Question

Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator is evaluating the operational behavior and port roles of Rapid PVST+ on Cisco Catalyst switches. Which two statements regarding Rapid PVST+ operations are correct?

  1. An Alternate port functions as a backup path to the root bridge and can rapidly transition to the root port role if the active root port fails.Answer
  2. Designated ports utilize an explicit proposal and agreement handshake on point-to-point links to transition directly to the forwarding state.Answer
  3. C
    PortFast should be configured on interswitch trunk links to eliminate spanning tree convergence delays between switches.
  4. D
    The switch bridge priority for a VLAN can be set to any arbitrary integer value between 0 and 65535, such as 5000.

Answer

The two correct statements are that an Alternate port acts as a backup path to the root bridge, and that Designated ports use a proposal and agreement handshake on point-to-point links for rapid state transitions.
Rapid PVST+ defines an Alternate port as a discarded port receiving BPDUs from another switch, providing a backup path toward the root bridge. Additionally, Rapid PVST+ uses a proposal/agreement handshake on point-to-point links to bypass legacy 802.1D listening/learning timers and achieve rapid convergence.

Step-by-Step Solution

1
Analyze Rapid PVST+ port roles and state transition mechanisms.
Identified that Alternate ports offer immediate backup paths to the root bridge, and point-to-point links use proposal/agreement sync mechanisms for fast convergence.
Rapid PVST+ (802.1w implementation per VLAN) replaces timer-based convergence with explicit sync handshakes and active alternate backup paths.
2
Evaluate misconfigurations and priority calculations.
Confirmed that PortFast on trunk links creates loops and priority values must strictly follow 4096 increments due to the System ID Extension.
Bridge priorities consist of a 4-bit priority field (increments of 4096) combined with a 12-bit VLAN ID field.

Key Concept

Rapid PVST+ Port Roles and Sync Handshake Mechanics
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