 40 = 4136.

Step-by-Step Solution

1
Verify CLI input validation rules for Rapid PVST+ bridge priority
Cisco IOS requires configurable bridge priority values to be explicit multiples of 4096 (0,4096,8192,12288,…,614400, 4096, 8192, 12288, \dots, 61440).
The 16-bit Priority field in the Bridge ID (BID) uses the 4 most significant bits for configurable priority and the remaining 12 bits for the System ID Extension (VLAN ID).
2
Calculate the total advertised Bridge Priority field value
Base Priority (40964096) + System ID Extension (4040) = Total Priority (41364136).
Rapid PVST+ dynamically inserts the 12-bit VLAN ID into the System ID Extension field for per-VLAN spanning tree identification.

Key Concept

Rapid PVST+ Bridge ID Structure and System ID Extension Calculation
Question 15Question →

A network administrator is configuring a Cisco Catalyst switch running Rapid PVST+ to serve as a secondary root bridge for VLAN 20. The administrator intends to set a custom bridge priority using the command `spanning-tree vlan 20 priority <value>`. Which value will Cisco IOS accept for this command?

Show answer & explanation

Answer: 24576

Answer

The correct option is 24576 because base bridge priority in Rapid PVST+ must be configured as a strict multiple of 4096.
In Rapid PVST+, Cisco IOS enforces that bridge priority settings configured via CLI must be exact multiples of 4096 (range 0 to 61440). The value 24576 equals 6 × 4096, making it a valid base priority parameter.

Step-by-Step Solution

1
Understand Rapid PVST+ Priority Architecture
Rapid PVST+ uses the 802.1t System ID Extension, which splits the 16-bit Bridge Priority field into a 4-bit Priority field and a 12-bit System ID Extension (VLAN ID).
Because only the top 4 bits are allocated for configurable priority, priority values can only change in steps of 212=40962^{12} = 4096.
2
Evaluate CLI configuration requirements
When executing `spanning-tree vlan <vlan-id> priority <value>`, the `<value>` parameter must be a multiple of 4096 (e.g., 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, etc.).
Cisco IOS automatically adds the VLAN ID to the base priority when constructing the BPDU, so the user must specify the unshifted base priority.
3
Test candidate values
Calculating 24576/4096=624576 / 4096 = 6, which is an exact integer. Other choices (24596, 20000, 8212) leave non-zero remainders when divided by 4096.
Only 24576 satisfies the increment requirement and will be accepted by Cisco IOS.

Key Concept

Rapid PVST+ Bridge Priority Configuration and 4096 Increment Constraint
Question 16Question →

A network engineer is evaluating the Rapid PVST+ topology operational mechanics across a enterprise LAN segment. A switch has multiple active connections to adjacent switches and redundant interswitch links. Which two statements correctly describe Rapid PVST+ port role operations and behaviors in this environment? (Select two.)

Select all that apply

Show answer & explanation

Answer: An Alternate port receives BPDUs from a neighbor switch, provides an alternate path to the root bridge, and remains in the Discarding state.; A Backup port receives BPDUs originated by its own local switch over a shared collision domain or redundant link to the same segment and remains in the Discarding state.

Answer

The correct statements are that an Alternate port receives BPDUs from a neighbor switch to provide an alternate path toward the root bridge in the Discarding state, and a Backup port receives BPDUs sent by its own switch to provide redundancy for a Designated Port on the same collision domain.
In Rapid PVST+ (IEEE 802.1w implementation in Cisco switches), an Alternate port receives BPDUs from other switches and stands by to replace the Root Port if the active path to the root bridge fails. A Backup port receives BPDUs transmitted by its own switch over a shared media connection and stands by to replace the Designated Port for that specific segment. Both roles maintain a Discarding state to prevent Layer 2 loops.

Step-by-Step Solution

1
Analyze Alternate Port Role
Identified that an Alternate port receives BPDUs from other switches and acts as a standby path to the Root Bridge.
Rapid PVST+ defines Alternate ports to take over immediately if the primary Root Port loses connectivity to the root bridge.
2
Analyze Backup Port Role
Identified that a Backup port receives its own switch's BPDUs on a shared or redundant connection to the same network segment.
A Backup port backs up a local Designated Port on the same LAN segment when multiple interfaces from a single switch connect to the same shared collision domain.
3
Evaluate Distractors and Misconfigurations
Eliminated incorrect assertions regarding role substitution and PortFast trunk misapplications.
Alternate ports back up Root Ports (not Designated Ports), and PortFast is intended for edge host ports, not for bypassing Backup port role logic on interswitch links.

Key Concept

Rapid PVST+ Port Roles (Root, Designated, Alternate, Backup) and Operational States
Question 17Question →

A network administrator is manually configuring a Cisco switch to become the root bridge for VLAN 10 using Rapid PVST+. Which value is valid to specify with the command `spanning-tree vlan 10 priority <value>`?

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Answer: 24576

Answer

The valid bridge priority setting is 24576, as Rapid PVST+ requires configurable priority values to be in increments of 4096.
In Rapid PVST+, the configurable bridge priority must be a multiple of 4096 (0,4096,8192,…,614400, 4096, 8192, \dots, 61440). The value 24576 is 6×40966 \times 4096, making it a valid configuration parameter. The switch automatically adds the VLAN ID (10) to the base priority to create the combined Bridge Identifier priority of 24586.

Step-by-Step Solution

1
Identify the Rapid PVST+ bridge priority structure requirement
Rapid PVST+ uses the 802.1t System ID Extension, dividing the 16-bit Bridge Priority field into a 4-bit priority component and a 12-bit VLAN ID component.
Because 4 bits are reserved for priority, values can only change by steps of 212=40962^{12} = 4096.
2
Evaluate the candidate values for multiples of 4096
24576/4096=624576 / 4096 = 6, which is an exact integer multiple.
The CLI requires entering the base priority multiplier (6×4096=245766 \times 4096 = 24576), and the switch will internally add VLAN 10 to yield a total priority of 24586.

Key Concept

Rapid PVST+ Bridge Priority Increment Calculation
Estimated Time:45s
Question 18Question →

A network engineer wants to configure a switch as the primary root bridge for VLAN 20 using Rapid PVST+. What is the lowest valid base bridge priority value that can be specified in the configuration command before the System ID Extension is added?

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Answer: 0

Answer

The lowest valid base bridge priority value is 0.
In Rapid PVST+, the base bridge priority is configured in increments of 4096 due to the 12-bit System ID Extension field. The valid range for base priorities is 0 to 61440 in steps of 4096. Therefore, 0 is a valid value and represents the lowest possible base priority.

Step-by-Step Solution

1
Identify the Rapid PVST+ bridge priority configuration rules.
Rapid PVST+ uses the 802.1t System ID Extension, which forces base bridge priority values to be configured in increments of 4096.
The 16-bit Bridge Priority field is divided into a 4-bit Priority field and a 12-bit System ID Extension (VLAN ID).
2
Determine the range of valid base priority values.
Valid base priority values are 0, 4096, 8192, 12288, ..., 61440.
The 4-bit priority multiplier allows values from 0×40960 \times 4096 up to 15×409615 \times 4096.
3
Select the lowest valid configured value.
The minimum allowed base priority is 0 (which results in a total priority of 0+20=200 + 20 = 20 for VLAN 20).
0 is the lowest multiple of 4096.

Key Concept

Rapid PVST+ Bridge Priority and System ID Extension Calculation
Question 19Question →

A network engineer is operating a four-switch topology (Switch-A, Switch-B, Switch-C, and Switch-D) running Rapid PVST+ for VLAN 50. All inter-switch links are 1 Gbps 802.1Q trunks operating with standard IEEE 802.1D short path costs (cost = 4).

The configured base bridge priorities and MAC addresses for VLAN 50 are as follows:
• Switch-A: Base Priority 24576, MAC 0010.1111.1111
• Switch-B: Base Priority 24576, MAC 0010.2222.2222
• Switch-C: Base Priority 28672, MAC 0010.3333.3333
• Switch-D: Base Priority 32768, MAC 0010.4444.4444

Link connections:
��� Switch-C connects to Switch-A via GigabitEthernet0/1.
• Switch-C connects to Switch-B via GigabitEthernet0/2.
• Switch-D connects directly to Switch-A via GigabitEthernet0/1.
• Switch-D connects directly to Switch-B via GigabitEthernet0/2.
• Switch-A and Switch-B are connected together via two parallel 1 Gbps trunks: Trunk-1 (Gi0/1 on both) and Trunk-2 (Gi0/2 on both). On Switch-A, interface Gi0/1 has port priority 112, and Gi0/2 has port priority 128 (default).

Based on Rapid PVST+ operations for VLAN 50, which port role and state are assigned to interface GigabitEthernet0/2 on Switch-C?

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Answer: Alternate port in the Discarding state

Answer

Interface GigabitEthernet0/2 on Switch-C is assigned the Alternate port role and is placed in the Discarding state.
Switch-A is elected Root Bridge for VLAN 50 due to having the lowest Bridge ID (Priority 24576+50=2462624576 + 50 = 24626, MAC 0010.1111.1111). Switch-C's interface Gi0/1 connects directly to Switch-A with a cumulative path cost of 4, making Gi0/1 the Root Port. Interface Gi0/2 on Switch-C connects to Switch-B. Both Switch-B and Switch-C have a Root Path Cost of 4 to reach Switch-A. When electing the Designated Port for the segment connecting Switch-B and Switch-C, Switch-B wins because its Bridge ID (24626.0010.2222.222224626.0010.2222.2222) is lower than Switch-C's Bridge ID (28722.0010.3333.333328722.0010.3333.3333). Therefore, Switch-B's interface becomes Designated Forwarding, and Switch-C's interface Gi0/2 becomes an Alternate port in the Discarding state.

Step-by-Step Solution

1
Calculate the Bridge ID for each switch for VLAN 50 by adding the System ID Extension (VLAN ID 50) to the base bridge priority.
Switch-A Bridge ID: 24576+50=2462624576 + 50 = 24626 with MAC 0010.1111.1111.
Switch-B Bridge ID: 24576+50=2462624576 + 50 = 24626 with MAC 0010.2222.2222.
Switch-C Bridge ID: 28672+50=2872228672 + 50 = 28722 with MAC 0010.3333.3333.
Switch-D Bridge ID: 32768+50=3281832768 + 50 = 32818 with MAC 0010.4444.4444.
Rapid PVST+ uses the 12-bit System ID Extension added to the 4-bit priority field to uniquely identify the bridge priority for each VLAN.
2
Determine the Root Bridge for VLAN 50.
Switch-A is elected Root Bridge because Switch-A and Switch-B tie on priority (24626), but Switch-A has the lower MAC address (0010.1111.1111 < 0010.2222.2222).
The switch with the lowest numerical Bridge ID (Priority + Sys-ID + MAC) becomes the Root Bridge.
3
Determine the Root Port for Switch-C.
Switch-C Gi0/1 connects directly to Root Bridge Switch-A with path cost 4. Switch-C Gi0/2 connects to Switch-B, which has a path cost of 4 to Switch-A (via Gi0/1), making the path via Gi0/2 cost 4+4=84 + 4 = 8. Thus, Switch-C Gi0/1 is the Root Port (Forwarding).
Root Port is selected based on the lowest cumulative Root Path Cost.
4
Determine the role of Switch-C GigabitEthernet0/2 on the link connecting Switch-C and Switch-B.
Both Switch-B and Switch-C advertise a Root Path Cost of 4 on this segment. To elect the Designated Port for the link, compare Bridge IDs of the advertising switches: Switch-B (24626.0010.2222.2222) vs Switch-C (28722.0010.3333.3333). Switch-B has the lower Bridge ID, so Switch-B Gi0/1 becomes the Designated Port (Forwarding). Consequently, Switch-C Gi0/2 becomes an Alternate Port and enters the Discarding state.
In Rapid PVST+, a non-root port receiving superior BPDUs from another switch on a point-to-point link becomes an Alternate port in the Discarding state to maintain a loop-free topology.

Key Concept

Rapid PVST+ Root Bridge Election, Path Cost Calculation, and Alternate Port Role Assignment
Question 20Question →

A network administrator manually configures a switch running Rapid PVST+ with a base bridge priority of 28672 for VLAN 100. What total priority value will be transmitted in the Bridge Identifier (BID) within BPDUs generated for VLAN 100 on this switch?

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Answer: 28772

Answer

28772 is the total bridge priority advertised in the BPDUs for VLAN 100 because Rapid PVST+ adds the VLAN ID (100) to the configured base bridge priority (28672).
In Rapid PVST+, Cisco switches append the 12-bit VLAN ID (System ID Extension) to the 4-bit configurable base priority. Adding the VLAN ID of 100 to the base priority of 28672 yields a total priority value of 28772.

Step-by-Step Solution

1
Identify the base bridge priority and the VLAN ID.
Base priority is 28672 and VLAN ID is 100.
Rapid PVST+ uses the 802.1t System ID Extension mechanism.
2
Calculate total priority by adding the System ID Extension (VLAN ID) to the base priority.
28672+100=2877228672 + 100 = 28772
The 16-bit Priority field in the BID consists of a 4-bit Priority field (multiples of 4096) and a 12-bit System ID Extension field.

Key Concept

Rapid PVST+ System ID Extension and Bridge Identifier (BID) Calculation
Estimated Time:45s
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