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Difficulty: EasyRapid PVST+ Spanning Tree Protocol Operations and Roles

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

  1. An Alternate port provides an alternate path toward the Root Bridge and remains in the Discarding state under normal operation.Answer
  2. Rapid PVST+ consolidates the legacy 802.1D Listening, Blocking, and Disabled states into a single Discarding state.Answer
  3. C
    PortFast should be enabled on interswitch trunk links to achieve fast Rapid PVST+ convergence between switches.
  4. D
    The bridge priority value for a VLAN can be configured to any arbitrary integer between 0 and 65535.

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
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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